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| Boilerpipe Text | 3.1.
Objects, values and types
¶
Objects
are Pythonâs abstraction for data. All data in a Python program
is represented by objects or by relations between objects. Even code is
represented by objects.
Every object has an identity, a type and a value. An objectâs
identity
never
changes once it has been created; you may think of it as the objectâs address in
memory. The
is
operator compares the identity of two objects; the
id()
function returns an integer representing its identity.
CPython implementation detail:
For CPython,
id(x)
is the memory address where
x
is stored.
An objectâs type determines the operations that the object supports (e.g., âdoes
it have a length?â) and also defines the possible values for objects of that
type. The
type()
function returns an objectâs type (which is an object
itself). Like its identity, an objectâs
type
is also unchangeable.
[
1
]
The
value
of some objects can change. Objects whose value can
change are said to be
mutable
; objects whose value is unchangeable once they
are created are called
immutable
. (The value of an immutable container object
that contains a reference to a mutable object can change when the latterâs value
is changed; however the container is still considered immutable, because the
collection of objects it contains cannot be changed. So, immutability is not
strictly the same as having an unchangeable value, it is more subtle.) An
objectâs mutability is determined by its type; for instance, numbers, strings
and tuples are immutable, while dictionaries and lists are mutable.
Objects are never explicitly destroyed; however, when they become unreachable
they may be garbage-collected. An implementation is allowed to postpone garbage
collection or omit it altogether â it is a matter of implementation quality
how garbage collection is implemented, as long as no objects are collected that
are still reachable.
CPython implementation detail:
CPython currently uses a reference-counting scheme with (optional) delayed
detection of cyclically linked garbage, which collects most objects as soon
as they become unreachable, but is not guaranteed to collect garbage
containing circular references. See the documentation of the
gc
module for information on controlling the collection of cyclic garbage.
Other implementations act differently and CPython may change.
Do not depend on immediate finalization of objects when they become
unreachable (so you should always close files explicitly).
Note that the use of the implementationâs tracing or debugging facilities may
keep objects alive that would normally be collectable. Also note that catching
an exception with a
try
âŠ
except
statement may keep
objects alive.
Some objects contain references to âexternalâ resources such as open files or
windows. It is understood that these resources are freed when the object is
garbage-collected, but since garbage collection is not guaranteed to happen,
such objects also provide an explicit way to release the external resource,
usually a
close()
method. Programs are strongly recommended to explicitly
close such objects. The
try
âŠ
finally
statement
and the
with
statement provide convenient ways to do this.
Some objects contain references to other objects; these are called
containers
.
Examples of containers are tuples, lists and dictionaries. The references are
part of a containerâs value. In most cases, when we talk about the value of a
container, we imply the values, not the identities of the contained objects;
however, when we talk about the mutability of a container, only the identities
of the immediately contained objects are implied. So, if an immutable container
(like a tuple) contains a reference to a mutable object, its value changes if
that mutable object is changed.
Types affect almost all aspects of object behavior. Even the importance of
object identity is affected in some sense: for immutable types, operations that
compute new values may actually return a reference to any existing object with
the same type and value, while for mutable objects this is not allowed.
For example, after
a
=
1;
b
=
1
,
a
and
b
may or may not refer to
the same object with the value one, depending on the implementation.
This is because
int
is an immutable type, so the reference to
1
can be reused. This behaviour depends on the implementation used, so should
not be relied upon, but is something to be aware of when making use of object
identity tests.
However, after
c
=
[];
d
=
[]
,
c
and
d
are guaranteed to refer to two
different, unique, newly created empty lists. (Note that
e
=
f
=
[]
assigns
the
same
object to both
e
and
f
.)
3.2.
The standard type hierarchy
¶
Below is a list of the types that are built into Python. Extension modules
(written in C, Java, or other languages, depending on the implementation) can
define additional types. Future versions of Python may add types to the type
hierarchy (e.g., rational numbers, efficiently stored arrays of integers, etc.),
although such additions will often be provided via the standard library instead.
Some of the type descriptions below contain a paragraph listing âspecial
attributes.â These are attributes that provide access to the implementation and
are not intended for general use. Their definition may change in the future.
3.2.1.
None
¶
This type has a single value. There is a single object with this value. This
object is accessed through the built-in name
None
. It is used to signify the
absence of a value in many situations, e.g., it is returned from functions that
donât explicitly return anything. Its truth value is false.
3.2.2.
NotImplemented
¶
This type has a single value. There is a single object with this value. This
object is accessed through the built-in name
NotImplemented
. Numeric methods
and rich comparison methods should return this value if they do not implement the
operation for the operands provided. (The interpreter will then try the
reflected operation, or some other fallback, depending on the operator.) It
should not be evaluated in a boolean context.
See
Implementing the arithmetic operations
for more details.
Changed in version 3.9:
Evaluating
NotImplemented
in a boolean context was deprecated.
3.2.3.
Ellipsis
¶
This type has a single value. There is a single object with this value. This
object is accessed through the literal
...
or the built-in name
Ellipsis
. Its truth value is true.
3.2.4.
numbers.Number
¶
These are created by numeric literals and returned as results by arithmetic
operators and arithmetic built-in functions. Numeric objects are immutable;
once created their value never changes. Python numbers are of course strongly
related to mathematical numbers, but subject to the limitations of numerical
representation in computers.
The string representations of the numeric classes, computed by
__repr__()
and
__str__()
, have the following
properties:
They are valid numeric literals which, when passed to their
class constructor, produce an object having the value of the
original numeric.
The representation is in base 10, when possible.
Leading zeros, possibly excepting a single zero before a
decimal point, are not shown.
Trailing zeros, possibly excepting a single zero after a
decimal point, are not shown.
A sign is shown only when the number is negative.
Python distinguishes between integers, floating-point numbers, and complex
numbers:
3.2.4.1.
numbers.Integral
¶
These represent elements from the mathematical set of integers (positive and
negative).
Note
The rules for integer representation are intended to give the most meaningful
interpretation of shift and mask operations involving negative integers.
There are two types of integers:
Integers (
int
)
These represent numbers in an unlimited range, subject to available (virtual)
memory only. For the purpose of shift and mask operations, a binary
representation is assumed, and negative numbers are represented in a variant of
2âs complement which gives the illusion of an infinite string of sign bits
extending to the left.
Booleans (
bool
)
These represent the truth values False and True. The two objects representing
the values
False
and
True
are the only Boolean objects. The Boolean type is a
subtype of the integer type, and Boolean values behave like the values 0 and 1,
respectively, in almost all contexts, the exception being that when converted to
a string, the strings
"False"
or
"True"
are returned, respectively.
3.2.4.2.
numbers.Real
(
float
)
¶
These represent machine-level double precision floating-point numbers. You are
at the mercy of the underlying machine architecture (and C or Java
implementation) for the accepted range and handling of overflow. Python does not
support single-precision floating-point numbers; the savings in processor and
memory usage that are usually the reason for using these are dwarfed by the
overhead of using objects in Python, so there is no reason to complicate the
language with two kinds of floating-point numbers.
3.2.4.3.
numbers.Complex
(
complex
)
¶
These represent complex numbers as a pair of machine-level double precision
floating-point numbers. The same caveats apply as for floating-point numbers.
The real and imaginary parts of a complex number
z
can be retrieved through
the read-only attributes
z.real
and
z.imag
.
3.2.5.
Sequences
¶
These represent finite ordered sets indexed by non-negative numbers. The
built-in function
len()
returns the number of items of a sequence. When
the length of a sequence is
n
, the index set contains the numbers 0, 1,
âŠ,
n
-1. Item
i
of sequence
a
is selected by
a[i]
. Some sequences,
including built-in sequences, interpret negative subscripts by adding the
sequence length. For example,
a[-2]
equals
a[n-2]
, the second to last
item of sequence a with length
n
.
The resulting value must be a nonnegative integer less than the number of items
in the sequence. If it is not, an
IndexError
is raised.
Sequences also support slicing:
a[start:stop]
selects all items with index
k
such
that
start
<=
k
<
stop
. When used as an expression, a slice is a
sequence of the same type. The comment above about negative subscripts also applies
to negative slice positions.
Note that no error is raised if a slice position is less than zero or larger
than the length of the sequence.
If
start
is missing or
None
, slicing behaves as if
start
was zero.
If
stop
is missing or
None
, slicing behaves as if
stop
was equal to
the length of the sequence.
Some sequences also support âextended slicingâ with a third âstepâ parameter:
a[i:j:k]
selects all items of
a
with index
x
where
x
=
i
+
n*k
,
n
>=
0
and
i
<=
x
<
j
.
Sequences are distinguished according to their mutability:
3.2.5.1.
Immutable sequences
¶
An object of an immutable sequence type cannot change once it is created. (If
the object contains references to other objects, these other objects may be
mutable and may be changed; however, the collection of objects directly
referenced by an immutable object cannot change.)
The following types are immutable sequences:
Strings
A string (
str
) is a sequence of values that represent
characters
, or more formally,
Unicode code points
.
All the code points in the range
0
to
0x10FFFF
can be
represented in a string.
Python doesnât have a dedicated
character
type.
Instead, every code point in the string is represented as a string
object with length
1
.
The built-in function
ord()
converts a code point from its string form to an integer in the
range
0
to
0x10FFFF
;
chr()
converts an integer in the range
0
to
0x10FFFF
to the corresponding length
1
string object.
str.encode()
can be used to convert a
str
to
bytes
using the given text encoding, and
bytes.decode()
can be used to achieve the opposite.
Tuples
The items of a
tuple
are arbitrary Python objects. Tuples of two or
more items are formed by comma-separated lists of expressions. A tuple
of one item (a âsingletonâ) can be formed by affixing a comma to an
expression (an expression by itself does not create a tuple, since
parentheses must be usable for grouping of expressions). An empty
tuple can be formed by an empty pair of parentheses.
Bytes
A
bytes
object is an immutable array. The items are 8-bit bytes,
represented by integers in the range 0 <= x < 256. Bytes literals
(like
b'abc'
) and the built-in
bytes()
constructor
can be used to create bytes objects. Also, bytes objects can be
decoded to strings via the
decode()
method.
3.2.5.2.
Mutable sequences
¶
Mutable sequences can be changed after they are created. The subscription and
slicing notations can be used as the target of assignment and
del
(delete) statements.
Note
The
collections
and
array
module provide
additional examples of mutable sequence types.
There are currently two intrinsic mutable sequence types:
Lists
The items of a list are arbitrary Python objects. Lists are formed by
placing a comma-separated list of expressions in square brackets. (Note
that there are no special cases needed to form lists of length 0 or 1.)
Byte Arrays
A bytearray object is a mutable array. They are created by the built-in
bytearray()
constructor. Aside from being mutable
(and hence unhashable), byte arrays otherwise provide the same interface
and functionality as immutable
bytes
objects.
3.2.6.
Set types
¶
These represent unordered, finite sets of unique, immutable objects. As such,
they cannot be indexed by any subscript. However, they can be iterated over, and
the built-in function
len()
returns the number of items in a set. Common
uses for sets are fast membership testing, removing duplicates from a sequence,
and computing mathematical operations such as intersection, union, difference,
and symmetric difference.
For set elements, the same immutability rules apply as for dictionary keys. Note
that numeric types obey the normal rules for numeric comparison: if two numbers
compare equal (e.g.,
1
and
1.0
), only one of them can be contained in a
set.
There are currently two intrinsic set types:
Sets
These represent a mutable set. They are created by the built-in
set()
constructor and can be modified afterwards by several methods, such as
add()
.
Frozen sets
These represent an immutable set. They are created by the built-in
frozenset()
constructor. As a frozenset is immutable and
hashable
, it can be used again as an element of another set, or as
a dictionary key.
3.2.7.
Mappings
¶
These represent finite sets of objects indexed by arbitrary index sets. The
subscript notation
a[k]
selects the item indexed by
k
from the mapping
a
; this can be used in expressions and as the target of assignments or
del
statements. The built-in function
len()
returns the number
of items in a mapping.
There is currently a single intrinsic mapping type:
3.2.7.1.
Dictionaries
¶
These represent finite sets of objects indexed by nearly arbitrary values. The
only types of values not acceptable as keys are values containing lists or
dictionaries or other mutable types that are compared by value rather than by
object identity, the reason being that the efficient implementation of
dictionaries requires a keyâs hash value to remain constant. Numeric types used
for keys obey the normal rules for numeric comparison: if two numbers compare
equal (e.g.,
1
and
1.0
) then they can be used interchangeably to index
the same dictionary entry.
Dictionaries preserve insertion order, meaning that keys will be produced
in the same order they were added sequentially over the dictionary.
Replacing an existing key does not change the order, however removing a key
and re-inserting it will add it to the end instead of keeping its old place.
Dictionaries are mutable; they can be created by the
{}
notation (see
section
Dictionary displays
).
The extension modules
dbm.ndbm
and
dbm.gnu
provide
additional examples of mapping types, as does the
collections
module.
Changed in version 3.7:
Dictionaries did not preserve insertion order in versions of Python before 3.6.
In CPython 3.6, insertion order was preserved, but it was considered
an implementation detail at that time rather than a language guarantee.
3.2.8.
Callable types
¶
These are the types to which the function call operation (see section
Calls
) can be applied:
3.2.8.1.
User-defined functions
¶
A user-defined function object is created by a function definition (see
section
Function definitions
). It should be called with an argument list
containing the same number of items as the functionâs formal parameter
list.
3.2.8.1.1.
Special read-only attributes
¶
Attribute
Meaning
function.
__builtins__
¶
A reference to the
dictionary
that holds the functionâs
builtins namespace.
Added in version 3.10.
function.
__globals__
¶
A reference to the
dictionary
that holds the functionâs
global variables
â the global namespace of the module
in which the function was defined.
function.
__closure__
¶
None
or a
tuple
of cells that contain bindings for the names specified
in the
co_freevars
attribute of the functionâs
code
object
.
A cell object has the attribute
cell_contents
.
This can be used to get the value of the cell, as well as set the value.
3.2.8.1.2.
Special writable attributes
¶
Most of these attributes check the type of the assigned value:
Attribute
Meaning
function.
__doc__
¶
The functionâs documentation string, or
None
if unavailable.
function.
__name__
¶
The functionâs name.
See also:
__name__
attributes
.
function.
__qualname__
¶
The functionâs
qualified name
.
See also:
__qualname__
attributes
.
Added in version 3.3.
function.
__module__
¶
The name of the module the function was defined in,
or
None
if unavailable.
function.
__defaults__
¶
A
tuple
containing default
parameter
values
for those parameters that have defaults,
or
None
if no parameters have a default value.
function.
__code__
¶
The
code object
representing
the compiled function body.
function.
__dict__
¶
The namespace supporting arbitrary function attributes.
See also:
__dict__
attributes
.
function.
__annotations__
¶
A
dictionary
containing annotations of
parameters
.
The keys of the dictionary are the parameter names,
and
'return'
for the return annotation, if provided.
See also:
object.__annotations__
.
Changed in version 3.14:
Annotations are now
lazily evaluated
.
See
PEP 649
.
function.
__annotate__
¶
The
annotate function
for this function, or
None
if the function has no annotations. See
object.__annotate__
.
Added in version 3.14.
function.
__kwdefaults__
¶
A
dictionary
containing defaults for keyword-only
parameters
.
function.
__type_params__
¶
A
tuple
containing the
type parameters
of
a
generic function
.
Added in version 3.12.
Function objects also support getting and setting arbitrary attributes, which
can be used, for example, to attach metadata to functions. Regular attribute
dot-notation is used to get and set such attributes.
CPython implementation detail:
CPythonâs current implementation only supports function attributes
on user-defined functions. Function attributes on
built-in functions
may be supported in the
future.
Additional information about a functionâs definition can be retrieved from its
code object
(accessible via the
__code__
attribute).
3.2.8.2.
Instance methods
¶
An instance method object combines a class, a class instance and any
callable object (normally a user-defined function).
Special read-only attributes:
method.
__self__
¶
Refers to the class instance object to which the method is
bound
method.
__func__
¶
Refers to the original
function object
method.
__doc__
¶
The methodâs documentation
(same as
method.__func__.__doc__
).
A
string
if the original function had a docstring, else
None
.
method.
__name__
¶
The name of the method
(same as
method.__func__.__name__
)
method.
__module__
¶
The name of the module the method was defined in, or
None
if
unavailable.
Methods also support accessing (but not setting) the arbitrary function
attributes on the underlying
function object
.
User-defined method objects may be created when getting an attribute of a
class (perhaps via an instance of that class), if that attribute is a
user-defined
function object
or a
classmethod
object.
When an instance method object is created by retrieving a user-defined
function object
from a class via one of its
instances, its
__self__
attribute is the instance, and the
method object is said to be
bound
. The new methodâs
__func__
attribute is the original function object.
When an instance method object is created by retrieving a
classmethod
object from a class or instance, its
__self__
attribute is the
class itself, and its
__func__
attribute is the function object
underlying the class method.
When an instance method object is called, the underlying function
(
__func__
) is called, inserting the class instance
(
__self__
) in front of the argument list. For instance, when
C
is a class which contains a definition for a function
f()
, and
x
is an instance of
C
, calling
x.f(1)
is
equivalent to calling
C.f(x,
1)
.
When an instance method object is derived from a
classmethod
object, the
âclass instanceâ stored in
__self__
will actually be the class
itself, so that calling either
x.f(1)
or
C.f(1)
is equivalent to
calling
f(C,1)
where
f
is the underlying function.
It is important to note that user-defined functions
which are attributes of a class instance are not converted to bound
methods; this
only
happens when the function is an attribute of the
class.
3.2.8.3.
Generator functions
¶
A function or method which uses the
yield
statement (see section
The yield statement
) is called a
generator function
. Such a function, when
called, always returns an
iterator
object which can be used to
execute the body of the function: calling the iteratorâs
iterator.__next__()
method will cause the function to execute until
it provides a value using the
yield
statement. When the
function executes a
return
statement or falls off the end, a
StopIteration
exception is raised and the iterator will have
reached the end of the set of values to be returned.
3.2.8.4.
Coroutine functions
¶
A function or method which is defined using
async
def
is called
a
coroutine function
. Such a function, when called, returns a
coroutine
object. It may contain
await
expressions,
as well as
async
with
and
async
for
statements. See
also the
Coroutine Objects
section.
3.2.8.5.
Asynchronous generator functions
¶
A function or method which is defined using
async
def
and
which uses the
yield
statement is called a
asynchronous generator function
. Such a function, when called,
returns an
asynchronous iterator
object which can be used in an
async
for
statement to execute the body of the function.
Calling the asynchronous iteratorâs
aiterator.__anext__
method
will return an
awaitable
which when awaited
will execute until it provides a value using the
yield
expression. When the function executes an empty
return
statement or falls off the end, a
StopAsyncIteration
exception
is raised and the asynchronous iterator will have reached the end of
the set of values to be yielded.
3.2.8.6.
Built-in functions
¶
A built-in function object is a wrapper around a C function. Examples of
built-in functions are
len()
and
math.sin()
(
math
is a
standard built-in module). The number and type of the arguments are
determined by the C function. Special read-only attributes:
__doc__
is the functionâs documentation string, or
None
if
unavailable. See
function.__doc__
.
__name__
is the functionâs name. See
function.__name__
.
__self__
is set to
None
(but see the next item).
__module__
is the name of
the module the function was defined in or
None
if unavailable.
See
function.__module__
.
3.2.8.7.
Built-in methods
¶
This is really a different disguise of a built-in function, this time containing
an object passed to the C function as an implicit extra argument. An example of
a built-in method is
alist.append()
, assuming
alist
is a list object. In
this case, the special read-only attribute
__self__
is set to the object
denoted by
alist
. (The attribute has the same semantics as it does with
other
instance
methods
.)
3.2.8.8.
Classes
¶
Classes are callable. These objects normally act as factories for new
instances of themselves, but variations are possible for class types that
override
__new__()
. The arguments of the call are passed to
__new__()
and, in the typical case, to
__init__()
to
initialize the new instance.
3.2.8.9.
Class Instances
¶
Instances of arbitrary classes can be made callable by defining a
__call__()
method in their class.
3.2.9.
Modules
¶
Modules are a basic organizational unit of Python code, and are created by
the
import system
as invoked either by the
import
statement, or by calling
functions such as
importlib.import_module()
and built-in
__import__()
. A module object has a namespace implemented by a
dictionary
object (this is the dictionary referenced by the
__globals__
attribute of functions defined in the module). Attribute references are
translated to lookups in this dictionary, e.g.,
m.x
is equivalent to
m.__dict__["x"]
. A module object does not contain the code object used
to initialize the module (since it isnât needed once the initialization is
done).
Attribute assignment updates the moduleâs namespace dictionary, e.g.,
m.x
=
1
is equivalent to
m.__dict__["x"]
=
1
.
3.2.9.2.
Other writable attributes on module objects
¶
As well as the import-related attributes listed above, module objects also have
the following writable attributes:
module.
__doc__
¶
The moduleâs documentation string, or
None
if unavailable.
See also:
__doc__
attributes
.
module.
__annotations__
¶
A dictionary containing
variable annotations
collected during module body execution. For best practices on working with
__annotations__
, see
annotationlib
.
module.
__annotate__
¶
The
annotate function
for this module, or
None
if the module has
no annotations. See also:
__annotate__
attributes.
Added in version 3.14.
3.2.9.3.
Module dictionaries
¶
Module objects also have the following special read-only attribute:
module.
__dict__
¶
The moduleâs namespace as a dictionary object. Uniquely among the attributes
listed here,
__dict__
cannot be accessed as a global variable from
within a module; it can only be accessed as an attribute on module objects.
CPython implementation detail:
Because of the way CPython clears module dictionaries, the module
dictionary will be cleared when the module falls out of scope even if the
dictionary still has live references. To avoid this, copy the dictionary
or keep the module around while using its dictionary directly.
3.2.10.
Custom classes
¶
Custom class types are typically created by class definitions (see section
Class definitions
). A class has a namespace implemented by a dictionary object.
Class attribute references are translated to lookups in this dictionary, e.g.,
C.x
is translated to
C.__dict__["x"]
(although there are a number of
hooks which allow for other means of locating attributes). When the attribute
name is not found there, the attribute search continues in the base classes.
This search of the base classes uses the C3 method resolution order which
behaves correctly even in the presence of âdiamondâ inheritance structures
where there are multiple inheritance paths leading back to a common ancestor.
Additional details on the C3 MRO used by Python can be found at
The Python 2.3 Method Resolution Order
.
When a class attribute reference (for class
C
, say) would yield a
class method object, it is transformed into an instance method object whose
__self__
attribute is
C
.
When it would yield a
staticmethod
object,
it is transformed into the object wrapped by the static method
object. See section
Implementing Descriptors
for another way in which attributes
retrieved from a class may differ from those actually contained in its
__dict__
.
Class attribute assignments update the classâs dictionary, never the dictionary
of a base class.
A class object can be called (see above) to yield a class instance (see below).
3.2.10.1.
Special attributes
¶
Attribute
Meaning
type.
__name__
¶
The classâs name.
See also:
__name__
attributes
.
type.
__qualname__
¶
The classâs
qualified name
.
See also:
__qualname__
attributes
.
type.
__module__
¶
The name of the module in which the class was defined.
type.
__dict__
¶
A
mapping
proxy
providing a read-only view of the classâs namespace.
See also:
__dict__
attributes
.
type.
__bases__
¶
A
tuple
containing the classâs bases.
In most cases, for a class defined as
class
X(A,
B,
C)
,
X.__bases__
will be exactly equal to
(A,
B,
C)
.
type.
__base__
¶
CPython implementation detail:
The single base class in the inheritance chain that is responsible
for the memory layout of instances. This attribute corresponds to
tp_base
at the C level.
type.
__doc__
¶
The classâs documentation string, or
None
if undefined.
Not inherited by subclasses.
type.
__annotations__
¶
A dictionary containing
variable annotations
collected during class body execution. See also:
__annotations__
attributes
.
For best practices on working with
__annotations__
,
please see
annotationlib
. Use
annotationlib.get_annotations()
instead of accessing this
attribute directly.
Warning
Accessing the
__annotations__
attribute directly
on a class object may return annotations for the wrong class, specifically
in certain cases where the class, its base class, or a metaclass
is defined under
from
__future__
import
annotations
.
See
749
for details.
This attribute does not exist on certain builtin classes. On
user-defined classes without
__annotations__
, it is an
empty dictionary.
Changed in version 3.14:
Annotations are now
lazily evaluated
.
See
PEP 649
.
type.
__annotate__
(
)
¶
The
annotate function
for this class, or
None
if the class has no annotations.
See also:
__annotate__
attributes
.
Added in version 3.14.
type.
__type_params__
¶
A
tuple
containing the
type parameters
of
a
generic class
.
Added in version 3.12.
type.
__static_attributes__
¶
A
tuple
containing names of attributes of this class which are
assigned through
self.X
from any function in its body.
Added in version 3.13.
type.
__firstlineno__
¶
The line number of the first line of the class definition,
including decorators.
Setting the
__module__
attribute removes the
__firstlineno__
item from the typeâs dictionary.
Added in version 3.13.
type.
__mro__
¶
The
tuple
of classes that are considered when looking for
base classes during method resolution.
3.2.10.2.
Special methods
¶
In addition to the special attributes described above, all Python classes also
have the following two methods available:
type.
mro
(
)
¶
This method can be overridden by a metaclass to customize the method
resolution order for its instances. It is called at class instantiation,
and its result is stored in
__mro__
.
type.
__subclasses__
(
)
¶
Each class keeps a list of weak references to its immediate subclasses. This
method returns a list of all those references still alive. The list is in
definition order. Example:
>>>
class
A
:
pass
>>>
class
B
(
A
):
pass
>>>
A
.
__subclasses__
()
[<class 'B'>]
3.2.11.
Class instances
¶
A class instance is created by calling a class object (see above). A class
instance has a namespace implemented as a dictionary which is the first place
in which attribute references are searched. When an attribute is not found
there, and the instanceâs class has an attribute by that name, the search
continues with the class attributes. If a class attribute is found that is a
user-defined function object, it is transformed into an instance method
object whose
__self__
attribute is the instance. Static method and
class method objects are also transformed; see above under âClassesâ. See
section
Implementing Descriptors
for another way in which attributes of a class
retrieved via its instances may differ from the objects actually stored in
the classâs
__dict__
. If no class attribute is found, and the
objectâs class has a
__getattr__()
method, that is called to satisfy
the lookup.
Attribute assignments and deletions update the instanceâs dictionary, never a
classâs dictionary. If the class has a
__setattr__()
or
__delattr__()
method, this is called instead of updating the instance
dictionary directly.
Class instances can pretend to be numbers, sequences, or mappings if they have
methods with certain special names. See section
Special method names
.
3.2.11.1.
Special attributes
¶
object.
__class__
¶
The class to which a class instance belongs.
object.
__dict__
¶
A dictionary or other mapping object used to store an objectâs (writable)
attributes. Not all instances have a
__dict__
attribute; see the
section on
__slots__
for more details.
3.2.12.
I/O objects (also known as file objects)
¶
A
file object
represents an open file. Various shortcuts are
available to create file objects: the
open()
built-in function, and
also
os.popen()
,
os.fdopen()
, and the
makefile()
method of socket objects (and perhaps by
other functions or methods provided by extension modules).
File objects implement common methods, listed below, to simplify usage in
generic code. They are expected to be
With Statement Context Managers
.
The objects
sys.stdin
,
sys.stdout
and
sys.stderr
are
initialized to file objects corresponding to the interpreterâs standard
input, output and error streams; they are all open in text mode and
therefore follow the interface defined by the
io.TextIOBase
abstract class.
file.
read
(
size
=
-1
,
/
)
¶
Retrieve up to
size
data from the file. As a convenience if
size
is
unspecified or -1 retrieve all data available.
file.
write
(
data
,
/
)
¶
Store
data
to the file.
file.
close
(
)
¶
Flush any buffers and close the underlying file.
3.2.13.
Internal types
¶
A few types used internally by the interpreter are exposed to the user. Their
definitions may change with future versions of the interpreter, but they are
mentioned here for completeness.
3.2.13.1.
Code objects
¶
Code objects represent
byte-compiled
executable Python code, or
bytecode
.
The difference between a code object and a function object is that the function
object contains an explicit reference to the functionâs globals (the module in
which it was defined), while a code object contains no context; also the default
argument values are stored in the function object, not in the code object
(because they represent values calculated at run-time). Unlike function
objects, code objects are immutable and contain no references (directly or
indirectly) to mutable objects.
3.2.13.1.1.
Special read-only attributes
¶
codeobject.
co_name
¶
The function name
codeobject.
co_qualname
¶
The fully qualified function name
Added in version 3.11.
codeobject.
co_argcount
¶
The total number of positional
parameters
(including positional-only parameters and parameters with default values)
that the function has
codeobject.
co_posonlyargcount
¶
The number of positional-only
parameters
(including arguments with default values) that the function has
codeobject.
co_kwonlyargcount
¶
The number of keyword-only
parameters
(including arguments with default values) that the function has
codeobject.
co_nlocals
¶
The number of
local variables
used by the function
(including parameters)
codeobject.
co_varnames
¶
A
tuple
containing the names of the local variables in the
function (starting with the parameter names)
codeobject.
co_cellvars
¶
A
tuple
containing the names of
local variables
that are referenced from at least one
nested scope
inside the function
codeobject.
co_freevars
¶
A
tuple
containing the names of
free (closure) variables
that a
nested scope
references in an outer scope. See also
function.__closure__
.
Note: references to global and builtin names are
not
included.
codeobject.
co_code
¶
A string representing the sequence of
bytecode
instructions in
the function
codeobject.
co_consts
¶
A
tuple
containing the literals used by the
bytecode
in
the function
codeobject.
co_names
¶
A
tuple
containing the names used by the
bytecode
in
the function
codeobject.
co_filename
¶
The name of the file from which the code was compiled
codeobject.
co_firstlineno
¶
The line number of the first line of the function
codeobject.
co_lnotab
¶
A string encoding the mapping from
bytecode
offsets to line
numbers. For details, see the source code of the interpreter.
Deprecated since version 3.12:
This attribute of code objects is deprecated, and may be removed in
Python 3.15.
codeobject.
co_stacksize
¶
The required stack size of the code object
codeobject.
co_flags
¶
An
integer
encoding a number of flags for the
interpreter.
The following flag bits are defined for
co_flags
:
bit
0x04
is set if
the function uses the
*arguments
syntax to accept an arbitrary number of
positional arguments; bit
0x08
is set if the function uses the
**keywords
syntax to accept arbitrary keyword arguments; bit
0x20
is set
if the function is a generator. See
Code Objects Bit Flags
for details
on the semantics of each flags that might be present.
Future feature declarations (for example,
from
__future__
import
division
) also use bits
in
co_flags
to indicate whether a code object was compiled with a
particular feature enabled. See
compiler_flag
.
Other bits in
co_flags
are reserved for internal use.
If a code object represents a function and has a docstring,
the
CO_HAS_DOCSTRING
bit is set in
co_flags
and the first item in
co_consts
is
the docstring of the function.
3.2.13.1.2.
Methods on code objects
¶
codeobject.
co_positions
(
)
¶
Returns an iterable over the source code positions of each
bytecode
instruction in the code object.
The iterator returns
tuple
s containing the
(start_line,
end_line,
start_column,
end_column)
. The
i-th
tuple corresponds to the
position of the source code that compiled to the
i-th
code unit.
Column information is 0-indexed utf-8 byte offsets on the given source
line.
This positional information can be missing. A non-exhaustive lists of
cases where this may happen:
Running the interpreter with
-X
no_debug_ranges
.
Loading a pyc file compiled while using
-X
no_debug_ranges
.
Position tuples corresponding to artificial instructions.
Line and column numbers that canât be represented due to
implementation specific limitations.
When this occurs, some or all of the tuple elements can be
None
.
Added in version 3.11.
Note
This feature requires storing column positions in code objects which may
result in a small increase of disk usage of compiled Python files or
interpreter memory usage. To avoid storing the extra information and/or
deactivate printing the extra traceback information, the
-X
no_debug_ranges
command line flag or the
PYTHONNODEBUGRANGES
environment variable can be used.
codeobject.
co_lines
(
)
¶
Returns an iterator that yields information about successive ranges of
bytecode
s. Each item yielded is a
(start,
end,
lineno)
tuple
:
start
(an
int
) represents the offset (inclusive) of the start
of the
bytecode
range
end
(an
int
) represents the offset (exclusive) of the end of
the
bytecode
range
lineno
is an
int
representing the line number of the
bytecode
range, or
None
if the bytecodes in the given range
have no line number
The items yielded will have the following properties:
The first range yielded will have a
start
of 0.
The
(start,
end)
ranges will be non-decreasing and consecutive. That
is, for any pair of
tuple
s, the
start
of the second will be
equal to the
end
of the first.
No range will be backwards:
end
>=
start
for all triples.
The last
tuple
yielded will have
end
equal to the size of the
bytecode
.
Zero-width ranges, where
start
==
end
, are allowed. Zero-width ranges
are used for lines that are present in the source code, but have been
eliminated by the
bytecode
compiler.
Added in version 3.10.
See also
PEP 626
- Precise line numbers for debugging and other tools.
The PEP that introduced the
co_lines()
method.
codeobject.
replace
(
**
kwargs
)
¶
Return a copy of the code object with new values for the specified fields.
Code objects are also supported by the generic function
copy.replace()
.
Added in version 3.8.
3.2.13.2.
Frame objects
¶
Frame objects represent execution frames. They may occur in
traceback objects
,
and are also passed to registered trace functions.
3.2.13.2.1.
Special read-only attributes
¶
frame.
f_back
¶
Points to the previous stack frame (towards the caller),
or
None
if this is the bottom stack frame
frame.
f_code
¶
The
code object
being executed in this frame.
Accessing this attribute raises an
auditing event
object.__getattr__
with arguments
obj
and
"f_code"
.
frame.
f_locals
¶
The mapping used by the frame to look up
local variables
.
If the frame refers to an
optimized scope
,
this may return a write-through proxy object.
Changed in version 3.13:
Return a proxy for optimized scopes.
frame.
f_globals
¶
The dictionary used by the frame to look up
global variables
frame.
f_builtins
¶
The dictionary used by the frame to look up
built-in (intrinsic) names
frame.
f_lasti
¶
The âprecise instructionâ of the frame object
(this is an index into the
bytecode
string of the
code object
)
frame.
f_generator
¶
The
generator
or
coroutine
object that owns this frame,
or
None
if the frame is a normal function.
Added in version 3.14.
3.2.13.2.2.
Special writable attributes
¶
frame.
f_trace
¶
If not
None
, this is a function called for various events during
code execution (this is used by debuggers). Normally an event is
triggered for each new source line (see
f_trace_lines
).
frame.
f_trace_lines
¶
Set this attribute to
False
to disable triggering a tracing
event for each source line.
frame.
f_trace_opcodes
¶
Set this attribute to
True
to allow per-opcode events to be
requested. Note that this may lead to
undefined interpreter behaviour if exceptions raised by the trace
function escape to the function being traced.
frame.
f_lineno
¶
The current line number of the frame â writing to this
from within a trace function jumps to the given line (only for the bottom-most
frame). A debugger can implement a Jump command (aka Set Next Statement)
by writing to this attribute.
3.2.13.2.3.
Frame object methods
¶
Frame objects support one method:
frame.
clear
(
)
¶
This method clears all references to
local variables
held by the
frame. Also, if the frame belonged to a
generator
, the generator
is finalized. This helps break reference cycles involving frame
objects (for example when catching an
exception
and storing its
traceback
for later use).
RuntimeError
is raised if the frame is currently executing
or suspended.
Added in version 3.4.
Changed in version 3.13:
Attempting to clear a suspended frame raises
RuntimeError
(as has always been the case for executing frames).
3.2.13.3.
Traceback objects
¶
Traceback objects represent the stack trace of an
exception
.
A traceback object
is implicitly created when an exception occurs, and may also be explicitly
created by calling
types.TracebackType
.
Changed in version 3.7:
Traceback objects can now be explicitly instantiated from Python code.
For implicitly created tracebacks, when the search for an exception handler
unwinds the execution stack, at each unwound level a traceback object is
inserted in front of the current traceback. When an exception handler is
entered, the stack trace is made available to the program. (See section
The try statement
.) It is accessible as the third item of the
tuple returned by
sys.exc_info()
, and as the
__traceback__
attribute
of the caught exception.
When the program contains no suitable
handler, the stack trace is written (nicely formatted) to the standard error
stream; if the interpreter is interactive, it is also made available to the user
as
sys.last_traceback
.
For explicitly created tracebacks, it is up to the creator of the traceback
to determine how the
tb_next
attributes should be linked to
form a full stack trace.
Special read-only attributes:
traceback.
tb_frame
¶
Points to the execution
frame
of the current
level.
Accessing this attribute raises an
auditing event
object.__getattr__
with arguments
obj
and
"tb_frame"
.
traceback.
tb_lineno
¶
Gives the line number where the exception occurred
traceback.
tb_lasti
¶
Indicates the âprecise instructionâ.
The line number and last instruction in the traceback may differ from the
line number of its
frame object
if the exception
occurred in a
try
statement with no matching except clause or with a
finally
clause.
traceback.
tb_next
¶
The special writable attribute
tb_next
is the next level in the
stack trace (towards the frame where the exception occurred), or
None
if
there is no next level.
Changed in version 3.7:
This attribute is now writable
3.2.13.4.
Slice objects
¶
Slice objects are used to represent slices for
__getitem__()
methods. They are also created by the built-in
slice()
function.
Special read-only attributes:
start
is the lower bound;
stop
is the upper bound;
step
is the step
value; each is
None
if omitted. These attributes can have any type.
Slice objects support one method:
slice.
indices
(
self
,
length
)
¶
This method takes a single integer argument
length
and computes
information about the slice that the slice object would describe if
applied to a sequence of
length
items. It returns a tuple of three
integers; respectively these are the
start
and
stop
indices and the
step
or stride length of the slice. Missing or out-of-bounds indices
are handled in a manner consistent with regular slices.
3.2.13.5.
Static method objects
¶
Static method objects provide a way of defeating the transformation of function
objects to method objects described above. A static method object is a wrapper
around any other object, usually a user-defined method object. When a static
method object is retrieved from a class or a class instance, the object actually
returned is the wrapped object, which is not subject to any further
transformation. Static method objects are also callable. Static method
objects are created by the built-in
staticmethod()
constructor.
3.2.13.6.
Class method objects
¶
A class method object, like a static method object, is a wrapper around another
object that alters the way in which that object is retrieved from classes and
class instances. The behaviour of class method objects upon such retrieval is
described above, under
âinstance methodsâ
. Class method objects are created
by the built-in
classmethod()
constructor.
3.3.
Special method names
¶
A class can implement certain operations that are invoked by special syntax
(such as arithmetic operations or subscripting and slicing) by defining methods
with special names. This is Pythonâs approach to
operator overloading
,
allowing classes to define their own behavior with respect to language
operators. For instance, if a class defines a method named
__getitem__()
,
and
x
is an instance of this class, then
x[i]
is roughly equivalent
to
type(x).__getitem__(x,
i)
. Except where mentioned, attempts to execute an
operation raise an exception when no appropriate method is defined (typically
AttributeError
or
TypeError
).
Setting a special method to
None
indicates that the corresponding
operation is not available. For example, if a class sets
__iter__()
to
None
, the class is not iterable, so calling
iter()
on its instances will raise a
TypeError
(without
falling back to
__getitem__()
).
[
2
]
When implementing a class that emulates any built-in type, it is important that
the emulation only be implemented to the degree that it makes sense for the
object being modelled. For example, some sequences may work well with retrieval
of individual elements, but extracting a slice may not make sense.
(One example of this is the
NodeList
interface
in the W3Câs Document Object Model.)
3.3.1.
Basic customization
¶
object.
__new__
(
cls
[
,
...
]
)
¶
Called to create a new instance of class
cls
.
__new__()
is a static
method (special-cased so you need not declare it as such) that takes the class
of which an instance was requested as its first argument. The remaining
arguments are those passed to the object constructor expression (the call to the
class). The return value of
__new__()
should be the new object instance
(usually an instance of
cls
).
Typical implementations create a new instance of the class by invoking the
superclassâs
__new__()
method using
super().__new__(cls[,
...])
with appropriate arguments and then modifying the newly created instance
as necessary before returning it.
If
__new__()
is invoked during object construction and it returns an
instance of
cls
, then the new instanceâs
__init__()
method
will be invoked like
__init__(self[,
...])
, where
self
is the new instance
and the remaining arguments are the same as were passed to the object constructor.
If
__new__()
does not return an instance of
cls
, then the new instanceâs
__init__()
method will not be invoked.
__new__()
is intended mainly to allow subclasses of immutable types (like
int, str, or tuple) to customize instance creation. It is also commonly
overridden in custom metaclasses in order to customize class creation.
object.
__init__
(
self
[
,
...
]
)
¶
Called after the instance has been created (by
__new__()
), but before
it is returned to the caller. The arguments are those passed to the
class constructor expression. If a base class has an
__init__()
method, the derived classâs
__init__()
method, if any, must explicitly
call it to ensure proper initialization of the base class part of the
instance; for example:
super().__init__([args...])
.
Because
__new__()
and
__init__()
work together in constructing
objects (
__new__()
to create it, and
__init__()
to customize it),
no non-
None
value may be returned by
__init__()
; doing so will
cause a
TypeError
to be raised at runtime.
object.
__del__
(
self
)
¶
Called when the instance is about to be destroyed. This is also called a
finalizer or (improperly) a destructor. If a base class has a
__del__()
method, the derived classâs
__del__()
method,
if any, must explicitly call it to ensure proper deletion of the base
class part of the instance.
It is possible (though not recommended!) for the
__del__()
method
to postpone destruction of the instance by creating a new reference to
it. This is called object
resurrection
. It is implementation-dependent
whether
__del__()
is called a second time when a resurrected object
is about to be destroyed; the current
CPython
implementation
only calls it once.
It is not guaranteed that
__del__()
methods are called for objects
that still exist when the interpreter exits.
weakref.finalize
provides a straightforward way to register
a cleanup function to be called when an object is garbage collected.
Note
del
x
doesnât directly call
x.__del__()
â the former decrements
the reference count for
x
by one, and the latter is only called when
x
âs reference count reaches zero.
CPython implementation detail:
It is possible for a reference cycle to prevent the reference count
of an object from going to zero. In this case, the cycle will be
later detected and deleted by the
cyclic garbage collector
. A common cause of reference cycles is when
an exception has been caught in a local variable. The frameâs
locals then reference the exception, which references its own
traceback, which references the locals of all frames caught in the
traceback.
See also
Documentation for the
gc
module.
Warning
Due to the precarious circumstances under which
__del__()
methods are
invoked, exceptions that occur during their execution are ignored, and a warning
is printed to
sys.stderr
instead. In particular:
__del__()
can be invoked when arbitrary code is being executed,
including from any arbitrary thread. If
__del__()
needs to take
a lock or invoke any other blocking resource, it may deadlock as
the resource may already be taken by the code that gets interrupted
to execute
__del__()
.
__del__()
can be executed during interpreter shutdown. As a
consequence, the global variables it needs to access (including other
modules) may already have been deleted or set to
None
. Python
guarantees that globals whose name begins with a single underscore
are deleted from their module before other globals are deleted; if
no other references to such globals exist, this may help in assuring
that imported modules are still available at the time when the
__del__()
method is called.
object.
__repr__
(
self
)
¶
Called by the
repr()
built-in function to compute the âofficialâ string
representation of an object. If at all possible, this should look like a
valid Python expression that could be used to recreate an object with the
same value (given an appropriate environment). If this is not possible, a
string of the form
<...some
useful
description...>
should be returned.
The return value must be a string object. If a class defines
__repr__()
but not
__str__()
, then
__repr__()
is also used when an
âinformalâ string representation of instances of that class is required.
This is typically used for debugging, so it is important that the representation
is information-rich and unambiguous. A default implementation is provided by the
object
class itself.
object.
__str__
(
self
)
¶
Called by
str(object)
, the default
__format__()
implementation,
and the built-in function
print()
, to compute the âinformalâ or nicely
printable string representation of an object. The return value must be a
str
object.
This method differs from
object.__repr__()
in that there is no
expectation that
__str__()
return a valid Python expression: a more
convenient or concise representation can be used.
The default implementation defined by the built-in type
object
calls
object.__repr__()
.
object.
__bytes__
(
self
)
¶
Called by
bytes
to compute a byte-string representation
of an object. This should return a
bytes
object. The
object
class itself does not provide this method.
object.
__format__
(
self
,
format_spec
)
¶
Called by the
format()
built-in function,
and by extension, evaluation of
formatted string literals
and the
str.format()
method, to produce a âformattedâ
string representation of an object. The
format_spec
argument is
a string that contains a description of the formatting options desired.
The interpretation of the
format_spec
argument is up to the type
implementing
__format__()
, however most classes will either
delegate formatting to one of the built-in types, or use a similar
formatting option syntax.
See
Format specification mini-language
for a description of the standard formatting syntax.
The return value must be a string object.
The default implementation by the
object
class should be given
an empty
format_spec
string. It delegates to
__str__()
.
Changed in version 3.4:
The __format__ method of
object
itself raises a
TypeError
if passed any non-empty string.
Changed in version 3.7:
object.__format__(x,
'')
is now equivalent to
str(x)
rather
than
format(str(x),
'')
.
object.
__lt__
(
self
,
other
)
¶
object.
__le__
(
self
,
other
)
¶
object.
__eq__
(
self
,
other
)
¶
object.
__ne__
(
self
,
other
)
¶
object.
__gt__
(
self
,
other
)
¶
object.
__ge__
(
self
,
other
)
¶
These are the so-called ârich comparisonâ methods. The correspondence between
operator symbols and method names is as follows:
x<y
calls
x.__lt__(y)
,
x<=y
calls
x.__le__(y)
,
x==y
calls
x.__eq__(y)
,
x!=y
calls
x.__ne__(y)
,
x>y
calls
x.__gt__(y)
, and
x>=y
calls
x.__ge__(y)
.
A rich comparison method may return the singleton
NotImplemented
if it does
not implement the operation for a given pair of arguments. By convention,
False
and
True
are returned for a successful comparison. However, these
methods can return any value, so if the comparison operator is used in a Boolean
context (e.g., in the condition of an
if
statement), Python will call
bool()
on the value to determine if the result is true or false.
By default,
object
implements
__eq__()
by using
is
, returning
NotImplemented
in the case of a false comparison:
True
if
x
is
y
else
NotImplemented
. For
__ne__()
, by default it
delegates to
__eq__()
and inverts the result unless it is
NotImplemented
. There are no other implied relationships among the
comparison operators or default implementations; for example, the truth of
(x<y
or
x==y)
does not imply
x<=y
. To automatically generate ordering
operations from a single root operation, see
functools.total_ordering()
.
By default, the
object
class provides implementations consistent
with
Value comparisons
: equality compares according to
object identity, and order comparisons raise
TypeError
. Each default
method may generate these results directly, but may also return
NotImplemented
.
See the paragraph on
__hash__()
for
some important notes on creating
hashable
objects which support
custom comparison operations and are usable as dictionary keys.
There are no swapped-argument versions of these methods (to be used when the
left argument does not support the operation but the right argument does);
rather,
__lt__()
and
__gt__()
are each otherâs reflection,
__le__()
and
__ge__()
are each otherâs reflection, and
__eq__()
and
__ne__()
are their own reflection.
If the operands are of different types, and the right operandâs type is
a direct or indirect subclass of the left operandâs type,
the reflected method of the right operand has priority, otherwise
the left operandâs method has priority. Virtual subclassing is
not considered.
When no appropriate method returns any value other than
NotImplemented
, the
==
and
!=
operators will fall back to
is
and
is
not
, respectively.
object.
__hash__
(
self
)
¶
Called by built-in function
hash()
and for operations on members of
hashed collections including
set
,
frozenset
, and
dict
. The
__hash__()
method should return an integer. The only required
property is that objects which compare equal have the same hash value; it is
advised to mix together the hash values of the components of the object that
also play a part in comparison of objects by packing them into a tuple and
hashing the tuple. Example:
def
__hash__
(
self
):
return
hash
((
self
.
name
,
self
.
nick
,
self
.
color
))
Note
hash()
truncates the value returned from an objectâs custom
__hash__()
method to the size of a
Py_ssize_t
. This is
typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds. If an
objectâs
__hash__()
must interoperate on builds of different bit
sizes, be sure to check the width on all supported builds. An easy way
to do this is with
python
-c
"import
sys;
print(sys.hash_info.width)"
.
If a class does not define an
__eq__()
method it should not define a
__hash__()
operation either; if it defines
__eq__()
but not
__hash__()
, its instances will not be usable as items in hashable
collections. If a class defines mutable objects and implements an
__eq__()
method, it should not implement
__hash__()
, since the
implementation of
hashable
collections requires that a keyâs hash value is
immutable (if the objectâs hash value changes, it will be in the wrong hash
bucket).
User-defined classes have
__eq__()
and
__hash__()
methods
by default (inherited from the
object
class); with them, all objects compare
unequal (except with themselves) and
x.__hash__()
returns an appropriate
value such that
x
==
y
implies both that
x
is
y
and
hash(x)
==
hash(y)
.
A class that overrides
__eq__()
and does not define
__hash__()
will have its
__hash__()
implicitly set to
None
. When the
__hash__()
method of a class is
None
, instances of the class will
raise an appropriate
TypeError
when a program attempts to retrieve
their hash value, and will also be correctly identified as unhashable when
checking
isinstance(obj,
collections.abc.Hashable)
.
If a class that overrides
__eq__()
needs to retain the implementation
of
__hash__()
from a parent class, the interpreter must be told this
explicitly by setting
__hash__
=
<ParentClass>.__hash__
.
If a class that does not override
__eq__()
wishes to suppress hash
support, it should include
__hash__
=
None
in the class definition.
A class which defines its own
__hash__()
that explicitly raises
a
TypeError
would be incorrectly identified as hashable by
an
isinstance(obj,
collections.abc.Hashable)
call.
Note
By default, the
__hash__()
values of str and bytes objects are
âsaltedâ with an unpredictable random value. Although they
remain constant within an individual Python process, they are not
predictable between repeated invocations of Python.
This is intended to provide protection against a denial-of-service caused
by carefully chosen inputs that exploit the worst case performance of a
dict insertion,
O
(
n
2
) complexity. See
https://ocert.org/advisories/ocert-2011-003.html
for details.
Changing hash values affects the iteration order of sets.
Python has never made guarantees about this ordering
(and it typically varies between 32-bit and 64-bit builds).
See also
PYTHONHASHSEED
.
Changed in version 3.3:
Hash randomization is enabled by default.
object.
__bool__
(
self
)
¶
Called to implement truth value testing and the built-in operation
bool()
; should return
False
or
True
. When this method is not
defined,
__len__()
is called, if it is defined, and the object is
considered true if its result is nonzero. If a class defines neither
__len__()
nor
__bool__()
(which is true of the
object
class itself), all its instances are considered true.
3.3.2.
Customizing attribute access
¶
The following methods can be defined to customize the meaning of attribute
access (use of, assignment to, or deletion of
x.name
) for class instances.
object.
__getattr__
(
self
,
name
)
¶
Called when the default attribute access fails with an
AttributeError
(either
__getattribute__()
raises an
AttributeError
because
name
is not an instance attribute or an attribute in the class tree
for
self
; or
__get__()
of a
name
property raises
AttributeError
). This method should either return the (computed)
attribute value or raise an
AttributeError
exception.
The
object
class itself does not provide this method.
Note that if the attribute is found through the normal mechanism,
__getattr__()
is not called. (This is an intentional asymmetry between
__getattr__()
and
__setattr__()
.) This is done both for efficiency
reasons and because otherwise
__getattr__()
would have no way to access
other attributes of the instance. Note that at least for instance variables,
you can take total control by not inserting any values in the instance attribute
dictionary (but instead inserting them in another object). See the
__getattribute__()
method below for a way to actually get total control
over attribute access.
object.
__getattribute__
(
self
,
name
)
¶
Called unconditionally to implement attribute accesses for instances of the
class. If the class also defines
__getattr__()
, the latter will not be
called unless
__getattribute__()
either calls it explicitly or raises an
AttributeError
. This method should return the (computed) attribute value
or raise an
AttributeError
exception. In order to avoid infinite
recursion in this method, its implementation should always call the base class
method with the same name to access any attributes it needs, for example,
object.__getattribute__(self,
name)
.
Note
This method may still be bypassed when looking up special methods as the
result of implicit invocation via language syntax or
built-in functions
.
See
Special method lookup
.
For certain sensitive attribute accesses, raises an
auditing event
object.__getattr__
with arguments
obj
and
name
.
object.
__setattr__
(
self
,
name
,
value
)
¶
Called when an attribute assignment is attempted. This is called instead of
the normal mechanism (i.e. store the value in the instance dictionary).
name
is the attribute name,
value
is the value to be assigned to it.
If
__setattr__()
wants to assign to an instance attribute, it should
call the base class method with the same name, for example,
object.__setattr__(self,
name,
value)
.
For certain sensitive attribute assignments, raises an
auditing event
object.__setattr__
with arguments
obj
,
name
,
value
.
object.
__delattr__
(
self
,
name
)
¶
Like
__setattr__()
but for attribute deletion instead of assignment. This
should only be implemented if
del
obj.name
is meaningful for the object.
For certain sensitive attribute deletions, raises an
auditing event
object.__delattr__
with arguments
obj
and
name
.
object.
__dir__
(
self
)
¶
Called when
dir()
is called on the object. An iterable must be
returned.
dir()
converts the returned iterable to a list and sorts it.
3.3.2.1.
Customizing module attribute access
¶
module.
__getattr__
(
)
¶
module.
__dir__
(
)
¶
Special names
__getattr__
and
__dir__
can be also used to customize
access to module attributes. The
__getattr__
function at the module level
should accept one argument which is the name of an attribute and return the
computed value or raise an
AttributeError
. If an attribute is
not found on a module object through the normal lookup, i.e.
object.__getattribute__()
, then
__getattr__
is searched in
the module
__dict__
before raising an
AttributeError
. If found,
it is called with the attribute name and the result is returned.
The
__dir__
function should accept no arguments, and return an iterable of
strings that represents the names accessible on module. If present, this
function overrides the standard
dir()
search on a module.
module.
__class__
¶
For a more fine grained customization of the module behavior (setting
attributes, properties, etc.), one can set the
__class__
attribute of
a module object to a subclass of
types.ModuleType
. For example:
import
sys
from
types
import
ModuleType
class
VerboseModule
(
ModuleType
):
def
__repr__
(
self
):
return
f
'Verbose
{
self
.
__name__
}
'
def
__setattr__
(
self
,
attr
,
value
):
print
(
f
'Setting
{
attr
}
...'
)
super
()
.
__setattr__
(
attr
,
value
)
sys
.
modules
[
__name__
]
.
__class__
=
VerboseModule
Note
Defining module
__getattr__
and setting module
__class__
only
affect lookups made using the attribute access syntax â directly accessing
the module globals (whether by code within the module, or via a reference
to the moduleâs globals dictionary) is unaffected.
Changed in version 3.5:
__class__
module attribute is now writable.
Added in version 3.7:
__getattr__
and
__dir__
module attributes.
See also
PEP 562
- Module __getattr__ and __dir__
Describes the
__getattr__
and
__dir__
functions on modules.
3.3.2.2.
Implementing Descriptors
¶
The following methods only apply when an instance of the class containing the
method (a so-called
descriptor
class) appears in an
owner
class (the
descriptor must be in either the ownerâs class dictionary or in the class
dictionary for one of its parents). In the examples below, âthe attributeâ
refers to the attribute whose name is the key of the property in the owner
classâ
__dict__
. The
object
class itself does not
implement any of these protocols.
object.
__get__
(
self
,
instance
,
owner
=
None
)
¶
Called to get the attribute of the owner class (class attribute access) or
of an instance of that class (instance attribute access). The optional
owner
argument is the owner class, while
instance
is the instance that
the attribute was accessed through, or
None
when the attribute is
accessed through the
owner
.
This method should return the computed attribute value or raise an
AttributeError
exception.
PEP 252
specifies that
__get__()
is callable with one or two
arguments. Pythonâs own built-in descriptors support this specification;
however, it is likely that some third-party tools have descriptors
that require both arguments. Pythonâs own
__getattribute__()
implementation always passes in both arguments whether they are required
or not.
object.
__set__
(
self
,
instance
,
value
)
¶
Called to set the attribute on an instance
instance
of the owner class to a
new value,
value
.
Note, adding
__set__()
or
__delete__()
changes the kind of
descriptor to a âdata descriptorâ. See
Invoking Descriptors
for
more details.
object.
__delete__
(
self
,
instance
)
¶
Called to delete the attribute on an instance
instance
of the owner class.
Instances of descriptors may also have the
__objclass__
attribute
present:
object.
__objclass__
¶
The attribute
__objclass__
is interpreted by the
inspect
module
as specifying the class where this object was defined (setting this
appropriately can assist in runtime introspection of dynamic class attributes).
For callables, it may indicate that an instance of the given type (or a
subclass) is expected or required as the first positional argument (for example,
CPython sets this attribute for unbound methods that are implemented in C).
3.3.2.3.
Invoking Descriptors
¶
In general, a descriptor is an object attribute with âbinding behaviorâ, one
whose attribute access has been overridden by methods in the descriptor
protocol:
__get__()
,
__set__()
, and
__delete__()
. If any of
those methods are defined for an object, it is said to be a descriptor.
The default behavior for attribute access is to get, set, or delete the
attribute from an objectâs dictionary. For instance,
a.x
has a lookup chain
starting with
a.__dict__['x']
, then
type(a).__dict__['x']
, and
continuing through the base classes of
type(a)
excluding metaclasses.
However, if the looked-up value is an object defining one of the descriptor
methods, then Python may override the default behavior and invoke the descriptor
method instead. Where this occurs in the precedence chain depends on which
descriptor methods were defined and how they were called.
The starting point for descriptor invocation is a binding,
a.x
. How the
arguments are assembled depends on
a
:
Direct Call
The simplest and least common call is when user code directly invokes a
descriptor method:
x.__get__(a)
.
Instance Binding
If binding to an object instance,
a.x
is transformed into the call:
type(a).__dict__['x'].__get__(a,
type(a))
.
Class Binding
If binding to a class,
A.x
is transformed into the call:
A.__dict__['x'].__get__(None,
A)
.
Super Binding
A dotted lookup such as
super(A,
a).x
searches
a.__class__.__mro__
for a base class
B
following
A
and then
returns
B.__dict__['x'].__get__(a,
A)
. If not a descriptor,
x
is
returned unchanged.
For instance bindings, the precedence of descriptor invocation depends on
which descriptor methods are defined. A descriptor can define any combination
of
__get__()
,
__set__()
and
__delete__()
. If it does not
define
__get__()
, then accessing the attribute will return the descriptor
object itself unless there is a value in the objectâs instance dictionary. If
the descriptor defines
__set__()
and/or
__delete__()
, it is a data
descriptor; if it defines neither, it is a non-data descriptor. Normally, data
descriptors define both
__get__()
and
__set__()
, while non-data
descriptors have just the
__get__()
method. Data descriptors with
__get__()
and
__set__()
(and/or
__delete__()
) defined
always override a redefinition in an
instance dictionary. In contrast, non-data descriptors can be overridden by
instances.
Python methods (including those decorated with
@staticmethod
and
@classmethod
) are
implemented as non-data descriptors. Accordingly, instances can redefine and
override methods. This allows individual instances to acquire behaviors that
differ from other instances of the same class.
The
property()
function is implemented as a data descriptor. Accordingly,
instances cannot override the behavior of a property.
3.3.2.4.
__slots__
¶
__slots__
allow us to explicitly declare data members (like
properties) and deny the creation of
__dict__
and
__weakref__
(unless explicitly declared in
__slots__
or available in a parent.)
The space saved over using
__dict__
can be significant.
Attribute lookup speed can be significantly improved as well.
object.
__slots__
¶
This class variable can be assigned a string, iterable, or sequence of
strings with variable names used by instances.
__slots__
reserves space
for the declared variables and prevents the automatic creation of
__dict__
and
__weakref__
for each instance.
Notes on using
__slots__
:
When inheriting from a class without
__slots__
, the
__dict__
and
__weakref__
attribute of the instances will always be accessible.
Without a
__dict__
variable, instances cannot be assigned new
variables not
listed in the
__slots__
definition. Attempts to assign to an unlisted
variable name raises
AttributeError
. If dynamic assignment of new
variables is desired, then add
'__dict__'
to the sequence of strings in
the
__slots__
declaration.
Without a
__weakref__
variable for each instance, classes defining
__slots__
do not support
weak
references
to its instances.
If weak reference
support is needed, then add
'__weakref__'
to the sequence of strings in the
__slots__
declaration.
__slots__
are implemented at the class level by creating
descriptors
for each variable name. As a result, class attributes
cannot be used to set default values for instance variables defined by
__slots__
; otherwise, the class attribute would overwrite the descriptor
assignment.
The action of a
__slots__
declaration is not limited to the class
where it is defined.
__slots__
declared in parents are available in
child classes. However, instances of a child subclass will get a
__dict__
and
__weakref__
unless the subclass also defines
__slots__
(which should only contain names of any
additional
slots).
If a class defines a slot also defined in a base class, the instance variable
defined by the base class slot is inaccessible (except by retrieving its
descriptor directly from the base class). This renders the meaning of the
program undefined. In the future, a check may be added to prevent this.
TypeError
will be raised if nonempty
__slots__
are defined for a
class derived from a
"variable-length"
built-in
type
such as
int
,
bytes
, and
tuple
.
Any non-string
iterable
may be assigned to
__slots__
.
If a
dictionary
is used to assign
__slots__
, the dictionary
keys will be used as the slot names. The values of the dictionary can be used
to provide per-attribute docstrings that will be recognised by
inspect.getdoc()
and displayed in the output of
help()
.
__class__
assignment works only if both classes have the
same
__slots__
.
Multiple inheritance
with multiple slotted parent
classes can be used,
but only one parent is allowed to have attributes created by slots
(the other bases must have empty slot layouts) - violations raise
TypeError
.
If an
iterator
is used for
__slots__
then a
descriptor
is
created for each
of the iteratorâs values. However, the
__slots__
attribute will be an empty
iterator.
3.3.3.
Customizing class creation
¶
Whenever a class inherits from another class,
__init_subclass__()
is
called on the parent class. This way, it is possible to write classes which
change the behavior of subclasses. This is closely related to class
decorators, but where class decorators only affect the specific class theyâre
applied to,
__init_subclass__
solely applies to future subclasses of the
class defining the method.
classmethod
object.
__init_subclass__
(
cls
)
¶
This method is called whenever the containing class is subclassed.
cls
is then the new subclass. If defined as a normal instance method,
this method is implicitly converted to a class method.
Keyword arguments which are given to a new class are passed to
the parent classâs
__init_subclass__
. For compatibility with
other classes using
__init_subclass__
, one should take out the
needed keyword arguments and pass the others over to the base
class, as in:
class
Philosopher
:
def
__init_subclass__
(
cls
,
/
,
default_name
,
**
kwargs
):
super
()
.
__init_subclass__
(
**
kwargs
)
cls
.
default_name
=
default_name
class
AustralianPhilosopher
(
Philosopher
,
default_name
=
"Bruce"
):
pass
The default implementation
object.__init_subclass__
does
nothing, but raises an error if it is called with any arguments.
Note
The metaclass hint
metaclass
is consumed by the rest of the type
machinery, and is never passed to
__init_subclass__
implementations.
The actual metaclass (rather than the explicit hint) can be accessed as
type(cls)
.
Added in version 3.6.
When a class is created,
type.__new__()
scans the class variables
and makes callbacks to those with a
__set_name__()
hook.
object.
__set_name__
(
self
,
owner
,
name
)
¶
Automatically called at the time the owning class
owner
is
created. The object has been assigned to
name
in that class:
class
A
:
x
=
C
()
# Automatically calls: x.__set_name__(A, 'x')
If the class variable is assigned after the class is created,
__set_name__()
will not be called automatically.
If needed,
__set_name__()
can be called directly:
class
A
:
pass
c
=
C
()
A
.
x
=
c
# The hook is not called
c
.
__set_name__
(
A
,
'x'
)
# Manually invoke the hook
See
Creating the class object
for more details.
Added in version 3.6.
3.3.3.1.
Metaclasses
¶
By default, classes are constructed using
type()
. The class body is
executed in a new namespace and the class name is bound locally to the
result of
type(name,
bases,
namespace)
.
The class creation process can be customized by passing the
metaclass
keyword argument in the class definition line, or by inheriting from an
existing class that included such an argument. In the following example,
both
MyClass
and
MySubclass
are instances of
Meta
:
class
Meta
(
type
):
pass
class
MyClass
(
metaclass
=
Meta
):
pass
class
MySubclass
(
MyClass
):
pass
Any other keyword arguments that are specified in the class definition are
passed through to all metaclass operations described below.
When a class definition is executed, the following steps occur:
MRO entries are resolved;
the appropriate metaclass is determined;
the class namespace is prepared;
the class body is executed;
the class object is created.
3.3.3.2.
Resolving MRO entries
¶
object.
__mro_entries__
(
self
,
bases
)
¶
If a base that appears in a class definition is not an instance of
type
, then an
__mro_entries__()
method is searched on the base.
If an
__mro_entries__()
method is found, the base is substituted with the
result of a call to
__mro_entries__()
when creating the class.
The method is called with the original bases tuple
passed to the
bases
parameter, and must return a tuple
of classes that will be used instead of the base. The returned tuple may be
empty: in these cases, the original base is ignored.
3.3.3.3.
Determining the appropriate metaclass
¶
The appropriate metaclass for a class definition is determined as follows:
if no bases and no explicit metaclass are given, then
type()
is used;
if an explicit metaclass is given and it is
not
an instance of
type()
, then it is used directly as the metaclass;
if an instance of
type()
is given as the explicit metaclass, or
bases are defined, then the most derived metaclass is used.
The most derived metaclass is selected from the explicitly specified
metaclass (if any) and the metaclasses (i.e.
type(cls)
) of all specified
base classes. The most derived metaclass is one which is a subtype of
all
of these candidate metaclasses. If none of the candidate metaclasses meets
that criterion, then the class definition will fail with
TypeError
.
3.3.3.4.
Preparing the class namespace
¶
Once the appropriate metaclass has been identified, then the class namespace
is prepared. If the metaclass has a
__prepare__
attribute, it is called
as
namespace
=
metaclass.__prepare__(name,
bases,
**kwds)
(where the
additional keyword arguments, if any, come from the class definition). The
__prepare__
method should be implemented as a
classmethod
. The
namespace returned by
__prepare__
is passed in to
__new__
, but when
the final class object is created the namespace is copied into a new
dict
.
If the metaclass has no
__prepare__
attribute, then the class namespace
is initialised as an empty ordered mapping.
See also
PEP 3115
- Metaclasses in Python 3000
Introduced the
__prepare__
namespace hook
3.3.3.5.
Executing the class body
¶
The class body is executed (approximately) as
exec(body,
globals(),
namespace)
. The key difference from a normal
call to
exec()
is that lexical scoping allows the class body (including
any methods) to reference names from the current and outer scopes when the
class definition occurs inside a function.
However, even when the class definition occurs inside the function, methods
defined inside the class still cannot see names defined at the class scope.
Class variables must be accessed through the first parameter of instance or
class methods, or through the implicit lexically scoped
__class__
reference
described in the next section.
3.3.3.6.
Creating the class object
¶
Once the class namespace has been populated by executing the class body,
the class object is created by calling
metaclass(name,
bases,
namespace,
**kwds)
(the additional keywords
passed here are the same as those passed to
__prepare__
).
This class object is the one that will be referenced by the zero-argument
form of
super()
.
__class__
is an implicit closure reference
created by the compiler if any methods in a class body refer to either
__class__
or
super
. This allows the zero argument form of
super()
to correctly identify the class being defined based on
lexical scoping, while the class or instance that was used to make the
current call is identified based on the first argument passed to the method.
CPython implementation detail:
In CPython 3.6 and later, the
__class__
cell is passed to the metaclass
as a
__classcell__
entry in the class namespace. If present, this must
be propagated up to the
type.__new__
call in order for the class to be
initialised correctly.
Failing to do so will result in a
RuntimeError
in Python 3.8.
When using the default metaclass
type
, or any metaclass that ultimately
calls
type.__new__
, the following additional customization steps are
invoked after creating the class object:
The
type.__new__
method collects all of the attributes in the class
namespace that define a
__set_name__()
method;
Those
__set_name__
methods are called with the class
being defined and the assigned name of that particular attribute;
The
__init_subclass__()
hook is called on the
immediate parent of the new class in its method resolution order.
After the class object is created, it is passed to the class decorators
included in the class definition (if any) and the resulting object is bound
in the local namespace as the defined class.
When a new class is created by
type.__new__
, the object provided as the
namespace parameter is copied to a new ordered mapping and the original
object is discarded. The new copy is wrapped in a read-only proxy, which
becomes the
__dict__
attribute of the class object.
See also
PEP 3135
- New super
Describes the implicit
__class__
closure reference
3.3.3.7.
Uses for metaclasses
¶
The potential uses for metaclasses are boundless. Some ideas that have been
explored include enum, logging, interface checking, automatic delegation,
automatic property creation, proxies, frameworks, and automatic resource
locking/synchronization.
3.3.4.
Customizing instance and subclass checks
¶
The following methods are used to override the default behavior of the
isinstance()
and
issubclass()
built-in functions.
In particular, the metaclass
abc.ABCMeta
implements these methods in
order to allow the addition of Abstract Base Classes (ABCs) as âvirtual base
classesâ to any class or type (including built-in types), including other
ABCs.
type.
__instancecheck__
(
self
,
instance
)
¶
Return true if
instance
should be considered a (direct or indirect)
instance of
class
. If defined, called to implement
isinstance(instance,
class)
.
type.
__subclasscheck__
(
self
,
subclass
)
¶
Return true if
subclass
should be considered a (direct or indirect)
subclass of
class
. If defined, called to implement
issubclass(subclass,
class)
.
Note that these methods are looked up on the type (metaclass) of a class. They
cannot be defined as class methods in the actual class. This is consistent with
the lookup of special methods that are called on instances, only in this
case the instance is itself a class.
See also
PEP 3119
- Introducing Abstract Base Classes
Includes the specification for customizing
isinstance()
and
issubclass()
behavior through
__instancecheck__()
and
__subclasscheck__()
, with motivation for this functionality
in the context of adding Abstract Base Classes (see the
abc
module) to the language.
3.3.5.
Emulating generic types
¶
When using
type annotations
, it is often useful to
parameterize
a
generic type
using Pythonâs square-brackets notation.
For example, the annotation
list[int]
might be used to signify a
list
in which all the elements are of type
int
.
See also
PEP 484
- Type Hints
Introducing Pythonâs framework for type annotations
Generic Alias Types
Documentation for objects representing parameterized generic classes
Generics
,
user-defined generics
and
typing.Generic
Documentation on how to implement generic classes that can be
parameterized at runtime and understood by static type-checkers.
A class can
generally
only be parameterized if it defines the special
class method
__class_getitem__()
.
classmethod
object.
__class_getitem__
(
cls
,
key
)
¶
Return an object representing the specialization of a generic class
by type arguments found in
key
.
When defined on a class,
__class_getitem__()
is automatically a class
method. As such, there is no need for it to be decorated with
@classmethod
when it is defined.
3.3.5.1.
The purpose of
__class_getitem__
¶
The purpose of
__class_getitem__()
is to allow runtime
parameterization of standard-library generic classes in order to more easily
apply
type hints
to these classes.
To implement custom generic classes that can be parameterized at runtime and
understood by static type-checkers, users should either inherit from a standard
library class that already implements
__class_getitem__()
, or
inherit from
typing.Generic
, which has its own implementation of
__class_getitem__()
.
Custom implementations of
__class_getitem__()
on classes defined
outside of the standard library may not be understood by third-party
type-checkers such as mypy. Using
__class_getitem__()
on any class for
purposes other than type hinting is discouraged.
3.3.5.2.
__class_getitem__
versus
__getitem__
¶
Usually, the
subscription
of an object using square
brackets will call the
__getitem__()
instance method defined on
the objectâs class. However, if the object being subscribed is itself a class,
the class method
__class_getitem__()
may be called instead.
__class_getitem__()
should return a
GenericAlias
object if it is properly defined.
Presented with the
expression
obj[x]
, the Python interpreter
follows something like the following process to decide whether
__getitem__()
or
__class_getitem__()
should be
called:
from
inspect
import
isclass
def
subscribe
(
obj
,
x
):
"""Return the result of the expression 'obj[x]'"""
class_of_obj
=
type
(
obj
)
# If the class of obj defines __getitem__,
# call class_of_obj.__getitem__(obj, x)
if
hasattr
(
class_of_obj
,
'__getitem__'
):
return
class_of_obj
.
__getitem__
(
obj
,
x
)
# Else, if obj is a class and defines __class_getitem__,
# call obj.__class_getitem__(x)
elif
isclass
(
obj
)
and
hasattr
(
obj
,
'__class_getitem__'
):
return
obj
.
__class_getitem__
(
x
)
# Else, raise an exception
else
:
raise
TypeError
(
f
"'
{
class_of_obj
.
__name__
}
' object is not subscriptable"
)
In Python, all classes are themselves instances of other classes. The class of
a class is known as that classâs
metaclass
, and most classes have the
type
class as their metaclass.
type
does not define
__getitem__()
, meaning that expressions such as
list[int]
,
dict[str,
float]
and
tuple[str,
bytes]
all result in
__class_getitem__()
being called:
>>>
# list has class "type" as its metaclass, like most classes:
>>>
type
(
list
)
<class 'type'>
>>>
type
(
dict
)
==
type
(
list
)
==
type
(
tuple
)
==
type
(
str
)
==
type
(
bytes
)
True
>>>
# "list[int]" calls "list.__class_getitem__(int)"
>>>
list
[
int
]
list[int]
>>>
# list.__class_getitem__ returns a GenericAlias object:
>>>
type
(
list
[
int
])
<class 'types.GenericAlias'>
However, if a class has a custom metaclass that defines
__getitem__()
, subscribing the class may result in different
behaviour. An example of this can be found in the
enum
module:
>>>
from
enum
import
Enum
>>>
class
Menu
(
Enum
):
...
"""A breakfast menu"""
...
SPAM
=
'spam'
...
BACON
=
'bacon'
...
>>>
# Enum classes have a custom metaclass:
>>>
type
(
Menu
)
<class 'enum.EnumMeta'>
>>>
# EnumMeta defines __getitem__,
>>>
# so __class_getitem__ is not called,
>>>
# and the result is not a GenericAlias object:
>>>
Menu
[
'SPAM'
]
<Menu.SPAM: 'spam'>
>>>
type
(
Menu
[
'SPAM'
])
<enum 'Menu'>
See also
PEP 560
- Core Support for typing module and generic types
Introducing
__class_getitem__()
, and outlining when a
subscription
results in
__class_getitem__()
being called instead of
__getitem__()
3.3.6.
Emulating callable objects
¶
object.
__call__
(
self
[
,
args...
]
)
¶
Called when the instance is âcalledâ as a function; if this method is defined,
x(arg1,
arg2,
...)
roughly translates to
type(x).__call__(x,
arg1,
...)
.
The
object
class itself does not provide this method.
3.3.7.
Emulating container types
¶
The following methods can be defined to implement container objects. None of them
are provided by the
object
class itself. Containers usually are
sequences
(such as
lists
or
tuples
) or
mappings
(like
dictionaries
),
but can represent other containers as well. The first set of methods is used
either to emulate a sequence or to emulate a mapping; the difference is that for
a sequence, the allowable keys should be the integers
k
for which
0
<=
k
<
N
where
N
is the length of the sequence, or
slice
objects, which define a
range of items. It is also recommended that mappings provide the methods
keys()
,
values()
,
items()
,
get()
,
clear()
,
setdefault()
,
pop()
,
popitem()
,
copy()
, and
update()
behaving similar to those for Pythonâs standard
dictionary
objects. The
collections.abc
module provides a
MutableMapping
abstract base class
to help create those methods from a base set of
__getitem__()
,
__setitem__()
,
__delitem__()
, and
keys()
.
Mutable sequences should provide methods
append()
,
clear()
,
count()
,
extend()
,
index()
,
insert()
,
pop()
,
remove()
, and
reverse()
,
like Python standard
list
objects.
Finally, sequence types should implement addition (meaning concatenation) and
multiplication (meaning repetition) by defining the methods
__add__()
,
__radd__()
,
__iadd__()
,
__mul__()
,
__rmul__()
and
__imul__()
described below; they should not define other numerical
operators.
It is recommended that both mappings and sequences implement the
__contains__()
method to allow efficient use of the
in
operator; for
mappings,
in
should search the mappingâs keys; for sequences, it should
search through the values. It is further recommended that both mappings and
sequences implement the
__iter__()
method to allow efficient iteration
through the container; for mappings,
__iter__()
should iterate
through the objectâs keys; for sequences, it should iterate through the values.
object.
__len__
(
self
)
¶
Called to implement the built-in function
len()
. Should return the length
of the object, an integer
>=
0. Also, an object that doesnât define a
__bool__()
method and whose
__len__()
method returns zero is
considered to be false in a Boolean context.
CPython implementation detail:
In CPython, the length is required to be at most
sys.maxsize
.
If the length is larger than
sys.maxsize
some features (such as
len()
) may raise
OverflowError
. To prevent raising
OverflowError
by truth value testing, an object must define a
__bool__()
method.
object.
__length_hint__
(
self
)
¶
Called to implement
operator.length_hint()
. Should return an estimated
length for the object (which may be greater or less than the actual length).
The length must be an integer
>=
0. The return value may also be
NotImplemented
, which is treated the same as if the
__length_hint__
method didnât exist at all. This method is purely an
optimization and is never required for correctness.
Added in version 3.4.
object.
__getitem__
(
self
,
subscript
)
¶
Called to implement
subscription
, that is,
self[subscript]
.
See
Subscriptions and slicings
for details on the syntax.
There are two types of built-in objects that support subscription
via
__getitem__()
:
sequences
, where
subscript
(also called
index
) should be an integer or a
slice
object.
See the
sequence documentation
for the expected
behavior, including handling
slice
objects and negative indices.
mappings
, where
subscript
is also called the
key
.
See
mapping documentation
for the expected
behavior.
If
subscript
is of an inappropriate type,
__getitem__()
should raise
TypeError
.
If
subscript
has an inappropriate value,
__getitem__()
should raise an
LookupError
or one of its subclasses
(
IndexError
for sequences;
KeyError
for mappings).
Note
Slicing is handled by
__getitem__()
,
__setitem__()
,
and
__delitem__()
.
A call like
a
[
1
:
2
]
=
b
is translated to
a
[
slice
(
1
,
2
,
None
)]
=
b
and so forth. Missing slice items are always filled in with
None
.
Note
The sequence iteration protocol (used, for example, in
for
loops), expects that an
IndexError
will be raised for illegal
indexes to allow proper detection of the end of a sequence.
Note
When
subscripting
a
class
, the special
class method
__class_getitem__()
may be called instead of
__getitem__()
. See
__class_getitem__ versus __getitem__
for more
details.
object.
__setitem__
(
self
,
key
,
value
)
¶
Called to implement assignment to
self[key]
. Same note as for
__getitem__()
. This should only be implemented for mappings if the
objects support changes to the values for keys, or if new keys can be added, or
for sequences if elements can be replaced. The same exceptions should be raised
for improper
key
values as for the
__getitem__()
method.
object.
__delitem__
(
self
,
key
)
¶
Called to implement deletion of
self[key]
. Same note as for
__getitem__()
. This should only be implemented for mappings if the
objects support removal of keys, or for sequences if elements can be removed
from the sequence. The same exceptions should be raised for improper
key
values as for the
__getitem__()
method.
object.
__missing__
(
self
,
key
)
¶
Called by
dict
.
__getitem__()
to implement
self[key]
for dict subclasses
when key is not in the dictionary.
object.
__iter__
(
self
)
¶
This method is called when an
iterator
is required for a container.
This method should return a new iterator object that can iterate over all the
objects in the container. For mappings, it should iterate over the keys of
the container.
object.
__reversed__
(
self
)
¶
Called (if present) by the
reversed()
built-in to implement
reverse iteration. It should return a new iterator object that iterates
over all the objects in the container in reverse order.
If the
__reversed__()
method is not provided, the
reversed()
built-in will fall back to using the sequence protocol (
__len__()
and
__getitem__()
). Objects that support the sequence protocol should
only provide
__reversed__()
if they can provide an implementation
that is more efficient than the one provided by
reversed()
.
The membership test operators (
in
and
not
in
) are normally
implemented as an iteration through a container. However, container objects can
supply the following special method with a more efficient implementation, which
also does not require the object be iterable.
object.
__contains__
(
self
,
item
)
¶
Called to implement membership test operators. Should return true if
item
is in
self
, false otherwise. For mapping objects, this should consider the
keys of the mapping rather than the values or the key-item pairs.
For objects that donât define
__contains__()
, the membership test first
tries iteration via
__iter__()
, then the old sequence iteration
protocol via
__getitem__()
, see
this section in the language
reference
.
3.3.8.
Emulating numeric types
¶
The following methods can be defined to emulate numeric objects. Methods
corresponding to operations that are not supported by the particular kind of
number implemented (e.g., bitwise operations for non-integral numbers) should be
left undefined.
object.
__add__
(
self
,
other
)
¶
object.
__sub__
(
self
,
other
)
¶
object.
__mul__
(
self
,
other
)
¶
object.
__matmul__
(
self
,
other
)
¶
object.
__truediv__
(
self
,
other
)
¶
object.
__floordiv__
(
self
,
other
)
¶
object.
__mod__
(
self
,
other
)
¶
object.
__divmod__
(
self
,
other
)
¶
object.
__pow__
(
self
,
other
[
,
modulo
]
)
¶
object.
__lshift__
(
self
,
other
)
¶
object.
__rshift__
(
self
,
other
)
¶
object.
__and__
(
self
,
other
)
¶
object.
__xor__
(
self
,
other
)
¶
object.
__or__
(
self
,
other
)
¶
These methods are called to implement the binary arithmetic operations
(
+
,
-
,
*
,
@
,
/
,
//
,
%
,
divmod()
,
pow()
,
**
,
<<
,
>>
,
&
,
^
,
|
). For instance, to
evaluate the expression
x
+
y
, where
x
is an instance of a class that
has an
__add__()
method,
type(x).__add__(x,
y)
is called. The
__divmod__()
method should be the equivalent to using
__floordiv__()
and
__mod__()
; it should not be related to
__truediv__()
. Note that
__pow__()
should be defined to accept
an optional third argument if the three-argument version of the built-in
pow()
function is to be supported.
If one of those methods does not support the operation with the supplied
arguments, it should return
NotImplemented
.
object.
__radd__
(
self
,
other
)
¶
object.
__rsub__
(
self
,
other
)
¶
object.
__rmul__
(
self
,
other
)
¶
object.
__rmatmul__
(
self
,
other
)
¶
object.
__rtruediv__
(
self
,
other
)
¶
object.
__rfloordiv__
(
self
,
other
)
¶
object.
__rmod__
(
self
,
other
)
¶
object.
__rdivmod__
(
self
,
other
)
¶
object.
__rpow__
(
self
,
other
[
,
modulo
]
)
¶
object.
__rlshift__
(
self
,
other
)
¶
object.
__rrshift__
(
self
,
other
)
¶
object.
__rand__
(
self
,
other
)
¶
object.
__rxor__
(
self
,
other
)
¶
object.
__ror__
(
self
,
other
)
¶
These methods are called to implement the binary arithmetic operations
(
+
,
-
,
*
,
@
,
/
,
//
,
%
,
divmod()
,
pow()
,
**
,
<<
,
>>
,
&
,
^
,
|
) with reflected
(swapped) operands. These functions are only called if the operands
are of different types, when the left operand does not support the corresponding
operation
[
3
]
, or the right operandâs class is derived from the left operandâs
class.
[
4
]
For instance, to evaluate the expression
x
-
y
, where
y
is
an instance of a class that has an
__rsub__()
method,
type(y).__rsub__(y,
x)
is called if
type(x).__sub__(x,
y)
returns
NotImplemented
or
type(y)
is a subclass of
type(x)
.
[
5
]
Note that
__rpow__()
should be defined to accept an optional third
argument if the three-argument version of the built-in
pow()
function
is to be supported.
Changed in version 3.14:
Three-argument
pow()
now try calling
__rpow__()
if necessary.
Previously it was only called in two-argument
pow()
and the binary
power operator.
Note
If the right operandâs type is a subclass of the left operandâs type and
that subclass provides a different implementation of the reflected method
for the operation, this method will be called before the left operandâs
non-reflected method. This behavior allows subclasses to override their
ancestorsâ operations.
object.
__iadd__
(
self
,
other
)
¶
object.
__isub__
(
self
,
other
)
¶
object.
__imul__
(
self
,
other
)
¶
object.
__imatmul__
(
self
,
other
)
¶
object.
__itruediv__
(
self
,
other
)
¶
object.
__ifloordiv__
(
self
,
other
)
¶
object.
__imod__
(
self
,
other
)
¶
object.
__ipow__
(
self
,
other
[
,
modulo
]
)
¶
object.
__ilshift__
(
self
,
other
)
¶
object.
__irshift__
(
self
,
other
)
¶
object.
__iand__
(
self
,
other
)
¶
object.
__ixor__
(
self
,
other
)
¶
object.
__ior__
(
self
,
other
)
¶
These methods are called to implement the augmented arithmetic assignments
(
+=
,
-=
,
*=
,
@=
,
/=
,
//=
,
%=
,
**=
,
<<=
,
>>=
,
&=
,
^=
,
|=
). These methods should attempt to do the
operation in-place (modifying
self
) and return the result (which could be,
but does not have to be,
self
). If a specific method is not defined, or if
that method returns
NotImplemented
, the
augmented assignment falls back to the normal methods. For instance, if
x
is an instance of a class with an
__iadd__()
method,
x
+=
y
is
equivalent to
x
=
x.__iadd__(y)
. If
__iadd__()
does not exist, or if
x.__iadd__(y)
returns
NotImplemented
,
x.__add__(y)
and
y.__radd__(x)
are considered, as with the evaluation of
x
+
y
. In
certain situations, augmented assignment can result in unexpected errors (see
Why does a_tuple[i] += [âitemâ] raise an exception when the addition works?
), but this behavior is in fact
part of the data model.
object.
__neg__
(
self
)
¶
object.
__pos__
(
self
)
¶
object.
__abs__
(
self
)
¶
object.
__invert__
(
self
)
¶
Called to implement the unary arithmetic operations (
-
,
+
,
abs()
and
~
).
object.
__complex__
(
self
)
¶
object.
__int__
(
self
)
¶
object.
__float__
(
self
)
¶
Called to implement the built-in functions
complex()
,
int()
and
float()
. Should return a value
of the appropriate type.
object.
__index__
(
self
)
¶
Called to implement
operator.index()
, and whenever Python needs to
losslessly convert the numeric object to an integer object (such as in
slicing, or in the built-in
bin()
,
hex()
and
oct()
functions). Presence of this method indicates that the numeric object is
an integer type. Must return an integer.
If
__int__()
,
__float__()
and
__complex__()
are not
defined then corresponding built-in functions
int()
,
float()
and
complex()
fall back to
__index__()
.
object.
__round__
(
self
[
,
ndigits
]
)
¶
object.
__trunc__
(
self
)
¶
object.
__floor__
(
self
)
¶
object.
__ceil__
(
self
)
¶
Called to implement the built-in function
round()
and
math
functions
trunc()
,
floor()
and
ceil()
.
Unless
ndigits
is passed to
__round__()
all these methods should
return the value of the object truncated to an
Integral
(typically an
int
).
Changed in version 3.14:
int()
no longer delegates to the
__trunc__()
method.
3.3.9.
With Statement Context Managers
¶
A
context manager
is an object that defines the runtime context to be
established when executing a
with
statement. The context manager
handles the entry into, and the exit from, the desired runtime context for the
execution of the block of code. Context managers are normally invoked using the
with
statement (described in section
The with statement
), but can also be
used by directly invoking their methods.
Typical uses of context managers include saving and restoring various kinds of
global state, locking and unlocking resources, closing opened files, etc.
For more information on context managers, see
Context Manager Types
.
The
object
class itself does not provide the context manager methods.
object.
__enter__
(
self
)
¶
Enter the runtime context related to this object. The
with
statement
will bind this methodâs return value to the target(s) specified in the
as
clause of the statement, if any.
object.
__exit__
(
self
,
exc_type
,
exc_value
,
traceback
)
¶
Exit the runtime context related to this object. The parameters describe the
exception that caused the context to be exited. If the context was exited
without an exception, all three arguments will be
None
.
If an exception is supplied, and the method wishes to suppress the exception
(i.e., prevent it from being propagated), it should return a true value.
Otherwise, the exception will be processed normally upon exit from this method.
Note that
__exit__()
methods should not reraise the passed-in exception;
this is the callerâs responsibility.
See also
PEP 343
- The âwithâ statement
The specification, background, and examples for the Python
with
statement.
3.3.10.
Customizing positional arguments in class pattern matching
¶
When using a class name in a pattern, positional arguments in the pattern are not
allowed by default, i.e.
case
MyClass(x,
y)
is typically invalid without special
support in
MyClass
. To be able to use that kind of pattern, the class needs to
define a
__match_args__
attribute.
object.
__match_args__
¶
This class variable can be assigned a tuple of strings. When this class is
used in a class pattern with positional arguments, each positional argument will
be converted into a keyword argument, using the corresponding value in
__match_args__
as the keyword. The absence of this attribute is equivalent to
setting it to
()
.
For example, if
MyClass.__match_args__
is
("left",
"center",
"right")
that means
that
case
MyClass(x,
y)
is equivalent to
case
MyClass(left=x,
center=y)
. Note
that the number of arguments in the pattern must be smaller than or equal to the number
of elements in
__match_args__
; if it is larger, the pattern match attempt will raise
a
TypeError
.
Added in version 3.10.
See also
PEP 634
- Structural Pattern Matching
The specification for the Python
match
statement.
3.3.11.
Emulating buffer types
¶
The
buffer protocol
provides a way for Python
objects to expose efficient access to a low-level memory array. This protocol
is implemented by builtin types such as
bytes
and
memoryview
,
and third-party libraries may define additional buffer types.
While buffer types are usually implemented in C, it is also possible to
implement the protocol in Python.
object.
__buffer__
(
self
,
flags
)
¶
Called when a buffer is requested from
self
(for example, by the
memoryview
constructor). The
flags
argument is an integer
representing the kind of buffer requested, affecting for example whether
the returned buffer is read-only or writable.
inspect.BufferFlags
provides a convenient way to interpret the flags. The method must return
a
memoryview
object.
Thread safety:
In
free-threaded
Python,
implementations must manage any internal export counter using atomic
operations. The method must be safe to call concurrently from multiple
threads, and the returned bufferâs underlying data must remain valid
until the corresponding
__release_buffer__()
call
completes. See
Thread safety for memoryview objects
for details.
object.
__release_buffer__
(
self
,
buffer
)
¶
Called when a buffer is no longer needed. The
buffer
argument is a
memoryview
object that was previously returned by
__buffer__()
. The method must release any resources associated
with the buffer. This method should return
None
.
Thread safety:
In
free-threaded
Python,
any export counter decrement must use atomic operations. Resource
cleanup must be thread-safe, as the final release may race with
concurrent releases from other threads.
Buffer objects that do not need to perform any cleanup are not required
to implement this method.
Added in version 3.12.
See also
PEP 688
- Making the buffer protocol accessible in Python
Introduces the Python
__buffer__
and
__release_buffer__
methods.
collections.abc.Buffer
ABC for buffer types.
3.3.12.
Annotations
¶
Functions, classes, and modules may contain
annotations
,
which are a way to associate information (usually
type hints
)
with a symbol.
object.
__annotations__
¶
This attribute contains the annotations for an object. It is
lazily evaluated
, so accessing the attribute may
execute arbitrary code and raise exceptions. If evaluation is successful, the
attribute is set to a dictionary mapping from variable names to annotations.
Changed in version 3.14:
Annotations are now lazily evaluated.
object.
__annotate__
(
format
)
¶
An
annotate function
.
Returns a new dictionary object mapping attribute/parameter names to their annotation values.
Takes a format parameter specifying the format in which annotations values should be provided.
It must be a member of the
annotationlib.Format
enum, or an integer with
a value corresponding to a member of the enum.
If an annotate function doesnât support the requested format, it must raise
NotImplementedError
. Annotate functions must always support
VALUE
format; they must not raise
NotImplementedError()
when called with this format.
When called with
VALUE
format, an annotate function may raise
NameError
; it must not raise
NameError
when called requesting any other format.
If an object does not have any annotations,
__annotate__
should preferably be set
to
None
(it canât be deleted), rather than set to a function that returns an empty dict.
Added in version 3.14.
See also
PEP 649
â Deferred evaluation of annotation using descriptors
Introduces lazy evaluation of annotations and the
__annotate__
function.
3.3.13.
Special method lookup
¶
For custom classes, implicit invocations of special methods are only guaranteed
to work correctly if defined on an objectâs type, not in the objectâs instance
dictionary. That behaviour is the reason why the following code raises an
exception:
>>>
class
C
:
...
pass
...
>>>
c
=
C
()
>>>
c
.
__len__
=
lambda
:
5
>>>
len
(
c
)
Traceback (most recent call last):
File
"<stdin>"
, line
1
, in
<module>
TypeError
:
object of type 'C' has no len()
The rationale behind this behaviour lies with a number of special methods such
as
__hash__()
and
__repr__()
that are implemented
by all objects,
including type objects. If the implicit lookup of these methods used the
conventional lookup process, they would fail when invoked on the type object
itself:
>>>
1
.
__hash__
()
==
hash
(
1
)
True
>>>
int
.
__hash__
()
==
hash
(
int
)
Traceback (most recent call last):
File
"<stdin>"
, line
1
, in
<module>
TypeError
:
descriptor '__hash__' of 'int' object needs an argument
Incorrectly attempting to invoke an unbound method of a class in this way is
sometimes referred to as âmetaclass confusionâ, and is avoided by bypassing
the instance when looking up special methods:
>>>
type
(
1
)
.
__hash__
(
1
)
==
hash
(
1
)
True
>>>
type
(
int
)
.
__hash__
(
int
)
==
hash
(
int
)
True
In addition to bypassing any instance attributes in the interest of
correctness, implicit special method lookup generally also bypasses the
__getattribute__()
method even of the objectâs metaclass:
>>>
class
Meta
(
type
):
...
def
__getattribute__
(
*
args
):
...
print
(
"Metaclass getattribute invoked"
)
...
return
type
.
__getattribute__
(
*
args
)
...
>>>
class
C
(
object
,
metaclass
=
Meta
):
...
def
__len__
(
self
):
...
return
10
...
def
__getattribute__
(
*
args
):
...
print
(
"Class getattribute invoked"
)
...
return
object
.
__getattribute__
(
*
args
)
...
>>>
c
=
C
()
>>>
c
.
__len__
()
# Explicit lookup via instance
Class getattribute invoked
10
>>>
type
(
c
)
.
__len__
(
c
)
# Explicit lookup via type
Metaclass getattribute invoked
10
>>>
len
(
c
)
# Implicit lookup
10
Bypassing the
__getattribute__()
machinery in this fashion
provides significant scope for speed optimisations within the
interpreter, at the cost of some flexibility in the handling of
special methods (the special method
must
be set on the class
object itself in order to be consistently invoked by the interpreter).
3.4.
Coroutines
¶
3.4.1.
Awaitable Objects
¶
An
awaitable
object generally implements an
__await__()
method.
Coroutine objects
returned from
async
def
functions
are awaitable.
object.
__await__
(
self
)
¶
Must return an
iterator
. Should be used to implement
awaitable
objects. For instance,
asyncio.Future
implements
this method to be compatible with the
await
expression.
The
object
class itself is not awaitable and does not provide
this method.
Note
The language doesnât place any restriction on the type or value of the
objects yielded by the iterator returned by
__await__
, as this is
specific to the implementation of the asynchronous execution framework
(e.g.
asyncio
) that will be managing the
awaitable
object.
Added in version 3.5.
See also
PEP 492
for additional information about awaitable objects.
3.4.2.
Coroutine Objects
¶
Coroutine objects
are
awaitable
objects.
A coroutineâs execution can be controlled by calling
__await__()
and
iterating over the result. When the coroutine has finished executing and
returns, the iterator raises
StopIteration
, and the exceptionâs
value
attribute holds the return value. If the
coroutine raises an exception, it is propagated by the iterator. Coroutines
should not directly raise unhandled
StopIteration
exceptions.
Coroutines also have the methods listed below, which are analogous to
those of generators (see
Generator-iterator methods
). However, unlike
generators, coroutines do not directly support iteration.
Changed in version 3.5.2:
It is a
RuntimeError
to await on a coroutine more than once.
coroutine.
send
(
value
)
¶
Starts or resumes execution of the coroutine. If
value
is
None
,
this is equivalent to advancing the iterator returned by
__await__()
. If
value
is not
None
, this method delegates
to the
send()
method of the iterator that caused
the coroutine to suspend. The result (return value,
StopIteration
, or other exception) is the same as when
iterating over the
__await__()
return value, described above.
coroutine.
throw
(
value
)
¶
coroutine.
throw
(
type
[
,
value
[
,
traceback
]
]
)
Raises the specified exception in the coroutine. This method delegates
to the
throw()
method of the iterator that caused
the coroutine to suspend, if it has such a method. Otherwise,
the exception is raised at the suspension point. The result
(return value,
StopIteration
, or other exception) is the same as
when iterating over the
__await__()
return value, described
above. If the exception is not caught in the coroutine, it propagates
back to the caller.
Changed in version 3.12:
The second signature (type[, value[, traceback]]) is deprecated and
may be removed in a future version of Python.
coroutine.
close
(
)
¶
Causes the coroutine to clean itself up and exit. If the coroutine
is suspended, this method first delegates to the
close()
method of the iterator that caused the coroutine to suspend, if it
has such a method. Then it raises
GeneratorExit
at the
suspension point, causing the coroutine to immediately clean itself up.
Finally, the coroutine is marked as having finished executing, even if
it was never started.
Coroutine objects are automatically closed using the above process when
they are about to be destroyed.
3.4.3.
Asynchronous Iterators
¶
An
asynchronous iterator
can call asynchronous code in
its
__anext__
method.
Asynchronous iterators can be used in an
async
for
statement.
The
object
class itself does not provide these methods.
object.
__aiter__
(
self
)
¶
Must return an
asynchronous iterator
object.
object.
__anext__
(
self
)
¶
Must return an
awaitable
resulting in a next value of the iterator. Should
raise a
StopAsyncIteration
error when the iteration is over.
An example of an asynchronous iterable object:
class
Reader
:
async
def
readline
(
self
):
...
def
__aiter__
(
self
):
return
self
async
def
__anext__
(
self
):
val
=
await
self
.
readline
()
if
val
==
b
''
:
raise
StopAsyncIteration
return
val
Added in version 3.5.
Changed in version 3.7:
Prior to Python 3.7,
__aiter__()
could return an
awaitable
that would resolve to an
asynchronous iterator
.
Starting with Python 3.7,
__aiter__()
must return an
asynchronous iterator object. Returning anything else
will result in a
TypeError
error.
3.4.4.
Asynchronous Context Managers
¶
An
asynchronous context manager
is a
context manager
that is able to
suspend execution in its
__aenter__
and
__aexit__
methods.
Asynchronous context managers can be used in an
async
with
statement.
The
object
class itself does not provide these methods.
object.
__aenter__
(
self
)
¶
Semantically similar to
__enter__()
, the only
difference being that it must return an
awaitable
.
object.
__aexit__
(
self
,
exc_type
,
exc_value
,
traceback
)
¶
Semantically similar to
__exit__()
, the only
difference being that it must return an
awaitable
.
An example of an asynchronous context manager class:
class
AsyncContextManager
:
async
def
__aenter__
(
self
):
await
log
(
'entering context'
)
async
def
__aexit__
(
self
,
exc_type
,
exc
,
tb
):
await
log
(
'exiting context'
)
Added in version 3.5.
Footnotes |
| Markdown | [](https://www.python.org/)
Theme
### [Table of Contents](https://docs.python.org/3/contents.html)
- [3\. Data model](https://docs.python.org/3/reference/datamodel.html)
- [3\.1. Objects, values and types](https://docs.python.org/3/reference/datamodel.html#objects-values-and-types)
- [3\.2. The standard type hierarchy](https://docs.python.org/3/reference/datamodel.html#the-standard-type-hierarchy)
- [3\.2.1. None](https://docs.python.org/3/reference/datamodel.html#none)
- [3\.2.2. NotImplemented](https://docs.python.org/3/reference/datamodel.html#notimplemented)
- [3\.2.3. Ellipsis](https://docs.python.org/3/reference/datamodel.html#ellipsis)
- [3\.2.4. `numbers.Number`](https://docs.python.org/3/reference/datamodel.html#numbers-number)
- [3\.2.4.1. `numbers.Integral`](https://docs.python.org/3/reference/datamodel.html#numbers-integral)
- [3\.2.4.2. `numbers.Real` (`float`)](https://docs.python.org/3/reference/datamodel.html#numbers-real-float)
- [3\.2.4.3. `numbers.Complex` (`complex`)](https://docs.python.org/3/reference/datamodel.html#numbers-complex-complex)
- [3\.2.5. Sequences](https://docs.python.org/3/reference/datamodel.html#sequences)
- [3\.2.5.1. Immutable sequences](https://docs.python.org/3/reference/datamodel.html#immutable-sequences)
- [3\.2.5.2. Mutable sequences](https://docs.python.org/3/reference/datamodel.html#mutable-sequences)
- [3\.2.6. Set types](https://docs.python.org/3/reference/datamodel.html#set-types)
- [3\.2.7. Mappings](https://docs.python.org/3/reference/datamodel.html#mappings)
- [3\.2.7.1. Dictionaries](https://docs.python.org/3/reference/datamodel.html#dictionaries)
- [3\.2.8. Callable types](https://docs.python.org/3/reference/datamodel.html#callable-types)
- [3\.2.8.1. User-defined functions](https://docs.python.org/3/reference/datamodel.html#user-defined-functions)
- [3\.2.8.1.1. Special read-only attributes](https://docs.python.org/3/reference/datamodel.html#special-read-only-attributes)
- [3\.2.8.1.2. Special writable attributes](https://docs.python.org/3/reference/datamodel.html#special-writable-attributes)
- [3\.2.8.2. Instance methods](https://docs.python.org/3/reference/datamodel.html#instance-methods)
- [3\.2.8.3. Generator functions](https://docs.python.org/3/reference/datamodel.html#generator-functions)
- [3\.2.8.4. Coroutine functions](https://docs.python.org/3/reference/datamodel.html#coroutine-functions)
- [3\.2.8.5. Asynchronous generator functions](https://docs.python.org/3/reference/datamodel.html#asynchronous-generator-functions)
- [3\.2.8.6. Built-in functions](https://docs.python.org/3/reference/datamodel.html#built-in-functions)
- [3\.2.8.7. Built-in methods](https://docs.python.org/3/reference/datamodel.html#built-in-methods)
- [3\.2.8.8. Classes](https://docs.python.org/3/reference/datamodel.html#classes)
- [3\.2.8.9. Class Instances](https://docs.python.org/3/reference/datamodel.html#class-instances)
- [3\.2.9. Modules](https://docs.python.org/3/reference/datamodel.html#modules)
- [3\.2.9.1. Import-related attributes on module objects](https://docs.python.org/3/reference/datamodel.html#import-related-attributes-on-module-objects)
- [3\.2.9.2. Other writable attributes on module objects](https://docs.python.org/3/reference/datamodel.html#other-writable-attributes-on-module-objects)
- [3\.2.9.3. Module dictionaries](https://docs.python.org/3/reference/datamodel.html#module-dictionaries)
- [3\.2.10. Custom classes](https://docs.python.org/3/reference/datamodel.html#custom-classes)
- [3\.2.10.1. Special attributes](https://docs.python.org/3/reference/datamodel.html#special-attributes)
- [3\.2.10.2. Special methods](https://docs.python.org/3/reference/datamodel.html#special-methods)
- [3\.2.11. Class instances](https://docs.python.org/3/reference/datamodel.html#id4)
- [3\.2.11.1. Special attributes](https://docs.python.org/3/reference/datamodel.html#id5)
- [3\.2.12. I/O objects (also known as file objects)](https://docs.python.org/3/reference/datamodel.html#i-o-objects-also-known-as-file-objects)
- [3\.2.13. Internal types](https://docs.python.org/3/reference/datamodel.html#internal-types)
- [3\.2.13.1. Code objects](https://docs.python.org/3/reference/datamodel.html#code-objects)
- [3\.2.13.1.1. Special read-only attributes](https://docs.python.org/3/reference/datamodel.html#index-64)
- [3\.2.13.1.2. Methods on code objects](https://docs.python.org/3/reference/datamodel.html#methods-on-code-objects)
- [3\.2.13.2. Frame objects](https://docs.python.org/3/reference/datamodel.html#frame-objects)
- [3\.2.13.2.1. Special read-only attributes](https://docs.python.org/3/reference/datamodel.html#index-70)
- [3\.2.13.2.2. Special writable attributes](https://docs.python.org/3/reference/datamodel.html#index-71)
- [3\.2.13.2.3. Frame object methods](https://docs.python.org/3/reference/datamodel.html#frame-object-methods)
- [3\.2.13.3. Traceback objects](https://docs.python.org/3/reference/datamodel.html#traceback-objects)
- [3\.2.13.4. Slice objects](https://docs.python.org/3/reference/datamodel.html#slice-objects)
- [3\.2.13.5. Static method objects](https://docs.python.org/3/reference/datamodel.html#static-method-objects)
- [3\.2.13.6. Class method objects](https://docs.python.org/3/reference/datamodel.html#class-method-objects)
- [3\.3. Special method names](https://docs.python.org/3/reference/datamodel.html#special-method-names)
- [3\.3.1. Basic customization](https://docs.python.org/3/reference/datamodel.html#basic-customization)
- [3\.3.2. Customizing attribute access](https://docs.python.org/3/reference/datamodel.html#customizing-attribute-access)
- [3\.3.2.1. Customizing module attribute access](https://docs.python.org/3/reference/datamodel.html#customizing-module-attribute-access)
- [3\.3.2.2. Implementing Descriptors](https://docs.python.org/3/reference/datamodel.html#implementing-descriptors)
- [3\.3.2.3. Invoking Descriptors](https://docs.python.org/3/reference/datamodel.html#invoking-descriptors)
- [3\.3.2.4. \_\_slots\_\_](https://docs.python.org/3/reference/datamodel.html#slots)
- [3\.3.3. Customizing class creation](https://docs.python.org/3/reference/datamodel.html#customizing-class-creation)
- [3\.3.3.1. Metaclasses](https://docs.python.org/3/reference/datamodel.html#metaclasses)
- [3\.3.3.2. Resolving MRO entries](https://docs.python.org/3/reference/datamodel.html#resolving-mro-entries)
- [3\.3.3.3. Determining the appropriate metaclass](https://docs.python.org/3/reference/datamodel.html#determining-the-appropriate-metaclass)
- [3\.3.3.4. Preparing the class namespace](https://docs.python.org/3/reference/datamodel.html#preparing-the-class-namespace)
- [3\.3.3.5. Executing the class body](https://docs.python.org/3/reference/datamodel.html#executing-the-class-body)
- [3\.3.3.6. Creating the class object](https://docs.python.org/3/reference/datamodel.html#creating-the-class-object)
- [3\.3.3.7. Uses for metaclasses](https://docs.python.org/3/reference/datamodel.html#uses-for-metaclasses)
- [3\.3.4. Customizing instance and subclass checks](https://docs.python.org/3/reference/datamodel.html#customizing-instance-and-subclass-checks)
- [3\.3.5. Emulating generic types](https://docs.python.org/3/reference/datamodel.html#emulating-generic-types)
- [3\.3.5.1. The purpose of *\_\_class\_getitem\_\_*](https://docs.python.org/3/reference/datamodel.html#the-purpose-of-class-getitem)
- [3\.3.5.2. *\_\_class\_getitem\_\_* versus *\_\_getitem\_\_*](https://docs.python.org/3/reference/datamodel.html#class-getitem-versus-getitem)
- [3\.3.6. Emulating callable objects](https://docs.python.org/3/reference/datamodel.html#emulating-callable-objects)
- [3\.3.7. Emulating container types](https://docs.python.org/3/reference/datamodel.html#emulating-container-types)
- [3\.3.8. Emulating numeric types](https://docs.python.org/3/reference/datamodel.html#emulating-numeric-types)
- [3\.3.9. With Statement Context Managers](https://docs.python.org/3/reference/datamodel.html#with-statement-context-managers)
- [3\.3.10. Customizing positional arguments in class pattern matching](https://docs.python.org/3/reference/datamodel.html#customizing-positional-arguments-in-class-pattern-matching)
- [3\.3.11. Emulating buffer types](https://docs.python.org/3/reference/datamodel.html#emulating-buffer-types)
- [3\.3.12. Annotations](https://docs.python.org/3/reference/datamodel.html#annotations)
- [3\.3.13. Special method lookup](https://docs.python.org/3/reference/datamodel.html#special-method-lookup)
- [3\.4. Coroutines](https://docs.python.org/3/reference/datamodel.html#coroutines)
- [3\.4.1. Awaitable Objects](https://docs.python.org/3/reference/datamodel.html#awaitable-objects)
- [3\.4.2. Coroutine Objects](https://docs.python.org/3/reference/datamodel.html#coroutine-objects)
- [3\.4.3. Asynchronous Iterators](https://docs.python.org/3/reference/datamodel.html#asynchronous-iterators)
- [3\.4.4. Asynchronous Context Managers](https://docs.python.org/3/reference/datamodel.html#asynchronous-context-managers)
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# 3\. Data model[¶](https://docs.python.org/3/reference/datamodel.html#data-model "Link to this heading")
## 3\.1. Objects, values and types[¶](https://docs.python.org/3/reference/datamodel.html#objects-values-and-types "Link to this heading")
*Objects* are Pythonâs abstraction for data. All data in a Python program is represented by objects or by relations between objects. Even code is represented by objects.
Every object has an identity, a type and a value. An objectâs *identity* never changes once it has been created; you may think of it as the objectâs address in memory. The [`is`](https://docs.python.org/3/reference/expressions.html#is) operator compares the identity of two objects; the [`id()`](https://docs.python.org/3/library/functions.html#id "id") function returns an integer representing its identity.
**CPython implementation detail:** For CPython, `id(x)` is the memory address where `x` is stored.
An objectâs type determines the operations that the object supports (e.g., âdoes it have a length?â) and also defines the possible values for objects of that type. The [`type()`](https://docs.python.org/3/library/functions.html#type "type") function returns an objectâs type (which is an object itself). Like its identity, an objectâs *type* is also unchangeable. [\[1\]](https://docs.python.org/3/reference/datamodel.html#id20)
The *value* of some objects can change. Objects whose value can change are said to be *mutable*; objects whose value is unchangeable once they are created are called *immutable*. (The value of an immutable container object that contains a reference to a mutable object can change when the latterâs value is changed; however the container is still considered immutable, because the collection of objects it contains cannot be changed. So, immutability is not strictly the same as having an unchangeable value, it is more subtle.) An objectâs mutability is determined by its type; for instance, numbers, strings and tuples are immutable, while dictionaries and lists are mutable.
Objects are never explicitly destroyed; however, when they become unreachable they may be garbage-collected. An implementation is allowed to postpone garbage collection or omit it altogether â it is a matter of implementation quality how garbage collection is implemented, as long as no objects are collected that are still reachable.
**CPython implementation detail:** CPython currently uses a reference-counting scheme with (optional) delayed detection of cyclically linked garbage, which collects most objects as soon as they become unreachable, but is not guaranteed to collect garbage containing circular references. See the documentation of the [`gc`](https://docs.python.org/3/library/gc.html#module-gc "gc: Interface to the cycle-detecting garbage collector.") module for information on controlling the collection of cyclic garbage. Other implementations act differently and CPython may change. Do not depend on immediate finalization of objects when they become unreachable (so you should always close files explicitly).
Note that the use of the implementationâs tracing or debugging facilities may keep objects alive that would normally be collectable. Also note that catching an exception with a [`try`](https://docs.python.org/3/reference/compound_stmts.html#try)âŠ[`except`](https://docs.python.org/3/reference/compound_stmts.html#except) statement may keep objects alive.
Some objects contain references to âexternalâ resources such as open files or windows. It is understood that these resources are freed when the object is garbage-collected, but since garbage collection is not guaranteed to happen, such objects also provide an explicit way to release the external resource, usually a `close()` method. Programs are strongly recommended to explicitly close such objects. The [`try`](https://docs.python.org/3/reference/compound_stmts.html#try)âŠ[`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) statement and the [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement provide convenient ways to do this.
Some objects contain references to other objects; these are called *containers*. Examples of containers are tuples, lists and dictionaries. The references are part of a containerâs value. In most cases, when we talk about the value of a container, we imply the values, not the identities of the contained objects; however, when we talk about the mutability of a container, only the identities of the immediately contained objects are implied. So, if an immutable container (like a tuple) contains a reference to a mutable object, its value changes if that mutable object is changed.
Types affect almost all aspects of object behavior. Even the importance of object identity is affected in some sense: for immutable types, operations that compute new values may actually return a reference to any existing object with the same type and value, while for mutable objects this is not allowed. For example, after `a = 1; b = 1`, *a* and *b* may or may not refer to the same object with the value one, depending on the implementation. This is because [`int`](https://docs.python.org/3/library/functions.html#int "int") is an immutable type, so the reference to `1` can be reused. This behaviour depends on the implementation used, so should not be relied upon, but is something to be aware of when making use of object identity tests. However, after `c = []; d = []`, *c* and *d* are guaranteed to refer to two different, unique, newly created empty lists. (Note that `e = f = []` assigns the *same* object to both *e* and *f*.)
## 3\.2. The standard type hierarchy[¶](https://docs.python.org/3/reference/datamodel.html#the-standard-type-hierarchy "Link to this heading")
Below is a list of the types that are built into Python. Extension modules (written in C, Java, or other languages, depending on the implementation) can define additional types. Future versions of Python may add types to the type hierarchy (e.g., rational numbers, efficiently stored arrays of integers, etc.), although such additions will often be provided via the standard library instead.
Some of the type descriptions below contain a paragraph listing âspecial attributes.â These are attributes that provide access to the implementation and are not intended for general use. Their definition may change in the future.
### 3\.2.1. None[¶](https://docs.python.org/3/reference/datamodel.html#none "Link to this heading")
This type has a single value. There is a single object with this value. This object is accessed through the built-in name `None`. It is used to signify the absence of a value in many situations, e.g., it is returned from functions that donât explicitly return anything. Its truth value is false.
### 3\.2.2. NotImplemented[¶](https://docs.python.org/3/reference/datamodel.html#notimplemented "Link to this heading")
This type has a single value. There is a single object with this value. This object is accessed through the built-in name [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented"). Numeric methods and rich comparison methods should return this value if they do not implement the operation for the operands provided. (The interpreter will then try the reflected operation, or some other fallback, depending on the operator.) It should not be evaluated in a boolean context.
See [Implementing the arithmetic operations](https://docs.python.org/3/library/numbers.html#implementing-the-arithmetic-operations) for more details.
Changed in version 3.9: Evaluating [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented") in a boolean context was deprecated.
Changed in version 3.14: Evaluating [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented") in a boolean context now raises a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError"). It previously evaluated to [`True`](https://docs.python.org/3/library/constants.html#True "True") and emitted a [`DeprecationWarning`](https://docs.python.org/3/library/exceptions.html#DeprecationWarning "DeprecationWarning") since Python 3.9.
### 3\.2.3. Ellipsis[¶](https://docs.python.org/3/reference/datamodel.html#ellipsis "Link to this heading")
This type has a single value. There is a single object with this value. This object is accessed through the literal `...` or the built-in name `Ellipsis`. Its truth value is true.
### 3\.2.4. [`numbers.Number`](https://docs.python.org/3/library/numbers.html#numbers.Number "numbers.Number")[¶](https://docs.python.org/3/reference/datamodel.html#numbers-number "Link to this heading")
These are created by numeric literals and returned as results by arithmetic operators and arithmetic built-in functions. Numeric objects are immutable; once created their value never changes. Python numbers are of course strongly related to mathematical numbers, but subject to the limitations of numerical representation in computers.
The string representations of the numeric classes, computed by [`__repr__()`](https://docs.python.org/3/reference/datamodel.html#object.__repr__ "object.__repr__") and [`__str__()`](https://docs.python.org/3/reference/datamodel.html#object.__str__ "object.__str__"), have the following properties:
- They are valid numeric literals which, when passed to their class constructor, produce an object having the value of the original numeric.
- The representation is in base 10, when possible.
- Leading zeros, possibly excepting a single zero before a decimal point, are not shown.
- Trailing zeros, possibly excepting a single zero after a decimal point, are not shown.
- A sign is shown only when the number is negative.
Python distinguishes between integers, floating-point numbers, and complex numbers:
#### 3\.2.4.1. [`numbers.Integral`](https://docs.python.org/3/library/numbers.html#numbers.Integral "numbers.Integral")[¶](https://docs.python.org/3/reference/datamodel.html#numbers-integral "Link to this heading")
These represent elements from the mathematical set of integers (positive and negative).
Note
The rules for integer representation are intended to give the most meaningful interpretation of shift and mask operations involving negative integers.
There are two types of integers:
Integers ([`int`](https://docs.python.org/3/library/functions.html#int "int"))
These represent numbers in an unlimited range, subject to available (virtual) memory only. For the purpose of shift and mask operations, a binary representation is assumed, and negative numbers are represented in a variant of 2âs complement which gives the illusion of an infinite string of sign bits extending to the left.
Booleans ([`bool`](https://docs.python.org/3/library/functions.html#bool "bool"))
These represent the truth values False and True. The two objects representing the values `False` and `True` are the only Boolean objects. The Boolean type is a subtype of the integer type, and Boolean values behave like the values 0 and 1, respectively, in almost all contexts, the exception being that when converted to a string, the strings `"False"` or `"True"` are returned, respectively.
#### 3\.2.4.2. [`numbers.Real`](https://docs.python.org/3/library/numbers.html#numbers.Real "numbers.Real") ([`float`](https://docs.python.org/3/library/functions.html#float "float"))[¶](https://docs.python.org/3/reference/datamodel.html#numbers-real-float "Link to this heading")
These represent machine-level double precision floating-point numbers. You are at the mercy of the underlying machine architecture (and C or Java implementation) for the accepted range and handling of overflow. Python does not support single-precision floating-point numbers; the savings in processor and memory usage that are usually the reason for using these are dwarfed by the overhead of using objects in Python, so there is no reason to complicate the language with two kinds of floating-point numbers.
#### 3\.2.4.3. [`numbers.Complex`](https://docs.python.org/3/library/numbers.html#numbers.Complex "numbers.Complex") ([`complex`](https://docs.python.org/3/library/functions.html#complex "complex"))[¶](https://docs.python.org/3/reference/datamodel.html#numbers-complex-complex "Link to this heading")
These represent complex numbers as a pair of machine-level double precision floating-point numbers. The same caveats apply as for floating-point numbers. The real and imaginary parts of a complex number `z` can be retrieved through the read-only attributes `z.real` and `z.imag`.
### 3\.2.5. Sequences[¶](https://docs.python.org/3/reference/datamodel.html#sequences "Link to this heading")
These represent finite ordered sets indexed by non-negative numbers. The built-in function [`len()`](https://docs.python.org/3/library/functions.html#len "len") returns the number of items of a sequence. When the length of a sequence is *n*, the index set contains the numbers 0, 1, âŠ, *n*\-1. Item *i* of sequence *a* is selected by `a[i]`. Some sequences, including built-in sequences, interpret negative subscripts by adding the sequence length. For example, `a[-2]` equals `a[n-2]`, the second to last item of sequence a with length `n`.
The resulting value must be a nonnegative integer less than the number of items in the sequence. If it is not, an [`IndexError`](https://docs.python.org/3/library/exceptions.html#IndexError "IndexError") is raised.
Sequences also support slicing: `a[start:stop]` selects all items with index *k* such that *start* `<=` *k* `<` *stop*. When used as an expression, a slice is a sequence of the same type. The comment above about negative subscripts also applies to negative slice positions. Note that no error is raised if a slice position is less than zero or larger than the length of the sequence.
If *start* is missing or [`None`](https://docs.python.org/3/library/constants.html#None "None"), slicing behaves as if *start* was zero. If *stop* is missing or `None`, slicing behaves as if *stop* was equal to the length of the sequence.
Some sequences also support âextended slicingâ with a third âstepâ parameter: `a[i:j:k]` selects all items of *a* with index *x* where `x = i + n*k`, *n* `>=` `0` and *i* `<=` *x* `<` *j*.
Sequences are distinguished according to their mutability:
#### 3\.2.5.1. Immutable sequences[¶](https://docs.python.org/3/reference/datamodel.html#immutable-sequences "Link to this heading")
An object of an immutable sequence type cannot change once it is created. (If the object contains references to other objects, these other objects may be mutable and may be changed; however, the collection of objects directly referenced by an immutable object cannot change.)
The following types are immutable sequences:
Strings
A string ([`str`](https://docs.python.org/3/library/stdtypes.html#str "str")) is a sequence of values that represent *characters*, or more formally, *Unicode code points*. All the code points in the range `0` to `0x10FFFF` can be represented in a string.
Python doesnât have a dedicated *character* type. Instead, every code point in the string is represented as a string object with length `1`.
The built-in function [`ord()`](https://docs.python.org/3/library/functions.html#ord "ord") converts a code point from its string form to an integer in the range `0` to `0x10FFFF`; [`chr()`](https://docs.python.org/3/library/functions.html#chr "chr") converts an integer in the range `0` to `0x10FFFF` to the corresponding length `1` string object. [`str.encode()`](https://docs.python.org/3/library/stdtypes.html#str.encode "str.encode") can be used to convert a [`str`](https://docs.python.org/3/library/stdtypes.html#str "str") to [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") using the given text encoding, and [`bytes.decode()`](https://docs.python.org/3/library/stdtypes.html#bytes.decode "bytes.decode") can be used to achieve the opposite.
Tuples
The items of a [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") are arbitrary Python objects. Tuples of two or more items are formed by comma-separated lists of expressions. A tuple of one item (a âsingletonâ) can be formed by affixing a comma to an expression (an expression by itself does not create a tuple, since parentheses must be usable for grouping of expressions). An empty tuple can be formed by an empty pair of parentheses.
Bytes
A [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") object is an immutable array. The items are 8-bit bytes, represented by integers in the range 0 \<= x \< 256. Bytes literals (like `b'abc'`) and the built-in [`bytes()`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") constructor can be used to create bytes objects. Also, bytes objects can be decoded to strings via the [`decode()`](https://docs.python.org/3/library/stdtypes.html#bytes.decode "bytes.decode") method.
#### 3\.2.5.2. Mutable sequences[¶](https://docs.python.org/3/reference/datamodel.html#mutable-sequences "Link to this heading")
Mutable sequences can be changed after they are created. The subscription and slicing notations can be used as the target of assignment and [`del`](https://docs.python.org/3/reference/simple_stmts.html#del) (delete) statements.
Note
The [`collections`](https://docs.python.org/3/library/collections.html#module-collections "collections: Container datatypes") and [`array`](https://docs.python.org/3/library/array.html#module-array "array: Space efficient arrays of uniformly typed numeric values.") module provide additional examples of mutable sequence types.
There are currently two intrinsic mutable sequence types:
Lists
The items of a list are arbitrary Python objects. Lists are formed by placing a comma-separated list of expressions in square brackets. (Note that there are no special cases needed to form lists of length 0 or 1.)
Byte Arrays
A bytearray object is a mutable array. They are created by the built-in [`bytearray()`](https://docs.python.org/3/library/stdtypes.html#bytearray "bytearray") constructor. Aside from being mutable (and hence unhashable), byte arrays otherwise provide the same interface and functionality as immutable [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") objects.
### 3\.2.6. Set types[¶](https://docs.python.org/3/reference/datamodel.html#set-types "Link to this heading")
These represent unordered, finite sets of unique, immutable objects. As such, they cannot be indexed by any subscript. However, they can be iterated over, and the built-in function [`len()`](https://docs.python.org/3/library/functions.html#len "len") returns the number of items in a set. Common uses for sets are fast membership testing, removing duplicates from a sequence, and computing mathematical operations such as intersection, union, difference, and symmetric difference.
For set elements, the same immutability rules apply as for dictionary keys. Note that numeric types obey the normal rules for numeric comparison: if two numbers compare equal (e.g., `1` and `1.0`), only one of them can be contained in a set.
There are currently two intrinsic set types:
Sets
These represent a mutable set. They are created by the built-in [`set()`](https://docs.python.org/3/library/stdtypes.html#set "set") constructor and can be modified afterwards by several methods, such as [`add()`](https://docs.python.org/3/library/stdtypes.html#set.add "set.add").
Frozen sets
These represent an immutable set. They are created by the built-in [`frozenset()`](https://docs.python.org/3/library/stdtypes.html#frozenset "frozenset") constructor. As a frozenset is immutable and [hashable](https://docs.python.org/3/glossary.html#term-hashable), it can be used again as an element of another set, or as a dictionary key.
### 3\.2.7. Mappings[¶](https://docs.python.org/3/reference/datamodel.html#mappings "Link to this heading")
These represent finite sets of objects indexed by arbitrary index sets. The subscript notation `a[k]` selects the item indexed by `k` from the mapping `a`; this can be used in expressions and as the target of assignments or [`del`](https://docs.python.org/3/reference/simple_stmts.html#del) statements. The built-in function [`len()`](https://docs.python.org/3/library/functions.html#len "len") returns the number of items in a mapping.
There is currently a single intrinsic mapping type:
#### 3\.2.7.1. Dictionaries[¶](https://docs.python.org/3/reference/datamodel.html#dictionaries "Link to this heading")
These represent finite sets of objects indexed by nearly arbitrary values. The only types of values not acceptable as keys are values containing lists or dictionaries or other mutable types that are compared by value rather than by object identity, the reason being that the efficient implementation of dictionaries requires a keyâs hash value to remain constant. Numeric types used for keys obey the normal rules for numeric comparison: if two numbers compare equal (e.g., `1` and `1.0`) then they can be used interchangeably to index the same dictionary entry.
Dictionaries preserve insertion order, meaning that keys will be produced in the same order they were added sequentially over the dictionary. Replacing an existing key does not change the order, however removing a key and re-inserting it will add it to the end instead of keeping its old place.
Dictionaries are mutable; they can be created by the `{}` notation (see section [Dictionary displays](https://docs.python.org/3/reference/expressions.html#dict)).
The extension modules [`dbm.ndbm`](https://docs.python.org/3/library/dbm.html#module-dbm.ndbm "dbm.ndbm: The New Database Manager") and [`dbm.gnu`](https://docs.python.org/3/library/dbm.html#module-dbm.gnu "dbm.gnu: GNU database manager") provide additional examples of mapping types, as does the [`collections`](https://docs.python.org/3/library/collections.html#module-collections "collections: Container datatypes") module.
Changed in version 3.7: Dictionaries did not preserve insertion order in versions of Python before 3.6. In CPython 3.6, insertion order was preserved, but it was considered an implementation detail at that time rather than a language guarantee.
### 3\.2.8. Callable types[¶](https://docs.python.org/3/reference/datamodel.html#callable-types "Link to this heading")
These are the types to which the function call operation (see section [Calls](https://docs.python.org/3/reference/expressions.html#calls)) can be applied:
#### 3\.2.8.1. User-defined functions[¶](https://docs.python.org/3/reference/datamodel.html#user-defined-functions "Link to this heading")
A user-defined function object is created by a function definition (see section [Function definitions](https://docs.python.org/3/reference/compound_stmts.html#function)). It should be called with an argument list containing the same number of items as the functionâs formal parameter list.
##### 3\.2.8.1.1. Special read-only attributes[¶](https://docs.python.org/3/reference/datamodel.html#special-read-only-attributes "Link to this heading")
| Attribute | Meaning |
|---|---|
| function.\_\_builtins\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__builtins__ "Link to this definition") | A reference to the [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") that holds the functionâs builtins namespace. Added in version 3.10. |
| function.\_\_globals\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__globals__ "Link to this definition") | A reference to the [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") that holds the functionâs [global variables](https://docs.python.org/3/reference/executionmodel.html#naming) â the global namespace of the module in which the function was defined. |
| function.\_\_closure\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__closure__ "Link to this definition") | `None` or a [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") of cells that contain bindings for the names specified in the [`co_freevars`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_freevars "codeobject.co_freevars") attribute of the functionâs [`code object`](https://docs.python.org/3/reference/datamodel.html#function.__code__ "function.__code__"). A cell object has the attribute `cell_contents`. This can be used to get the value of the cell, as well as set the value. |
##### 3\.2.8.1.2. Special writable attributes[¶](https://docs.python.org/3/reference/datamodel.html#special-writable-attributes "Link to this heading")
Most of these attributes check the type of the assigned value:
| Attribute | Meaning |
|---|---|
| function.\_\_doc\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__doc__ "Link to this definition") | The functionâs documentation string, or `None` if unavailable. |
| function.\_\_name\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__name__ "Link to this definition") | The functionâs name. See also: [`__name__ attributes`](https://docs.python.org/3/library/stdtypes.html#definition.__name__ "definition.__name__"). |
| function.\_\_qualname\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__qualname__ "Link to this definition") | The functionâs [qualified name](https://docs.python.org/3/glossary.html#term-qualified-name). See also: [`__qualname__ attributes`](https://docs.python.org/3/library/stdtypes.html#definition.__qualname__ "definition.__qualname__"). Added in version 3.3. |
| function.\_\_module\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__module__ "Link to this definition") | The name of the module the function was defined in, or `None` if unavailable. |
| function.\_\_defaults\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__defaults__ "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing default [parameter](https://docs.python.org/3/glossary.html#term-parameter) values for those parameters that have defaults, or `None` if no parameters have a default value. |
| function.\_\_code\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__code__ "Link to this definition") | The [code object](https://docs.python.org/3/reference/datamodel.html#code-objects) representing the compiled function body. |
| function.\_\_dict\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__dict__ "Link to this definition") | The namespace supporting arbitrary function attributes. See also: [`__dict__ attributes`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__"). |
| function.\_\_annotations\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__annotations__ "Link to this definition") | A [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") containing annotations of [parameters](https://docs.python.org/3/glossary.html#term-parameter). The keys of the dictionary are the parameter names, and `'return'` for the return annotation, if provided. See also: [`object.__annotations__`](https://docs.python.org/3/reference/datamodel.html#object.__annotations__ "object.__annotations__"). Changed in version 3.14: Annotations are now [lazily evaluated](https://docs.python.org/3/reference/executionmodel.html#lazy-evaluation). See [**PEP 649**](https://peps.python.org/pep-0649/). |
| function.\_\_annotate\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__annotate__ "Link to this definition") | The [annotate function](https://docs.python.org/3/glossary.html#term-annotate-function) for this function, or `None` if the function has no annotations. See [`object.__annotate__`](https://docs.python.org/3/reference/datamodel.html#object.__annotate__ "object.__annotate__"). Added in version 3.14. |
| function.\_\_kwdefaults\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__kwdefaults__ "Link to this definition") | A [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") containing defaults for keyword-only [parameters](https://docs.python.org/3/glossary.html#term-parameter). |
| function.\_\_type\_params\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__type_params__ "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the [type parameters](https://docs.python.org/3/reference/compound_stmts.html#type-params) of a [generic function](https://docs.python.org/3/reference/compound_stmts.html#generic-functions). Added in version 3.12. |
Function objects also support getting and setting arbitrary attributes, which can be used, for example, to attach metadata to functions. Regular attribute dot-notation is used to get and set such attributes.
**CPython implementation detail:** CPythonâs current implementation only supports function attributes on user-defined functions. Function attributes on [built-in functions](https://docs.python.org/3/reference/datamodel.html#builtin-functions) may be supported in the future.
Additional information about a functionâs definition can be retrieved from its [code object](https://docs.python.org/3/reference/datamodel.html#code-objects) (accessible via the [`__code__`](https://docs.python.org/3/reference/datamodel.html#function.__code__ "function.__code__") attribute).
#### 3\.2.8.2. Instance methods[¶](https://docs.python.org/3/reference/datamodel.html#instance-methods "Link to this heading")
An instance method object combines a class, a class instance and any callable object (normally a user-defined function).
Special read-only attributes:
| | |
|---|---|
| method.\_\_self\_\_[¶](https://docs.python.org/3/reference/datamodel.html#method.__self__ "Link to this definition") | Refers to the class instance object to which the method is [bound](https://docs.python.org/3/reference/datamodel.html#method-binding) |
| method.\_\_func\_\_[¶](https://docs.python.org/3/reference/datamodel.html#method.__func__ "Link to this definition") | Refers to the original [function object](https://docs.python.org/3/reference/datamodel.html#user-defined-funcs) |
| method.\_\_doc\_\_[¶](https://docs.python.org/3/reference/datamodel.html#method.__doc__ "Link to this definition") | The methodâs documentation (same as [`method.__func__.__doc__`](https://docs.python.org/3/reference/datamodel.html#function.__doc__ "function.__doc__")). A [`string`](https://docs.python.org/3/library/stdtypes.html#str "str") if the original function had a docstring, else `None`. |
| method.\_\_name\_\_[¶](https://docs.python.org/3/reference/datamodel.html#method.__name__ "Link to this definition") | The name of the method (same as [`method.__func__.__name__`](https://docs.python.org/3/reference/datamodel.html#function.__name__ "function.__name__")) |
| method.\_\_module\_\_[¶](https://docs.python.org/3/reference/datamodel.html#method.__module__ "Link to this definition") | The name of the module the method was defined in, or `None` if unavailable. |
Methods also support accessing (but not setting) the arbitrary function attributes on the underlying [function object](https://docs.python.org/3/reference/datamodel.html#user-defined-funcs).
User-defined method objects may be created when getting an attribute of a class (perhaps via an instance of that class), if that attribute is a user-defined [function object](https://docs.python.org/3/reference/datamodel.html#user-defined-funcs) or a [`classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod") object.
When an instance method object is created by retrieving a user-defined [function object](https://docs.python.org/3/reference/datamodel.html#user-defined-funcs) from a class via one of its instances, its [`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__") attribute is the instance, and the method object is said to be *bound*. The new methodâs [`__func__`](https://docs.python.org/3/reference/datamodel.html#method.__func__ "method.__func__") attribute is the original function object.
When an instance method object is created by retrieving a [`classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod") object from a class or instance, its [`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__") attribute is the class itself, and its [`__func__`](https://docs.python.org/3/reference/datamodel.html#method.__func__ "method.__func__") attribute is the function object underlying the class method.
When an instance method object is called, the underlying function ([`__func__`](https://docs.python.org/3/reference/datamodel.html#method.__func__ "method.__func__")) is called, inserting the class instance ([`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__")) in front of the argument list. For instance, when `C` is a class which contains a definition for a function `f()`, and `x` is an instance of `C`, calling `x.f(1)` is equivalent to calling `C.f(x, 1)`.
When an instance method object is derived from a [`classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod") object, the âclass instanceâ stored in [`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__") will actually be the class itself, so that calling either `x.f(1)` or `C.f(1)` is equivalent to calling `f(C,1)` where `f` is the underlying function.
It is important to note that user-defined functions which are attributes of a class instance are not converted to bound methods; this *only* happens when the function is an attribute of the class.
#### 3\.2.8.3. Generator functions[¶](https://docs.python.org/3/reference/datamodel.html#generator-functions "Link to this heading")
A function or method which uses the [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) statement (see section [The yield statement](https://docs.python.org/3/reference/simple_stmts.html#yield)) is called a *generator function*. Such a function, when called, always returns an [iterator](https://docs.python.org/3/glossary.html#term-iterator) object which can be used to execute the body of the function: calling the iteratorâs [`iterator.__next__()`](https://docs.python.org/3/library/stdtypes.html#iterator.__next__ "iterator.__next__") method will cause the function to execute until it provides a value using the `yield` statement. When the function executes a [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement or falls off the end, a [`StopIteration`](https://docs.python.org/3/library/exceptions.html#StopIteration "StopIteration") exception is raised and the iterator will have reached the end of the set of values to be returned.
#### 3\.2.8.4. Coroutine functions[¶](https://docs.python.org/3/reference/datamodel.html#coroutine-functions "Link to this heading")
A function or method which is defined using [`async def`](https://docs.python.org/3/reference/compound_stmts.html#async-def) is called a *coroutine function*. Such a function, when called, returns a [coroutine](https://docs.python.org/3/glossary.html#term-coroutine) object. It may contain [`await`](https://docs.python.org/3/reference/expressions.html#await) expressions, as well as [`async with`](https://docs.python.org/3/reference/compound_stmts.html#async-with) and [`async for`](https://docs.python.org/3/reference/compound_stmts.html#async-for) statements. See also the [Coroutine Objects](https://docs.python.org/3/reference/datamodel.html#coroutine-objects) section.
#### 3\.2.8.5. Asynchronous generator functions[¶](https://docs.python.org/3/reference/datamodel.html#asynchronous-generator-functions "Link to this heading")
A function or method which is defined using [`async def`](https://docs.python.org/3/reference/compound_stmts.html#async-def) and which uses the [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) statement is called a *asynchronous generator function*. Such a function, when called, returns an [asynchronous iterator](https://docs.python.org/3/glossary.html#term-asynchronous-iterator) object which can be used in an [`async for`](https://docs.python.org/3/reference/compound_stmts.html#async-for) statement to execute the body of the function.
Calling the asynchronous iteratorâs [`aiterator.__anext__`](https://docs.python.org/3/reference/datamodel.html#object.__anext__ "object.__anext__") method will return an [awaitable](https://docs.python.org/3/glossary.html#term-awaitable) which when awaited will execute until it provides a value using the [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) expression. When the function executes an empty [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement or falls off the end, a [`StopAsyncIteration`](https://docs.python.org/3/library/exceptions.html#StopAsyncIteration "StopAsyncIteration") exception is raised and the asynchronous iterator will have reached the end of the set of values to be yielded.
#### 3\.2.8.6. Built-in functions[¶](https://docs.python.org/3/reference/datamodel.html#built-in-functions "Link to this heading")
A built-in function object is a wrapper around a C function. Examples of built-in functions are [`len()`](https://docs.python.org/3/library/functions.html#len "len") and [`math.sin()`](https://docs.python.org/3/library/math.html#math.sin "math.sin") ([`math`](https://docs.python.org/3/library/math.html#module-math "math: Mathematical functions (sin() etc.).") is a standard built-in module). The number and type of the arguments are determined by the C function. Special read-only attributes:
- `__doc__` is the functionâs documentation string, or `None` if unavailable. See [`function.__doc__`](https://docs.python.org/3/reference/datamodel.html#function.__doc__ "function.__doc__").
- `__name__` is the functionâs name. See [`function.__name__`](https://docs.python.org/3/reference/datamodel.html#function.__name__ "function.__name__").
- `__self__` is set to `None` (but see the next item).
- `__module__` is the name of the module the function was defined in or `None` if unavailable. See [`function.__module__`](https://docs.python.org/3/reference/datamodel.html#function.__module__ "function.__module__").
#### 3\.2.8.7. Built-in methods[¶](https://docs.python.org/3/reference/datamodel.html#built-in-methods "Link to this heading")
This is really a different disguise of a built-in function, this time containing an object passed to the C function as an implicit extra argument. An example of a built-in method is `alist.append()`, assuming *alist* is a list object. In this case, the special read-only attribute `__self__` is set to the object denoted by *alist*. (The attribute has the same semantics as it does with [`other instance methods`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__").)
#### 3\.2.8.8. Classes[¶](https://docs.python.org/3/reference/datamodel.html#classes "Link to this heading")
Classes are callable. These objects normally act as factories for new instances of themselves, but variations are possible for class types that override [`__new__()`](https://docs.python.org/3/reference/datamodel.html#object.__new__ "object.__new__"). The arguments of the call are passed to `__new__()` and, in the typical case, to [`__init__()`](https://docs.python.org/3/reference/datamodel.html#object.__init__ "object.__init__") to initialize the new instance.
#### 3\.2.8.9. Class Instances[¶](https://docs.python.org/3/reference/datamodel.html#class-instances "Link to this heading")
Instances of arbitrary classes can be made callable by defining a [`__call__()`](https://docs.python.org/3/reference/datamodel.html#object.__call__ "object.__call__") method in their class.
### 3\.2.9. Modules[¶](https://docs.python.org/3/reference/datamodel.html#modules "Link to this heading")
Modules are a basic organizational unit of Python code, and are created by the [import system](https://docs.python.org/3/reference/import.html#importsystem) as invoked either by the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement, or by calling functions such as [`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module") and built-in [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__"). A module object has a namespace implemented by a [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") object (this is the dictionary referenced by the [`__globals__`](https://docs.python.org/3/reference/datamodel.html#function.__globals__ "function.__globals__") attribute of functions defined in the module). Attribute references are translated to lookups in this dictionary, e.g., `m.x` is equivalent to `m.__dict__["x"]`. A module object does not contain the code object used to initialize the module (since it isnât needed once the initialization is done).
Attribute assignment updates the moduleâs namespace dictionary, e.g., `m.x = 1` is equivalent to `m.__dict__["x"] = 1`.
#### 3\.2.9.1. Import-related attributes on module objects[¶](https://docs.python.org/3/reference/datamodel.html#import-related-attributes-on-module-objects "Link to this heading")
Module objects have the following attributes that relate to the [import system](https://docs.python.org/3/reference/import.html#importsystem). When a module is created using the machinery associated with the import system, these attributes are filled in based on the moduleâs [spec](https://docs.python.org/3/glossary.html#term-module-spec), before the [loader](https://docs.python.org/3/glossary.html#term-loader) executes and loads the module.
To create a module dynamically rather than using the import system, itâs recommended to use [`importlib.util.module_from_spec()`](https://docs.python.org/3/library/importlib.html#importlib.util.module_from_spec "importlib.util.module_from_spec"), which will set the various import-controlled attributes to appropriate values. Itâs also possible to use the [`types.ModuleType`](https://docs.python.org/3/library/types.html#types.ModuleType "types.ModuleType") constructor to create modules directly, but this technique is more error-prone, as most attributes must be manually set on the module object after it has been created when using this approach.
Caution
With the exception of [`__name__`](https://docs.python.org/3/reference/datamodel.html#module.__name__ "module.__name__"), it is **strongly** recommended that you rely on [`__spec__`](https://docs.python.org/3/reference/datamodel.html#module.__spec__ "module.__spec__") and its attributes instead of any of the other individual attributes listed in this subsection. Note that updating an attribute on `__spec__` will not update the corresponding attribute on the module itself:
Copy
```
>>> import typing
>>> typing.__name__, typing.__spec__.name
('typing', 'typing')
>>> typing.__spec__.name = 'spelling'
>>> typing.__name__, typing.__spec__.name
('typing', 'spelling')
>>> typing.__name__ = 'keyboard_smashing'
>>> typing.__name__, typing.__spec__.name
('keyboard_smashing', 'spelling')
```
module.\_\_name\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__name__ "Link to this definition")
The name used to uniquely identify the module in the import system. For a directly executed module, this will be set to `"__main__"`.
This attribute must be set to the fully qualified name of the module. It is expected to match the value of [`module.__spec__.name`](https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec.name "importlib.machinery.ModuleSpec.name").
module.\_\_spec\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__spec__ "Link to this definition")
A record of the moduleâs import-system-related state.
Set to the [`module spec`](https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec "importlib.machinery.ModuleSpec") that was used when importing the module. See [Module specs](https://docs.python.org/3/reference/import.html#module-specs) for more details.
Added in version 3.4.
module.\_\_package\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__package__ "Link to this definition")
The [package](https://docs.python.org/3/glossary.html#term-package) a module belongs to.
If the module is top-level (that is, not a part of any specific package) then the attribute should be set to `''` (the empty string). Otherwise, it should be set to the name of the moduleâs package (which can be equal to [`module.__name__`](https://docs.python.org/3/reference/datamodel.html#module.__name__ "module.__name__") if the module itself is a package). See [**PEP 366**](https://peps.python.org/pep-0366/) for further details.
This attribute is used instead of [`__name__`](https://docs.python.org/3/reference/datamodel.html#module.__name__ "module.__name__") to calculate explicit relative imports for main modules. It defaults to `None` for modules created dynamically using the [`types.ModuleType`](https://docs.python.org/3/library/types.html#types.ModuleType "types.ModuleType") constructor; use [`importlib.util.module_from_spec()`](https://docs.python.org/3/library/importlib.html#importlib.util.module_from_spec "importlib.util.module_from_spec") instead to ensure the attribute is set to a [`str`](https://docs.python.org/3/library/stdtypes.html#str "str").
It is **strongly** recommended that you use [`module.__spec__.parent`](https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec.parent "importlib.machinery.ModuleSpec.parent") instead of `module.__package__`. [`__package__`](https://docs.python.org/3/reference/datamodel.html#module.__package__ "module.__package__") is now only used as a fallback if `__spec__.parent` is not set, and this fallback path is deprecated.
Changed in version 3.4: This attribute now defaults to `None` for modules created dynamically using the [`types.ModuleType`](https://docs.python.org/3/library/types.html#types.ModuleType "types.ModuleType") constructor. Previously the attribute was optional.
Changed in version 3.6: The value of `__package__` is expected to be the same as [`__spec__.parent`](https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec.parent "importlib.machinery.ModuleSpec.parent"). [`__package__`](https://docs.python.org/3/reference/datamodel.html#module.__package__ "module.__package__") is now only used as a fallback during import resolution if `__spec__.parent` is not defined.
Changed in version 3.10: [`ImportWarning`](https://docs.python.org/3/library/exceptions.html#ImportWarning "ImportWarning") is raised if an import resolution falls back to `__package__` instead of [`__spec__.parent`](https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec.parent "importlib.machinery.ModuleSpec.parent").
Changed in version 3.12: Raise [`DeprecationWarning`](https://docs.python.org/3/library/exceptions.html#DeprecationWarning "DeprecationWarning") instead of [`ImportWarning`](https://docs.python.org/3/library/exceptions.html#ImportWarning "ImportWarning") when falling back to `__package__` during import resolution.
Deprecated since version 3.13, will be removed in version 3.15: `__package__` will cease to be set or taken into consideration by the import system or standard library.
module.\_\_loader\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__loader__ "Link to this definition")
The [loader](https://docs.python.org/3/glossary.html#term-loader) object that the import machinery used to load the module.
This attribute is mostly useful for introspection, but can be used for additional loader-specific functionality, for example getting data associated with a loader.
`__loader__` defaults to `None` for modules created dynamically using the [`types.ModuleType`](https://docs.python.org/3/library/types.html#types.ModuleType "types.ModuleType") constructor; use [`importlib.util.module_from_spec()`](https://docs.python.org/3/library/importlib.html#importlib.util.module_from_spec "importlib.util.module_from_spec") instead to ensure the attribute is set to a [loader](https://docs.python.org/3/glossary.html#term-loader) object.
It is **strongly** recommended that you use [`module.__spec__.loader`](https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec.loader "importlib.machinery.ModuleSpec.loader") instead of `module.__loader__`.
Changed in version 3.4: This attribute now defaults to `None` for modules created dynamically using the [`types.ModuleType`](https://docs.python.org/3/library/types.html#types.ModuleType "types.ModuleType") constructor. Previously the attribute was optional.
Deprecated since version 3.12, will be removed in version 3.16: Setting `__loader__` on a module while failing to set `__spec__.loader` is deprecated. In Python 3.16, `__loader__` will cease to be set or taken into consideration by the import system or the standard library.
module.\_\_path\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__path__ "Link to this definition")
A (possibly empty) [sequence](https://docs.python.org/3/glossary.html#term-sequence) of strings enumerating the locations where the packageâs submodules will be found. Non-package modules should not have a `__path__` attribute. See [\_\_path\_\_ attributes on modules](https://docs.python.org/3/reference/import.html#package-path-rules) for more details.
It is **strongly** recommended that you use [`module.__spec__.submodule_search_locations`](https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec.submodule_search_locations "importlib.machinery.ModuleSpec.submodule_search_locations") instead of `module.__path__`.
module.\_\_file\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__file__ "Link to this definition")
module.\_\_cached\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__cached__ "Link to this definition")
`__file__` and `__cached__` are both optional attributes that may or may not be set. Both attributes should be a [`str`](https://docs.python.org/3/library/stdtypes.html#str "str") when they are available.
`__file__` indicates the pathname of the file from which the module was loaded (if loaded from a file), or the pathname of the shared library file for extension modules loaded dynamically from a shared library. It might be missing for certain types of modules, such as C modules that are statically linked into the interpreter, and the [import system](https://docs.python.org/3/reference/import.html#importsystem) may opt to leave it unset if it has no semantic meaning (for example, a module loaded from a database).
If `__file__` is set then the `__cached__` attribute might also be set, which is the path to any compiled version of the code (for example, a byte-compiled file). The file does not need to exist to set this attribute; the path can simply point to where the compiled file *would* exist (see [**PEP 3147**](https://peps.python.org/pep-3147/)).
Note that `__cached__` may be set even if `__file__` is not set. However, that scenario is quite atypical. Ultimately, the [loader](https://docs.python.org/3/glossary.html#term-loader) is what makes use of the module spec provided by the [finder](https://docs.python.org/3/glossary.html#term-finder) (from which `__file__` and `__cached__` are derived). So if a loader can load from a cached module but otherwise does not load from a file, that atypical scenario may be appropriate.
It is **strongly** recommended that you use [`module.__spec__.cached`](https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec.cached "importlib.machinery.ModuleSpec.cached") instead of `module.__cached__`.
Deprecated since version 3.13, will be removed in version 3.15: Setting `__cached__` on a module while failing to set `__spec__.cached` is deprecated. In Python 3.15, `__cached__` will cease to be set or taken into consideration by the import system or standard library.
#### 3\.2.9.2. Other writable attributes on module objects[¶](https://docs.python.org/3/reference/datamodel.html#other-writable-attributes-on-module-objects "Link to this heading")
As well as the import-related attributes listed above, module objects also have the following writable attributes:
module.\_\_doc\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__doc__ "Link to this definition")
The moduleâs documentation string, or `None` if unavailable. See also: [`__doc__ attributes`](https://docs.python.org/3/library/stdtypes.html#definition.__doc__ "definition.__doc__").
module.\_\_annotations\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__annotations__ "Link to this definition")
A dictionary containing [variable annotations](https://docs.python.org/3/glossary.html#term-variable-annotation) collected during module body execution. For best practices on working with `__annotations__`, see [`annotationlib`](https://docs.python.org/3/library/annotationlib.html#module-annotationlib "annotationlib: Functionality for introspecting annotations").
Changed in version 3.14: Annotations are now [lazily evaluated](https://docs.python.org/3/reference/executionmodel.html#lazy-evaluation). See [**PEP 649**](https://peps.python.org/pep-0649/).
module.\_\_annotate\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__annotate__ "Link to this definition")
The [annotate function](https://docs.python.org/3/glossary.html#term-annotate-function) for this module, or `None` if the module has no annotations. See also: [`__annotate__`](https://docs.python.org/3/reference/datamodel.html#object.__annotate__ "object.__annotate__") attributes.
Added in version 3.14.
#### 3\.2.9.3. Module dictionaries[¶](https://docs.python.org/3/reference/datamodel.html#module-dictionaries "Link to this heading")
Module objects also have the following special read-only attribute:
module.\_\_dict\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__dict__ "Link to this definition")
The moduleâs namespace as a dictionary object. Uniquely among the attributes listed here, `__dict__` cannot be accessed as a global variable from within a module; it can only be accessed as an attribute on module objects.
**CPython implementation detail:** Because of the way CPython clears module dictionaries, the module dictionary will be cleared when the module falls out of scope even if the dictionary still has live references. To avoid this, copy the dictionary or keep the module around while using its dictionary directly.
### 3\.2.10. Custom classes[¶](https://docs.python.org/3/reference/datamodel.html#custom-classes "Link to this heading")
Custom class types are typically created by class definitions (see section [Class definitions](https://docs.python.org/3/reference/compound_stmts.html#class)). A class has a namespace implemented by a dictionary object. Class attribute references are translated to lookups in this dictionary, e.g., `C.x` is translated to `C.__dict__["x"]` (although there are a number of hooks which allow for other means of locating attributes). When the attribute name is not found there, the attribute search continues in the base classes. This search of the base classes uses the C3 method resolution order which behaves correctly even in the presence of âdiamondâ inheritance structures where there are multiple inheritance paths leading back to a common ancestor. Additional details on the C3 MRO used by Python can be found at [The Python 2.3 Method Resolution Order](https://docs.python.org/3/howto/mro.html#python-2-3-mro).
When a class attribute reference (for class `C`, say) would yield a class method object, it is transformed into an instance method object whose [`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__") attribute is `C`. When it would yield a [`staticmethod`](https://docs.python.org/3/library/functions.html#staticmethod "staticmethod") object, it is transformed into the object wrapped by the static method object. See section [Implementing Descriptors](https://docs.python.org/3/reference/datamodel.html#descriptors) for another way in which attributes retrieved from a class may differ from those actually contained in its [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__").
Class attribute assignments update the classâs dictionary, never the dictionary of a base class.
A class object can be called (see above) to yield a class instance (see below).
#### 3\.2.10.1. Special attributes[¶](https://docs.python.org/3/reference/datamodel.html#special-attributes "Link to this heading")
| Attribute | Meaning |
|---|---|
| type.\_\_name\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__name__ "Link to this definition") | The classâs name. See also: [`__name__ attributes`](https://docs.python.org/3/library/stdtypes.html#definition.__name__ "definition.__name__"). |
| type.\_\_qualname\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__qualname__ "Link to this definition") | The classâs [qualified name](https://docs.python.org/3/glossary.html#term-qualified-name). See also: [`__qualname__ attributes`](https://docs.python.org/3/library/stdtypes.html#definition.__qualname__ "definition.__qualname__"). |
| type.\_\_module\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__module__ "Link to this definition") | The name of the module in which the class was defined. |
| type.\_\_dict\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__dict__ "Link to this definition") | A [`mapping proxy`](https://docs.python.org/3/library/types.html#types.MappingProxyType "types.MappingProxyType") providing a read-only view of the classâs namespace. See also: [`__dict__ attributes`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__"). |
| type.\_\_bases\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__bases__ "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the classâs bases. In most cases, for a class defined as `class X(A, B, C)`, `X.__bases__` will be exactly equal to `(A, B, C)`. |
| type.\_\_base\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__base__ "Link to this definition") | **CPython implementation detail:** The single base class in the inheritance chain that is responsible for the memory layout of instances. This attribute corresponds to [`tp_base`](https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_base "PyTypeObject.tp_base") at the C level. |
| type.\_\_doc\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__doc__ "Link to this definition") | The classâs documentation string, or `None` if undefined. Not inherited by subclasses. |
| type.\_\_annotations\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__annotations__ "Link to this definition") | A dictionary containing [variable annotations](https://docs.python.org/3/glossary.html#term-variable-annotation) collected during class body execution. See also: [`__annotations__ attributes`](https://docs.python.org/3/reference/datamodel.html#object.__annotations__ "object.__annotations__"). For best practices on working with [`__annotations__`](https://docs.python.org/3/reference/datamodel.html#object.__annotations__ "object.__annotations__"), please see [`annotationlib`](https://docs.python.org/3/library/annotationlib.html#module-annotationlib "annotationlib: Functionality for introspecting annotations"). Use [`annotationlib.get_annotations()`](https://docs.python.org/3/library/annotationlib.html#annotationlib.get_annotations "annotationlib.get_annotations") instead of accessing this attribute directly. Warning Accessing the `__annotations__` attribute directly on a class object may return annotations for the wrong class, specifically in certain cases where the class, its base class, or a metaclass is defined under `from __future__ import annotations`. See [**749**](https://peps.python.org/pep-0749/#pep749-metaclasses) for details. This attribute does not exist on certain builtin classes. On user-defined classes without `__annotations__`, it is an empty dictionary. Changed in version 3.14: Annotations are now [lazily evaluated](https://docs.python.org/3/reference/executionmodel.html#lazy-evaluation). See [**PEP 649**](https://peps.python.org/pep-0649/). |
| type.\_\_annotate\_\_()[¶](https://docs.python.org/3/reference/datamodel.html#type.__annotate__ "Link to this definition") | The [annotate function](https://docs.python.org/3/glossary.html#term-annotate-function) for this class, or `None` if the class has no annotations. See also: [`__annotate__ attributes`](https://docs.python.org/3/reference/datamodel.html#object.__annotate__ "object.__annotate__"). Added in version 3.14. |
| type.\_\_type\_params\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__type_params__ "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the [type parameters](https://docs.python.org/3/reference/compound_stmts.html#type-params) of a [generic class](https://docs.python.org/3/reference/compound_stmts.html#generic-classes). Added in version 3.12. |
| type.\_\_static\_attributes\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__static_attributes__ "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing names of attributes of this class which are assigned through `self.X` from any function in its body. Added in version 3.13. |
| type.\_\_firstlineno\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__firstlineno__ "Link to this definition") | The line number of the first line of the class definition, including decorators. Setting the [`__module__`](https://docs.python.org/3/reference/datamodel.html#type.__module__ "type.__module__") attribute removes the `__firstlineno__` item from the typeâs dictionary. Added in version 3.13. |
| type.\_\_mro\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__mro__ "Link to this definition") | The [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") of classes that are considered when looking for base classes during method resolution. |
#### 3\.2.10.2. Special methods[¶](https://docs.python.org/3/reference/datamodel.html#special-methods "Link to this heading")
In addition to the special attributes described above, all Python classes also have the following two methods available:
type.mro()[¶](https://docs.python.org/3/reference/datamodel.html#type.mro "Link to this definition")
This method can be overridden by a metaclass to customize the method resolution order for its instances. It is called at class instantiation, and its result is stored in [`__mro__`](https://docs.python.org/3/reference/datamodel.html#type.__mro__ "type.__mro__").
type.\_\_subclasses\_\_()[¶](https://docs.python.org/3/reference/datamodel.html#type.__subclasses__ "Link to this definition")
Each class keeps a list of weak references to its immediate subclasses. This method returns a list of all those references still alive. The list is in definition order. Example:
Copy
```
>>> class A: pass
>>> class B(A): pass
>>> A.__subclasses__()
[<class 'B'>]
```
### 3\.2.11. Class instances[¶](https://docs.python.org/3/reference/datamodel.html#id4 "Link to this heading")
A class instance is created by calling a class object (see above). A class instance has a namespace implemented as a dictionary which is the first place in which attribute references are searched. When an attribute is not found there, and the instanceâs class has an attribute by that name, the search continues with the class attributes. If a class attribute is found that is a user-defined function object, it is transformed into an instance method object whose [`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__") attribute is the instance. Static method and class method objects are also transformed; see above under âClassesâ. See section [Implementing Descriptors](https://docs.python.org/3/reference/datamodel.html#descriptors) for another way in which attributes of a class retrieved via its instances may differ from the objects actually stored in the classâs [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__"). If no class attribute is found, and the objectâs class has a [`__getattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattr__ "object.__getattr__") method, that is called to satisfy the lookup.
Attribute assignments and deletions update the instanceâs dictionary, never a classâs dictionary. If the class has a [`__setattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__setattr__ "object.__setattr__") or [`__delattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__delattr__ "object.__delattr__") method, this is called instead of updating the instance dictionary directly.
Class instances can pretend to be numbers, sequences, or mappings if they have methods with certain special names. See section [Special method names](https://docs.python.org/3/reference/datamodel.html#specialnames).
#### 3\.2.11.1. Special attributes[¶](https://docs.python.org/3/reference/datamodel.html#id5 "Link to this heading")
object.\_\_class\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__class__ "Link to this definition")
The class to which a class instance belongs.
object.\_\_dict\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "Link to this definition")
A dictionary or other mapping object used to store an objectâs (writable) attributes. Not all instances have a `__dict__` attribute; see the section on [\_\_slots\_\_](https://docs.python.org/3/reference/datamodel.html#slots) for more details.
### 3\.2.12. I/O objects (also known as file objects)[¶](https://docs.python.org/3/reference/datamodel.html#i-o-objects-also-known-as-file-objects "Link to this heading")
A [file object](https://docs.python.org/3/glossary.html#term-file-object) represents an open file. Various shortcuts are available to create file objects: the [`open()`](https://docs.python.org/3/library/functions.html#open "open") built-in function, and also [`os.popen()`](https://docs.python.org/3/library/os.html#os.popen "os.popen"), [`os.fdopen()`](https://docs.python.org/3/library/os.html#os.fdopen "os.fdopen"), and the [`makefile()`](https://docs.python.org/3/library/socket.html#socket.socket.makefile "socket.socket.makefile") method of socket objects (and perhaps by other functions or methods provided by extension modules).
File objects implement common methods, listed below, to simplify usage in generic code. They are expected to be [With Statement Context Managers](https://docs.python.org/3/reference/datamodel.html#context-managers).
The objects `sys.stdin`, `sys.stdout` and `sys.stderr` are initialized to file objects corresponding to the interpreterâs standard input, output and error streams; they are all open in text mode and therefore follow the interface defined by the [`io.TextIOBase`](https://docs.python.org/3/library/io.html#io.TextIOBase "io.TextIOBase") abstract class.
file.read(*size\=\-1*, */*)[¶](https://docs.python.org/3/reference/datamodel.html#file.read "Link to this definition")
Retrieve up to *size* data from the file. As a convenience if *size* is unspecified or -1 retrieve all data available.
file.write(*data*, */*)[¶](https://docs.python.org/3/reference/datamodel.html#file.write "Link to this definition")
Store *data* to the file.
file.close()[¶](https://docs.python.org/3/reference/datamodel.html#file.close "Link to this definition")
Flush any buffers and close the underlying file.
### 3\.2.13. Internal types[¶](https://docs.python.org/3/reference/datamodel.html#internal-types "Link to this heading")
A few types used internally by the interpreter are exposed to the user. Their definitions may change with future versions of the interpreter, but they are mentioned here for completeness.
#### 3\.2.13.1. Code objects[¶](https://docs.python.org/3/reference/datamodel.html#code-objects "Link to this heading")
Code objects represent *byte-compiled* executable Python code, or [bytecode](https://docs.python.org/3/glossary.html#term-bytecode). The difference between a code object and a function object is that the function object contains an explicit reference to the functionâs globals (the module in which it was defined), while a code object contains no context; also the default argument values are stored in the function object, not in the code object (because they represent values calculated at run-time). Unlike function objects, code objects are immutable and contain no references (directly or indirectly) to mutable objects.
##### 3\.2.13.1.1. Special read-only attributes[¶](https://docs.python.org/3/reference/datamodel.html#index-64 "Link to this heading")
| | |
|---|---|
| codeobject.co\_name[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_name "Link to this definition") | The function name |
| codeobject.co\_qualname[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_qualname "Link to this definition") | The fully qualified function name Added in version 3.11. |
| codeobject.co\_argcount[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_argcount "Link to this definition") | The total number of positional [parameters](https://docs.python.org/3/glossary.html#term-parameter) (including positional-only parameters and parameters with default values) that the function has |
| codeobject.co\_posonlyargcount[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_posonlyargcount "Link to this definition") | The number of positional-only [parameters](https://docs.python.org/3/glossary.html#term-parameter) (including arguments with default values) that the function has |
| codeobject.co\_kwonlyargcount[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_kwonlyargcount "Link to this definition") | The number of keyword-only [parameters](https://docs.python.org/3/glossary.html#term-parameter) (including arguments with default values) that the function has |
| codeobject.co\_nlocals[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_nlocals "Link to this definition") | The number of [local variables](https://docs.python.org/3/reference/executionmodel.html#naming) used by the function (including parameters) |
| codeobject.co\_varnames[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_varnames "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the names of the local variables in the function (starting with the parameter names) |
| codeobject.co\_cellvars[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_cellvars "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the names of [local variables](https://docs.python.org/3/reference/executionmodel.html#naming) that are referenced from at least one [nested scope](https://docs.python.org/3/glossary.html#term-nested-scope) inside the function |
| codeobject.co\_freevars[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_freevars "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the names of [free (closure) variables](https://docs.python.org/3/glossary.html#term-closure-variable) that a [nested scope](https://docs.python.org/3/glossary.html#term-nested-scope) references in an outer scope. See also [`function.__closure__`](https://docs.python.org/3/reference/datamodel.html#function.__closure__ "function.__closure__"). Note: references to global and builtin names are *not* included. |
| codeobject.co\_code[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_code "Link to this definition") | A string representing the sequence of [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) instructions in the function |
| codeobject.co\_consts[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_consts "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the literals used by the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) in the function |
| codeobject.co\_names[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_names "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the names used by the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) in the function |
| codeobject.co\_filename[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_filename "Link to this definition") | The name of the file from which the code was compiled |
| codeobject.co\_firstlineno[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_firstlineno "Link to this definition") | The line number of the first line of the function |
| codeobject.co\_lnotab[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_lnotab "Link to this definition") | A string encoding the mapping from [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) offsets to line numbers. For details, see the source code of the interpreter. Deprecated since version 3.12: This attribute of code objects is deprecated, and may be removed in Python 3.15. |
| codeobject.co\_stacksize[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_stacksize "Link to this definition") | The required stack size of the code object |
| codeobject.co\_flags[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_flags "Link to this definition") | An [`integer`](https://docs.python.org/3/library/functions.html#int "int") encoding a number of flags for the interpreter. |
The following flag bits are defined for [`co_flags`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_flags "codeobject.co_flags"): bit `0x04` is set if the function uses the `*arguments` syntax to accept an arbitrary number of positional arguments; bit `0x08` is set if the function uses the `**keywords` syntax to accept arbitrary keyword arguments; bit `0x20` is set if the function is a generator. See [Code Objects Bit Flags](https://docs.python.org/3/library/inspect.html#inspect-module-co-flags) for details on the semantics of each flags that might be present.
Future feature declarations (for example, `from __future__ import division`) also use bits in [`co_flags`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_flags "codeobject.co_flags") to indicate whether a code object was compiled with a particular feature enabled. See [`compiler_flag`](https://docs.python.org/3/library/__future__.html#future__._Feature.compiler_flag "__future__._Feature.compiler_flag").
Other bits in [`co_flags`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_flags "codeobject.co_flags") are reserved for internal use.
If a code object represents a function and has a docstring, the [`CO_HAS_DOCSTRING`](https://docs.python.org/3/library/inspect.html#inspect.CO_HAS_DOCSTRING "inspect.CO_HAS_DOCSTRING") bit is set in [`co_flags`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_flags "codeobject.co_flags") and the first item in [`co_consts`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_consts "codeobject.co_consts") is the docstring of the function.
##### 3\.2.13.1.2. Methods on code objects[¶](https://docs.python.org/3/reference/datamodel.html#methods-on-code-objects "Link to this heading")
codeobject.co\_positions()[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_positions "Link to this definition")
Returns an iterable over the source code positions of each [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) instruction in the code object.
The iterator returns [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple")s containing the . The *i-th* tuple corresponds to the position of the source code that compiled to the *i-th* code unit. Column information is 0-indexed utf-8 byte offsets on the given source line.
This positional information can be missing. A non-exhaustive lists of cases where this may happen:
- Running the interpreter with [`-X`](https://docs.python.org/3/using/cmdline.html#cmdoption-X) `no_debug_ranges`.
- Loading a pyc file compiled while using [`-X`](https://docs.python.org/3/using/cmdline.html#cmdoption-X) `no_debug_ranges`.
- Position tuples corresponding to artificial instructions.
- Line and column numbers that canât be represented due to implementation specific limitations.
When this occurs, some or all of the tuple elements can be [`None`](https://docs.python.org/3/library/constants.html#None "None").
Added in version 3.11.
Note
This feature requires storing column positions in code objects which may result in a small increase of disk usage of compiled Python files or interpreter memory usage. To avoid storing the extra information and/or deactivate printing the extra traceback information, the [`-X`](https://docs.python.org/3/using/cmdline.html#cmdoption-X) `no_debug_ranges` command line flag or the [`PYTHONNODEBUGRANGES`](https://docs.python.org/3/using/cmdline.html#envvar-PYTHONNODEBUGRANGES) environment variable can be used.
codeobject.co\_lines()[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_lines "Link to this definition")
Returns an iterator that yields information about successive ranges of [bytecode](https://docs.python.org/3/glossary.html#term-bytecode)s. Each item yielded is a `(start, end, lineno)` [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple"):
- `start` (an [`int`](https://docs.python.org/3/library/functions.html#int "int")) represents the offset (inclusive) of the start of the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) range
- `end` (an [`int`](https://docs.python.org/3/library/functions.html#int "int")) represents the offset (exclusive) of the end of the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) range
- `lineno` is an [`int`](https://docs.python.org/3/library/functions.html#int "int") representing the line number of the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) range, or `None` if the bytecodes in the given range have no line number
The items yielded will have the following properties:
- The first range yielded will have a `start` of 0.
- The `(start, end)` ranges will be non-decreasing and consecutive. That is, for any pair of [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple")s, the `start` of the second will be equal to the `end` of the first.
- No range will be backwards: `end >= start` for all triples.
- The last [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") yielded will have `end` equal to the size of the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode).
Zero-width ranges, where `start == end`, are allowed. Zero-width ranges are used for lines that are present in the source code, but have been eliminated by the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) compiler.
Added in version 3.10.
See also
[**PEP 626**](https://peps.python.org/pep-0626/) - Precise line numbers for debugging and other tools.
The PEP that introduced the `co_lines()` method.
codeobject.replace(*\*\*kwargs*)[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.replace "Link to this definition")
Return a copy of the code object with new values for the specified fields.
Code objects are also supported by the generic function [`copy.replace()`](https://docs.python.org/3/library/copy.html#copy.replace "copy.replace").
Added in version 3.8.
#### 3\.2.13.2. Frame objects[¶](https://docs.python.org/3/reference/datamodel.html#frame-objects "Link to this heading")
Frame objects represent execution frames. They may occur in [traceback objects](https://docs.python.org/3/reference/datamodel.html#traceback-objects), and are also passed to registered trace functions.
##### 3\.2.13.2.1. Special read-only attributes[¶](https://docs.python.org/3/reference/datamodel.html#index-70 "Link to this heading")
| | |
|---|---|
| frame.f\_back[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_back "Link to this definition") | Points to the previous stack frame (towards the caller), or `None` if this is the bottom stack frame |
| frame.f\_code[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_code "Link to this definition") | The [code object](https://docs.python.org/3/reference/datamodel.html#code-objects) being executed in this frame. Accessing this attribute raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `object.__getattr__` with arguments `obj` and `"f_code"`. |
| frame.f\_locals[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_locals "Link to this definition") | The mapping used by the frame to look up [local variables](https://docs.python.org/3/reference/executionmodel.html#naming). If the frame refers to an [optimized scope](https://docs.python.org/3/glossary.html#term-optimized-scope), this may return a write-through proxy object. Changed in version 3.13: Return a proxy for optimized scopes. |
| frame.f\_globals[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_globals "Link to this definition") | The dictionary used by the frame to look up [global variables](https://docs.python.org/3/reference/executionmodel.html#naming) |
| frame.f\_builtins[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_builtins "Link to this definition") | The dictionary used by the frame to look up [built-in (intrinsic) names](https://docs.python.org/3/reference/executionmodel.html#naming) |
| frame.f\_lasti[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_lasti "Link to this definition") | The âprecise instructionâ of the frame object (this is an index into the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) string of the [code object](https://docs.python.org/3/reference/datamodel.html#code-objects)) |
| frame.f\_generator[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_generator "Link to this definition") | The [generator](https://docs.python.org/3/glossary.html#term-generator) or [coroutine](https://docs.python.org/3/glossary.html#term-coroutine) object that owns this frame, or `None` if the frame is a normal function. Added in version 3.14. |
##### 3\.2.13.2.2. Special writable attributes[¶](https://docs.python.org/3/reference/datamodel.html#index-71 "Link to this heading")
| | |
|---|---|
| frame.f\_trace[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_trace "Link to this definition") | If not `None`, this is a function called for various events during code execution (this is used by debuggers). Normally an event is triggered for each new source line (see [`f_trace_lines`](https://docs.python.org/3/reference/datamodel.html#frame.f_trace_lines "frame.f_trace_lines")). |
| frame.f\_trace\_lines[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_trace_lines "Link to this definition") | Set this attribute to [`False`](https://docs.python.org/3/library/constants.html#False "False") to disable triggering a tracing event for each source line. |
| frame.f\_trace\_opcodes[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_trace_opcodes "Link to this definition") | Set this attribute to [`True`](https://docs.python.org/3/library/constants.html#True "True") to allow per-opcode events to be requested. Note that this may lead to undefined interpreter behaviour if exceptions raised by the trace function escape to the function being traced. |
| frame.f\_lineno[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_lineno "Link to this definition") | The current line number of the frame â writing to this from within a trace function jumps to the given line (only for the bottom-most frame). A debugger can implement a Jump command (aka Set Next Statement) by writing to this attribute. |
##### 3\.2.13.2.3. Frame object methods[¶](https://docs.python.org/3/reference/datamodel.html#frame-object-methods "Link to this heading")
Frame objects support one method:
frame.clear()[¶](https://docs.python.org/3/reference/datamodel.html#frame.clear "Link to this definition")
This method clears all references to [local variables](https://docs.python.org/3/reference/executionmodel.html#naming) held by the frame. Also, if the frame belonged to a [generator](https://docs.python.org/3/glossary.html#term-generator), the generator is finalized. This helps break reference cycles involving frame objects (for example when catching an [exception](https://docs.python.org/3/library/exceptions.html#bltin-exceptions) and storing its [traceback](https://docs.python.org/3/reference/datamodel.html#traceback-objects) for later use).
[`RuntimeError`](https://docs.python.org/3/library/exceptions.html#RuntimeError "RuntimeError") is raised if the frame is currently executing or suspended.
Added in version 3.4.
Changed in version 3.13: Attempting to clear a suspended frame raises [`RuntimeError`](https://docs.python.org/3/library/exceptions.html#RuntimeError "RuntimeError") (as has always been the case for executing frames).
#### 3\.2.13.3. Traceback objects[¶](https://docs.python.org/3/reference/datamodel.html#traceback-objects "Link to this heading")
Traceback objects represent the stack trace of an [exception](https://docs.python.org/3/tutorial/errors.html#tut-errors). A traceback object is implicitly created when an exception occurs, and may also be explicitly created by calling [`types.TracebackType`](https://docs.python.org/3/library/types.html#types.TracebackType "types.TracebackType").
Changed in version 3.7: Traceback objects can now be explicitly instantiated from Python code.
For implicitly created tracebacks, when the search for an exception handler unwinds the execution stack, at each unwound level a traceback object is inserted in front of the current traceback. When an exception handler is entered, the stack trace is made available to the program. (See section [The try statement](https://docs.python.org/3/reference/compound_stmts.html#try).) It is accessible as the third item of the tuple returned by [`sys.exc_info()`](https://docs.python.org/3/library/sys.html#sys.exc_info "sys.exc_info"), and as the [`__traceback__`](https://docs.python.org/3/library/exceptions.html#BaseException.__traceback__ "BaseException.__traceback__") attribute of the caught exception.
When the program contains no suitable handler, the stack trace is written (nicely formatted) to the standard error stream; if the interpreter is interactive, it is also made available to the user as [`sys.last_traceback`](https://docs.python.org/3/library/sys.html#sys.last_traceback "sys.last_traceback").
For explicitly created tracebacks, it is up to the creator of the traceback to determine how the [`tb_next`](https://docs.python.org/3/reference/datamodel.html#traceback.tb_next "traceback.tb_next") attributes should be linked to form a full stack trace.
Special read-only attributes:
| | |
|---|---|
| traceback.tb\_frame[¶](https://docs.python.org/3/reference/datamodel.html#traceback.tb_frame "Link to this definition") | Points to the execution [frame](https://docs.python.org/3/reference/datamodel.html#frame-objects) of the current level. Accessing this attribute raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `object.__getattr__` with arguments `obj` and `"tb_frame"`. |
| traceback.tb\_lineno[¶](https://docs.python.org/3/reference/datamodel.html#traceback.tb_lineno "Link to this definition") | Gives the line number where the exception occurred |
| traceback.tb\_lasti[¶](https://docs.python.org/3/reference/datamodel.html#traceback.tb_lasti "Link to this definition") | Indicates the âprecise instructionâ. |
The line number and last instruction in the traceback may differ from the line number of its [frame object](https://docs.python.org/3/reference/datamodel.html#frame-objects) if the exception occurred in a [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement with no matching except clause or with a [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause.
traceback.tb\_next[¶](https://docs.python.org/3/reference/datamodel.html#traceback.tb_next "Link to this definition")
The special writable attribute `tb_next` is the next level in the stack trace (towards the frame where the exception occurred), or `None` if there is no next level.
Changed in version 3.7: This attribute is now writable
#### 3\.2.13.4. Slice objects[¶](https://docs.python.org/3/reference/datamodel.html#slice-objects "Link to this heading")
Slice objects are used to represent slices for [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__") methods. They are also created by the built-in [`slice()`](https://docs.python.org/3/library/functions.html#slice "slice") function.
Special read-only attributes: [`start`](https://docs.python.org/3/library/functions.html#slice.start "slice.start") is the lower bound; [`stop`](https://docs.python.org/3/library/functions.html#slice.stop "slice.stop") is the upper bound; [`step`](https://docs.python.org/3/library/functions.html#slice.step "slice.step") is the step value; each is `None` if omitted. These attributes can have any type.
Slice objects support one method:
slice.indices(*self*, *length*)[¶](https://docs.python.org/3/reference/datamodel.html#slice.indices "Link to this definition")
This method takes a single integer argument *length* and computes information about the slice that the slice object would describe if applied to a sequence of *length* items. It returns a tuple of three integers; respectively these are the *start* and *stop* indices and the *step* or stride length of the slice. Missing or out-of-bounds indices are handled in a manner consistent with regular slices.
#### 3\.2.13.5. Static method objects[¶](https://docs.python.org/3/reference/datamodel.html#static-method-objects "Link to this heading")
Static method objects provide a way of defeating the transformation of function objects to method objects described above. A static method object is a wrapper around any other object, usually a user-defined method object. When a static method object is retrieved from a class or a class instance, the object actually returned is the wrapped object, which is not subject to any further transformation. Static method objects are also callable. Static method objects are created by the built-in [`staticmethod()`](https://docs.python.org/3/library/functions.html#staticmethod "staticmethod") constructor.
#### 3\.2.13.6. Class method objects[¶](https://docs.python.org/3/reference/datamodel.html#class-method-objects "Link to this heading")
A class method object, like a static method object, is a wrapper around another object that alters the way in which that object is retrieved from classes and class instances. The behaviour of class method objects upon such retrieval is described above, under [âinstance methodsâ](https://docs.python.org/3/reference/datamodel.html#instance-methods). Class method objects are created by the built-in [`classmethod()`](https://docs.python.org/3/library/functions.html#classmethod "classmethod") constructor.
## 3\.3. Special method names[¶](https://docs.python.org/3/reference/datamodel.html#special-method-names "Link to this heading")
A class can implement certain operations that are invoked by special syntax (such as arithmetic operations or subscripting and slicing) by defining methods with special names. This is Pythonâs approach to *operator overloading*, allowing classes to define their own behavior with respect to language operators. For instance, if a class defines a method named [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"), and `x` is an instance of this class, then `x[i]` is roughly equivalent to `type(x).__getitem__(x, i)`. Except where mentioned, attempts to execute an operation raise an exception when no appropriate method is defined (typically [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError") or [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError")).
Setting a special method to `None` indicates that the corresponding operation is not available. For example, if a class sets [`__iter__()`](https://docs.python.org/3/reference/datamodel.html#object.__iter__ "object.__iter__") to `None`, the class is not iterable, so calling [`iter()`](https://docs.python.org/3/library/functions.html#iter "iter") on its instances will raise a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") (without falling back to [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__")). [\[2\]](https://docs.python.org/3/reference/datamodel.html#id21)
When implementing a class that emulates any built-in type, it is important that the emulation only be implemented to the degree that it makes sense for the object being modelled. For example, some sequences may work well with retrieval of individual elements, but extracting a slice may not make sense. (One example of this is the [NodeList](https://docs.python.org/3/library/xml.dom.html#dom-nodelist-objects) interface in the W3Câs Document Object Model.)
### 3\.3.1. Basic customization[¶](https://docs.python.org/3/reference/datamodel.html#basic-customization "Link to this heading")
object.\_\_new\_\_(*cls*\[, *...*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__new__ "Link to this definition")
Called to create a new instance of class *cls*. `__new__()` is a static method (special-cased so you need not declare it as such) that takes the class of which an instance was requested as its first argument. The remaining arguments are those passed to the object constructor expression (the call to the class). The return value of `__new__()` should be the new object instance (usually an instance of *cls*).
Typical implementations create a new instance of the class by invoking the superclassâs `__new__()` method using `super().__new__(cls[, ...])` with appropriate arguments and then modifying the newly created instance as necessary before returning it.
If `__new__()` is invoked during object construction and it returns an instance of *cls*, then the new instanceâs [`__init__()`](https://docs.python.org/3/reference/datamodel.html#object.__init__ "object.__init__") method will be invoked like `__init__(self[, ...])`, where *self* is the new instance and the remaining arguments are the same as were passed to the object constructor.
If `__new__()` does not return an instance of *cls*, then the new instanceâs [`__init__()`](https://docs.python.org/3/reference/datamodel.html#object.__init__ "object.__init__") method will not be invoked.
`__new__()` is intended mainly to allow subclasses of immutable types (like int, str, or tuple) to customize instance creation. It is also commonly overridden in custom metaclasses in order to customize class creation.
object.\_\_init\_\_(*self*\[, *...*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__init__ "Link to this definition")
Called after the instance has been created (by [`__new__()`](https://docs.python.org/3/reference/datamodel.html#object.__new__ "object.__new__")), but before it is returned to the caller. The arguments are those passed to the class constructor expression. If a base class has an `__init__()` method, the derived classâs `__init__()` method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example: `super().__init__([args...])`.
Because [`__new__()`](https://docs.python.org/3/reference/datamodel.html#object.__new__ "object.__new__") and `__init__()` work together in constructing objects (`__new__()` to create it, and `__init__()` to customize it), no non-`None` value may be returned by `__init__()`; doing so will cause a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") to be raised at runtime.
object.\_\_del\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__del__ "Link to this definition")
Called when the instance is about to be destroyed. This is also called a finalizer or (improperly) a destructor. If a base class has a `__del__()` method, the derived classâs `__del__()` method, if any, must explicitly call it to ensure proper deletion of the base class part of the instance.
It is possible (though not recommended!) for the `__del__()` method to postpone destruction of the instance by creating a new reference to it. This is called object *resurrection*. It is implementation-dependent whether `__del__()` is called a second time when a resurrected object is about to be destroyed; the current [CPython](https://docs.python.org/3/glossary.html#term-CPython) implementation only calls it once.
It is not guaranteed that `__del__()` methods are called for objects that still exist when the interpreter exits. [`weakref.finalize`](https://docs.python.org/3/library/weakref.html#weakref.finalize "weakref.finalize") provides a straightforward way to register a cleanup function to be called when an object is garbage collected.
Note
`del x` doesnât directly call `x.__del__()` â the former decrements the reference count for `x` by one, and the latter is only called when `x`âs reference count reaches zero.
**CPython implementation detail:** It is possible for a reference cycle to prevent the reference count of an object from going to zero. In this case, the cycle will be later detected and deleted by the [cyclic garbage collector](https://docs.python.org/3/glossary.html#term-garbage-collection). A common cause of reference cycles is when an exception has been caught in a local variable. The frameâs locals then reference the exception, which references its own traceback, which references the locals of all frames caught in the traceback.
See also
Documentation for the [`gc`](https://docs.python.org/3/library/gc.html#module-gc "gc: Interface to the cycle-detecting garbage collector.") module.
Warning
Due to the precarious circumstances under which `__del__()` methods are invoked, exceptions that occur during their execution are ignored, and a warning is printed to `sys.stderr` instead. In particular:
- `__del__()` can be invoked when arbitrary code is being executed, including from any arbitrary thread. If `__del__()` needs to take a lock or invoke any other blocking resource, it may deadlock as the resource may already be taken by the code that gets interrupted to execute `__del__()`.
- `__del__()` can be executed during interpreter shutdown. As a consequence, the global variables it needs to access (including other modules) may already have been deleted or set to `None`. Python guarantees that globals whose name begins with a single underscore are deleted from their module before other globals are deleted; if no other references to such globals exist, this may help in assuring that imported modules are still available at the time when the `__del__()` method is called.
object.\_\_repr\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__repr__ "Link to this definition")
Called by the [`repr()`](https://docs.python.org/3/library/functions.html#repr "repr") built-in function to compute the âofficialâ string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form `<...some useful description...>` should be returned. The return value must be a string object. If a class defines `__repr__()` but not [`__str__()`](https://docs.python.org/3/reference/datamodel.html#object.__str__ "object.__str__"), then `__repr__()` is also used when an âinformalâ string representation of instances of that class is required.
This is typically used for debugging, so it is important that the representation is information-rich and unambiguous. A default implementation is provided by the [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself.
object.\_\_str\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__str__ "Link to this definition")
Called by [`str(object)`](https://docs.python.org/3/library/stdtypes.html#str "str"), the default [`__format__()`](https://docs.python.org/3/reference/datamodel.html#object.__format__ "object.__format__") implementation, and the built-in function [`print()`](https://docs.python.org/3/library/functions.html#print "print"), to compute the âinformalâ or nicely printable string representation of an object. The return value must be a [str](https://docs.python.org/3/library/stdtypes.html#textseq) object.
This method differs from [`object.__repr__()`](https://docs.python.org/3/reference/datamodel.html#object.__repr__ "object.__repr__") in that there is no expectation that `__str__()` return a valid Python expression: a more convenient or concise representation can be used.
The default implementation defined by the built-in type [`object`](https://docs.python.org/3/library/functions.html#object "object") calls [`object.__repr__()`](https://docs.python.org/3/reference/datamodel.html#object.__repr__ "object.__repr__").
object.\_\_bytes\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__bytes__ "Link to this definition")
Called by [bytes](https://docs.python.org/3/library/functions.html#func-bytes) to compute a byte-string representation of an object. This should return a [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") object. The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide this method.
object.\_\_format\_\_(*self*, *format\_spec*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__format__ "Link to this definition")
Called by the [`format()`](https://docs.python.org/3/library/functions.html#format "format") built-in function, and by extension, evaluation of [formatted string literals](https://docs.python.org/3/reference/lexical_analysis.html#f-strings) and the [`str.format()`](https://docs.python.org/3/library/stdtypes.html#str.format "str.format") method, to produce a âformattedâ string representation of an object. The *format\_spec* argument is a string that contains a description of the formatting options desired. The interpretation of the *format\_spec* argument is up to the type implementing `__format__()`, however most classes will either delegate formatting to one of the built-in types, or use a similar formatting option syntax.
See [Format specification mini-language](https://docs.python.org/3/library/string.html#formatspec) for a description of the standard formatting syntax.
The return value must be a string object.
The default implementation by the [`object`](https://docs.python.org/3/library/functions.html#object "object") class should be given an empty *format\_spec* string. It delegates to [`__str__()`](https://docs.python.org/3/reference/datamodel.html#object.__str__ "object.__str__").
Changed in version 3.4: The \_\_format\_\_ method of `object` itself raises a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") if passed any non-empty string.
Changed in version 3.7: `object.__format__(x, '')` is now equivalent to `str(x)` rather than `format(str(x), '')`.
object.\_\_lt\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__lt__ "Link to this definition")
object.\_\_le\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__le__ "Link to this definition")
object.\_\_eq\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "Link to this definition")
object.\_\_ne\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ne__ "Link to this definition")
object.\_\_gt\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__gt__ "Link to this definition")
object.\_\_ge\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ge__ "Link to this definition")
These are the so-called ârich comparisonâ methods. The correspondence between operator symbols and method names is as follows: `x<y` calls `x.__lt__(y)`, `x<=y` calls `x.__le__(y)`, `x==y` calls `x.__eq__(y)`, `x!=y` calls `x.__ne__(y)`, `x>y` calls `x.__gt__(y)`, and `x>=y` calls `x.__ge__(y)`.
A rich comparison method may return the singleton [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented") if it does not implement the operation for a given pair of arguments. By convention, `False` and `True` are returned for a successful comparison. However, these methods can return any value, so if the comparison operator is used in a Boolean context (e.g., in the condition of an `if` statement), Python will call [`bool()`](https://docs.python.org/3/library/functions.html#bool "bool") on the value to determine if the result is true or false.
By default, `object` implements `__eq__()` by using `is`, returning [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented") in the case of a false comparison: `True if x is y else NotImplemented`. For `__ne__()`, by default it delegates to `__eq__()` and inverts the result unless it is `NotImplemented`. There are no other implied relationships among the comparison operators or default implementations; for example, the truth of `(x<y or x==y)` does not imply `x<=y`. To automatically generate ordering operations from a single root operation, see [`functools.total_ordering()`](https://docs.python.org/3/library/functools.html#functools.total_ordering "functools.total_ordering").
By default, the [`object`](https://docs.python.org/3/library/functions.html#object "object") class provides implementations consistent with [Value comparisons](https://docs.python.org/3/reference/expressions.html#expressions-value-comparisons): equality compares according to object identity, and order comparisons raise [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError"). Each default method may generate these results directly, but may also return [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented").
See the paragraph on [`__hash__()`](https://docs.python.org/3/reference/datamodel.html#object.__hash__ "object.__hash__") for some important notes on creating [hashable](https://docs.python.org/3/glossary.html#term-hashable) objects which support custom comparison operations and are usable as dictionary keys.
There are no swapped-argument versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, `__lt__()` and `__gt__()` are each otherâs reflection, `__le__()` and `__ge__()` are each otherâs reflection, and `__eq__()` and `__ne__()` are their own reflection. If the operands are of different types, and the right operandâs type is a direct or indirect subclass of the left operandâs type, the reflected method of the right operand has priority, otherwise the left operandâs method has priority. Virtual subclassing is not considered.
When no appropriate method returns any value other than [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented"), the `==` and `!=` operators will fall back to `is` and `is not`, respectively.
object.\_\_hash\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__hash__ "Link to this definition")
Called by built-in function [`hash()`](https://docs.python.org/3/library/functions.html#hash "hash") and for operations on members of hashed collections including [`set`](https://docs.python.org/3/library/stdtypes.html#set "set"), [`frozenset`](https://docs.python.org/3/library/stdtypes.html#frozenset "frozenset"), and [`dict`](https://docs.python.org/3/library/stdtypes.html#dict "dict"). The `__hash__()` method should return an integer. The only required property is that objects which compare equal have the same hash value; it is advised to mix together the hash values of the components of the object that also play a part in comparison of objects by packing them into a tuple and hashing the tuple. Example:
Copy
```
def __hash__(self):
return hash((self.name, self.nick, self.color))
```
Note
[`hash()`](https://docs.python.org/3/library/functions.html#hash "hash") truncates the value returned from an objectâs custom `__hash__()` method to the size of a [`Py_ssize_t`](https://docs.python.org/3/c-api/intro.html#c.Py_ssize_t "Py_ssize_t"). This is typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds. If an objectâs `__hash__()` must interoperate on builds of different bit sizes, be sure to check the width on all supported builds. An easy way to do this is with `python -c "import sys; print(sys.hash_info.width)"`.
If a class does not define an [`__eq__()`](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "object.__eq__") method it should not define a `__hash__()` operation either; if it defines `__eq__()` but not `__hash__()`, its instances will not be usable as items in hashable collections. If a class defines mutable objects and implements an `__eq__()` method, it should not implement `__hash__()`, since the implementation of [hashable](https://docs.python.org/3/glossary.html#term-hashable) collections requires that a keyâs hash value is immutable (if the objectâs hash value changes, it will be in the wrong hash bucket).
User-defined classes have [`__eq__()`](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "object.__eq__") and `__hash__()` methods by default (inherited from the [`object`](https://docs.python.org/3/library/functions.html#object "object") class); with them, all objects compare unequal (except with themselves) and `x.__hash__()` returns an appropriate value such that `x == y` implies both that `x is y` and `hash(x) == hash(y)`.
A class that overrides [`__eq__()`](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "object.__eq__") and does not define `__hash__()` will have its `__hash__()` implicitly set to `None`. When the `__hash__()` method of a class is `None`, instances of the class will raise an appropriate [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") when a program attempts to retrieve their hash value, and will also be correctly identified as unhashable when checking `isinstance(obj, collections.abc.Hashable)`.
If a class that overrides [`__eq__()`](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "object.__eq__") needs to retain the implementation of `__hash__()` from a parent class, the interpreter must be told this explicitly by setting `__hash__ = <ParentClass>.__hash__`.
If a class that does not override [`__eq__()`](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "object.__eq__") wishes to suppress hash support, it should include `__hash__ = None` in the class definition. A class which defines its own `__hash__()` that explicitly raises a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") would be incorrectly identified as hashable by an `isinstance(obj, collections.abc.Hashable)` call.
Note
By default, the `__hash__()` values of str and bytes objects are âsaltedâ with an unpredictable random value. Although they remain constant within an individual Python process, they are not predictable between repeated invocations of Python.
This is intended to provide protection against a denial-of-service caused by carefully chosen inputs that exploit the worst case performance of a dict insertion, *O*(*n*2) complexity. See <https://ocert.org/advisories/ocert-2011-003.html> for details.
Changing hash values affects the iteration order of sets. Python has never made guarantees about this ordering (and it typically varies between 32-bit and 64-bit builds).
See also [`PYTHONHASHSEED`](https://docs.python.org/3/using/cmdline.html#envvar-PYTHONHASHSEED).
Changed in version 3.3: Hash randomization is enabled by default.
object.\_\_bool\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__bool__ "Link to this definition")
Called to implement truth value testing and the built-in operation `bool()`; should return `False` or `True`. When this method is not defined, [`__len__()`](https://docs.python.org/3/reference/datamodel.html#object.__len__ "object.__len__") is called, if it is defined, and the object is considered true if its result is nonzero. If a class defines neither `__len__()` nor `__bool__()` (which is true of the [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself), all its instances are considered true.
### 3\.3.2. Customizing attribute access[¶](https://docs.python.org/3/reference/datamodel.html#customizing-attribute-access "Link to this heading")
The following methods can be defined to customize the meaning of attribute access (use of, assignment to, or deletion of `x.name`) for class instances.
object.\_\_getattr\_\_(*self*, *name*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__getattr__ "Link to this definition")
Called when the default attribute access fails with an [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError") (either [`__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__") raises an `AttributeError` because *name* is not an instance attribute or an attribute in the class tree for `self`; or [`__get__()`](https://docs.python.org/3/reference/datamodel.html#object.__get__ "object.__get__") of a *name* property raises `AttributeError`). This method should either return the (computed) attribute value or raise an `AttributeError` exception. The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide this method.
Note that if the attribute is found through the normal mechanism, `__getattr__()` is not called. (This is an intentional asymmetry between `__getattr__()` and [`__setattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__setattr__ "object.__setattr__").) This is done both for efficiency reasons and because otherwise `__getattr__()` would have no way to access other attributes of the instance. Note that at least for instance variables, you can take total control by not inserting any values in the instance attribute dictionary (but instead inserting them in another object). See the [`__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__") method below for a way to actually get total control over attribute access.
object.\_\_getattribute\_\_(*self*, *name*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "Link to this definition")
Called unconditionally to implement attribute accesses for instances of the class. If the class also defines [`__getattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattr__ "object.__getattr__"), the latter will not be called unless `__getattribute__()` either calls it explicitly or raises an [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError"). This method should return the (computed) attribute value or raise an `AttributeError` exception. In order to avoid infinite recursion in this method, its implementation should always call the base class method with the same name to access any attributes it needs, for example, `object.__getattribute__(self, name)`.
Note
This method may still be bypassed when looking up special methods as the result of implicit invocation via language syntax or [built-in functions](https://docs.python.org/3/reference/datamodel.html#builtin-functions). See [Special method lookup](https://docs.python.org/3/reference/datamodel.html#special-lookup).
For certain sensitive attribute accesses, raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `object.__getattr__` with arguments `obj` and `name`.
object.\_\_setattr\_\_(*self*, *name*, *value*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__setattr__ "Link to this definition")
Called when an attribute assignment is attempted. This is called instead of the normal mechanism (i.e. store the value in the instance dictionary). *name* is the attribute name, *value* is the value to be assigned to it.
If `__setattr__()` wants to assign to an instance attribute, it should call the base class method with the same name, for example, `object.__setattr__(self, name, value)`.
For certain sensitive attribute assignments, raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `object.__setattr__` with arguments `obj`, `name`, `value`.
object.\_\_delattr\_\_(*self*, *name*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__delattr__ "Link to this definition")
Like [`__setattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__setattr__ "object.__setattr__") but for attribute deletion instead of assignment. This should only be implemented if `del obj.name` is meaningful for the object.
For certain sensitive attribute deletions, raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `object.__delattr__` with arguments `obj` and `name`.
object.\_\_dir\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__dir__ "Link to this definition")
Called when [`dir()`](https://docs.python.org/3/library/functions.html#dir "dir") is called on the object. An iterable must be returned. `dir()` converts the returned iterable to a list and sorts it.
#### 3\.3.2.1. Customizing module attribute access[¶](https://docs.python.org/3/reference/datamodel.html#customizing-module-attribute-access "Link to this heading")
module.\_\_getattr\_\_()[¶](https://docs.python.org/3/reference/datamodel.html#module.__getattr__ "Link to this definition")
module.\_\_dir\_\_()[¶](https://docs.python.org/3/reference/datamodel.html#module.__dir__ "Link to this definition")
Special names `__getattr__` and `__dir__` can be also used to customize access to module attributes. The `__getattr__` function at the module level should accept one argument which is the name of an attribute and return the computed value or raise an [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError"). If an attribute is not found on a module object through the normal lookup, i.e. [`object.__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__"), then `__getattr__` is searched in the module `__dict__` before raising an `AttributeError`. If found, it is called with the attribute name and the result is returned.
The `__dir__` function should accept no arguments, and return an iterable of strings that represents the names accessible on module. If present, this function overrides the standard [`dir()`](https://docs.python.org/3/library/functions.html#dir "dir") search on a module.
module.\_\_class\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__class__ "Link to this definition")
For a more fine grained customization of the module behavior (setting attributes, properties, etc.), one can set the `__class__` attribute of a module object to a subclass of [`types.ModuleType`](https://docs.python.org/3/library/types.html#types.ModuleType "types.ModuleType"). For example:
Copy
```
import sys
from types import ModuleType
class VerboseModule(ModuleType):
def __repr__(self):
return f'Verbose {self.__name__}'
def __setattr__(self, attr, value):
print(f'Setting {attr}...')
super().__setattr__(attr, value)
sys.modules[__name__].__class__ = VerboseModule
```
Note
Defining module `__getattr__` and setting module `__class__` only affect lookups made using the attribute access syntax â directly accessing the module globals (whether by code within the module, or via a reference to the moduleâs globals dictionary) is unaffected.
Changed in version 3.5: `__class__` module attribute is now writable.
Added in version 3.7: `__getattr__` and `__dir__` module attributes.
See also
[**PEP 562**](https://peps.python.org/pep-0562/) - Module \_\_getattr\_\_ and \_\_dir\_\_
Describes the `__getattr__` and `__dir__` functions on modules.
#### 3\.3.2.2. Implementing Descriptors[¶](https://docs.python.org/3/reference/datamodel.html#implementing-descriptors "Link to this heading")
The following methods only apply when an instance of the class containing the method (a so-called *descriptor* class) appears in an *owner* class (the descriptor must be in either the ownerâs class dictionary or in the class dictionary for one of its parents). In the examples below, âthe attributeâ refers to the attribute whose name is the key of the property in the owner classâ [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__"). The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not implement any of these protocols.
object.\_\_get\_\_(*self*, *instance*, *owner\=None*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__get__ "Link to this definition")
Called to get the attribute of the owner class (class attribute access) or of an instance of that class (instance attribute access). The optional *owner* argument is the owner class, while *instance* is the instance that the attribute was accessed through, or `None` when the attribute is accessed through the *owner*.
This method should return the computed attribute value or raise an [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError") exception.
[**PEP 252**](https://peps.python.org/pep-0252/) specifies that `__get__()` is callable with one or two arguments. Pythonâs own built-in descriptors support this specification; however, it is likely that some third-party tools have descriptors that require both arguments. Pythonâs own [`__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__") implementation always passes in both arguments whether they are required or not.
object.\_\_set\_\_(*self*, *instance*, *value*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__set__ "Link to this definition")
Called to set the attribute on an instance *instance* of the owner class to a new value, *value*.
Note, adding `__set__()` or [`__delete__()`](https://docs.python.org/3/reference/datamodel.html#object.__delete__ "object.__delete__") changes the kind of descriptor to a âdata descriptorâ. See [Invoking Descriptors](https://docs.python.org/3/reference/datamodel.html#descriptor-invocation) for more details.
object.\_\_delete\_\_(*self*, *instance*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__delete__ "Link to this definition")
Called to delete the attribute on an instance *instance* of the owner class.
Instances of descriptors may also have the `__objclass__` attribute present:
object.\_\_objclass\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__objclass__ "Link to this definition")
The attribute `__objclass__` is interpreted by the [`inspect`](https://docs.python.org/3/library/inspect.html#module-inspect "inspect: Extract information and source code from live objects.") module as specifying the class where this object was defined (setting this appropriately can assist in runtime introspection of dynamic class attributes). For callables, it may indicate that an instance of the given type (or a subclass) is expected or required as the first positional argument (for example, CPython sets this attribute for unbound methods that are implemented in C).
#### 3\.3.2.3. Invoking Descriptors[¶](https://docs.python.org/3/reference/datamodel.html#invoking-descriptors "Link to this heading")
In general, a descriptor is an object attribute with âbinding behaviorâ, one whose attribute access has been overridden by methods in the descriptor protocol: [`__get__()`](https://docs.python.org/3/reference/datamodel.html#object.__get__ "object.__get__"), [`__set__()`](https://docs.python.org/3/reference/datamodel.html#object.__set__ "object.__set__"), and [`__delete__()`](https://docs.python.org/3/reference/datamodel.html#object.__delete__ "object.__delete__"). If any of those methods are defined for an object, it is said to be a descriptor.
The default behavior for attribute access is to get, set, or delete the attribute from an objectâs dictionary. For instance, `a.x` has a lookup chain starting with `a.__dict__['x']`, then `type(a).__dict__['x']`, and continuing through the base classes of `type(a)` excluding metaclasses.
However, if the looked-up value is an object defining one of the descriptor methods, then Python may override the default behavior and invoke the descriptor method instead. Where this occurs in the precedence chain depends on which descriptor methods were defined and how they were called.
The starting point for descriptor invocation is a binding, `a.x`. How the arguments are assembled depends on `a`:
Direct Call
The simplest and least common call is when user code directly invokes a descriptor method: `x.__get__(a)`.
Instance Binding
If binding to an object instance, `a.x` is transformed into the call: `type(a).__dict__['x'].__get__(a, type(a))`.
Class Binding
If binding to a class, `A.x` is transformed into the call: `A.__dict__['x'].__get__(None, A)`.
Super Binding
A dotted lookup such as `super(A, a).x` searches `a.__class__.__mro__` for a base class `B` following `A` and then returns `B.__dict__['x'].__get__(a, A)`. If not a descriptor, `x` is returned unchanged.
For instance bindings, the precedence of descriptor invocation depends on which descriptor methods are defined. A descriptor can define any combination of [`__get__()`](https://docs.python.org/3/reference/datamodel.html#object.__get__ "object.__get__"), [`__set__()`](https://docs.python.org/3/reference/datamodel.html#object.__set__ "object.__set__") and [`__delete__()`](https://docs.python.org/3/reference/datamodel.html#object.__delete__ "object.__delete__"). If it does not define `__get__()`, then accessing the attribute will return the descriptor object itself unless there is a value in the objectâs instance dictionary. If the descriptor defines `__set__()` and/or `__delete__()`, it is a data descriptor; if it defines neither, it is a non-data descriptor. Normally, data descriptors define both `__get__()` and `__set__()`, while non-data descriptors have just the `__get__()` method. Data descriptors with `__get__()` and `__set__()` (and/or `__delete__()`) defined always override a redefinition in an instance dictionary. In contrast, non-data descriptors can be overridden by instances.
Python methods (including those decorated with [`@staticmethod`](https://docs.python.org/3/library/functions.html#staticmethod "staticmethod") and [`@classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod")) are implemented as non-data descriptors. Accordingly, instances can redefine and override methods. This allows individual instances to acquire behaviors that differ from other instances of the same class.
The [`property()`](https://docs.python.org/3/library/functions.html#property "property") function is implemented as a data descriptor. Accordingly, instances cannot override the behavior of a property.
#### 3\.3.2.4. \_\_slots\_\_[¶](https://docs.python.org/3/reference/datamodel.html#slots "Link to this heading")
*\_\_slots\_\_* allow us to explicitly declare data members (like properties) and deny the creation of [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") and *\_\_weakref\_\_* (unless explicitly declared in *\_\_slots\_\_* or available in a parent.)
The space saved over using [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") can be significant. Attribute lookup speed can be significantly improved as well.
object.\_\_slots\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__slots__ "Link to this definition")
This class variable can be assigned a string, iterable, or sequence of strings with variable names used by instances. *\_\_slots\_\_* reserves space for the declared variables and prevents the automatic creation of [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") and *\_\_weakref\_\_* for each instance.
Notes on using *\_\_slots\_\_*:
- When inheriting from a class without *\_\_slots\_\_*, the [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") and *\_\_weakref\_\_* attribute of the instances will always be accessible.
- Without a [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") variable, instances cannot be assigned new variables not listed in the *\_\_slots\_\_* definition. Attempts to assign to an unlisted variable name raises [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError"). If dynamic assignment of new variables is desired, then add `'__dict__'` to the sequence of strings in the *\_\_slots\_\_* declaration.
- Without a *\_\_weakref\_\_* variable for each instance, classes defining *\_\_slots\_\_* do not support [`weak references`](https://docs.python.org/3/library/weakref.html#module-weakref "weakref: Support for weak references and weak dictionaries.") to its instances. If weak reference support is needed, then add `'__weakref__'` to the sequence of strings in the *\_\_slots\_\_* declaration.
- *\_\_slots\_\_* are implemented at the class level by creating [descriptors](https://docs.python.org/3/reference/datamodel.html#descriptors) for each variable name. As a result, class attributes cannot be used to set default values for instance variables defined by *\_\_slots\_\_*; otherwise, the class attribute would overwrite the descriptor assignment.
- The action of a *\_\_slots\_\_* declaration is not limited to the class where it is defined. *\_\_slots\_\_* declared in parents are available in child classes. However, instances of a child subclass will get a [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") and *\_\_weakref\_\_* unless the subclass also defines *\_\_slots\_\_* (which should only contain names of any *additional* slots).
- If a class defines a slot also defined in a base class, the instance variable defined by the base class slot is inaccessible (except by retrieving its descriptor directly from the base class). This renders the meaning of the program undefined. In the future, a check may be added to prevent this.
- [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") will be raised if nonempty *\_\_slots\_\_* are defined for a class derived from a [`"variable-length" built-in type`](https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_itemsize "PyTypeObject.tp_itemsize") such as [`int`](https://docs.python.org/3/library/functions.html#int "int"), [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes"), and [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple").
- Any non-string [iterable](https://docs.python.org/3/glossary.html#term-iterable) may be assigned to *\_\_slots\_\_*.
- If a [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") is used to assign *\_\_slots\_\_*, the dictionary keys will be used as the slot names. The values of the dictionary can be used to provide per-attribute docstrings that will be recognised by [`inspect.getdoc()`](https://docs.python.org/3/library/inspect.html#inspect.getdoc "inspect.getdoc") and displayed in the output of [`help()`](https://docs.python.org/3/library/functions.html#help "help").
- [`__class__`](https://docs.python.org/3/reference/datamodel.html#object.__class__ "object.__class__") assignment works only if both classes have the same *\_\_slots\_\_*.
- [Multiple inheritance](https://docs.python.org/3/tutorial/classes.html#tut-multiple) with multiple slotted parent classes can be used, but only one parent is allowed to have attributes created by slots (the other bases must have empty slot layouts) - violations raise [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError").
- If an [iterator](https://docs.python.org/3/glossary.html#term-iterator) is used for *\_\_slots\_\_* then a [descriptor](https://docs.python.org/3/glossary.html#term-descriptor) is created for each of the iteratorâs values. However, the *\_\_slots\_\_* attribute will be an empty iterator.
### 3\.3.3. Customizing class creation[¶](https://docs.python.org/3/reference/datamodel.html#customizing-class-creation "Link to this heading")
Whenever a class inherits from another class, [`__init_subclass__()`](https://docs.python.org/3/reference/datamodel.html#object.__init_subclass__ "object.__init_subclass__") is called on the parent class. This way, it is possible to write classes which change the behavior of subclasses. This is closely related to class decorators, but where class decorators only affect the specific class theyâre applied to, `__init_subclass__` solely applies to future subclasses of the class defining the method.
*classmethod* object.\_\_init\_subclass\_\_(*cls*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__init_subclass__ "Link to this definition")
This method is called whenever the containing class is subclassed. *cls* is then the new subclass. If defined as a normal instance method, this method is implicitly converted to a class method.
Keyword arguments which are given to a new class are passed to the parent classâs `__init_subclass__`. For compatibility with other classes using `__init_subclass__`, one should take out the needed keyword arguments and pass the others over to the base class, as in:
Copy
```
class Philosopher:
def __init_subclass__(cls, /, default_name, **kwargs):
super().__init_subclass__(**kwargs)
cls.default_name = default_name
class AustralianPhilosopher(Philosopher, default_name="Bruce"):
pass
```
The default implementation `object.__init_subclass__` does nothing, but raises an error if it is called with any arguments.
Note
The metaclass hint `metaclass` is consumed by the rest of the type machinery, and is never passed to `__init_subclass__` implementations. The actual metaclass (rather than the explicit hint) can be accessed as `type(cls)`.
Added in version 3.6.
When a class is created, `type.__new__()` scans the class variables and makes callbacks to those with a [`__set_name__()`](https://docs.python.org/3/reference/datamodel.html#object.__set_name__ "object.__set_name__") hook.
object.\_\_set\_name\_\_(*self*, *owner*, *name*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__set_name__ "Link to this definition")
Automatically called at the time the owning class *owner* is created. The object has been assigned to *name* in that class:
Copy
```
class A:
x = C() # Automatically calls: x.__set_name__(A, 'x')
```
If the class variable is assigned after the class is created, `__set_name__()` will not be called automatically. If needed, `__set_name__()` can be called directly:
Copy
```
class A:
pass
c = C()
A.x = c # The hook is not called
c.__set_name__(A, 'x') # Manually invoke the hook
```
See [Creating the class object](https://docs.python.org/3/reference/datamodel.html#class-object-creation) for more details.
Added in version 3.6.
#### 3\.3.3.1. Metaclasses[¶](https://docs.python.org/3/reference/datamodel.html#metaclasses "Link to this heading")
By default, classes are constructed using [`type()`](https://docs.python.org/3/library/functions.html#type "type"). The class body is executed in a new namespace and the class name is bound locally to the result of `type(name, bases, namespace)`.
The class creation process can be customized by passing the `metaclass` keyword argument in the class definition line, or by inheriting from an existing class that included such an argument. In the following example, both `MyClass` and `MySubclass` are instances of `Meta`:
Copy
```
class Meta(type):
pass
class MyClass(metaclass=Meta):
pass
class MySubclass(MyClass):
pass
```
Any other keyword arguments that are specified in the class definition are passed through to all metaclass operations described below.
When a class definition is executed, the following steps occur:
- MRO entries are resolved;
- the appropriate metaclass is determined;
- the class namespace is prepared;
- the class body is executed;
- the class object is created.
#### 3\.3.3.2. Resolving MRO entries[¶](https://docs.python.org/3/reference/datamodel.html#resolving-mro-entries "Link to this heading")
object.\_\_mro\_entries\_\_(*self*, *bases*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__mro_entries__ "Link to this definition")
If a base that appears in a class definition is not an instance of [`type`](https://docs.python.org/3/library/functions.html#type "type"), then an `__mro_entries__()` method is searched on the base. If an `__mro_entries__()` method is found, the base is substituted with the result of a call to `__mro_entries__()` when creating the class. The method is called with the original bases tuple passed to the *bases* parameter, and must return a tuple of classes that will be used instead of the base. The returned tuple may be empty: in these cases, the original base is ignored.
See also
[`types.resolve_bases()`](https://docs.python.org/3/library/types.html#types.resolve_bases "types.resolve_bases")
Dynamically resolve bases that are not instances of [`type`](https://docs.python.org/3/library/functions.html#type "type").
[`types.get_original_bases()`](https://docs.python.org/3/library/types.html#types.get_original_bases "types.get_original_bases")
Retrieve a classâs âoriginal basesâ prior to modifications by [`__mro_entries__()`](https://docs.python.org/3/reference/datamodel.html#object.__mro_entries__ "object.__mro_entries__").
[**PEP 560**](https://peps.python.org/pep-0560/)
Core support for typing module and generic types.
#### 3\.3.3.3. Determining the appropriate metaclass[¶](https://docs.python.org/3/reference/datamodel.html#determining-the-appropriate-metaclass "Link to this heading")
The appropriate metaclass for a class definition is determined as follows:
- if no bases and no explicit metaclass are given, then [`type()`](https://docs.python.org/3/library/functions.html#type "type") is used;
- if an explicit metaclass is given and it is *not* an instance of [`type()`](https://docs.python.org/3/library/functions.html#type "type"), then it is used directly as the metaclass;
- if an instance of [`type()`](https://docs.python.org/3/library/functions.html#type "type") is given as the explicit metaclass, or bases are defined, then the most derived metaclass is used.
The most derived metaclass is selected from the explicitly specified metaclass (if any) and the metaclasses (i.e. `type(cls)`) of all specified base classes. The most derived metaclass is one which is a subtype of *all* of these candidate metaclasses. If none of the candidate metaclasses meets that criterion, then the class definition will fail with `TypeError`.
#### 3\.3.3.4. Preparing the class namespace[¶](https://docs.python.org/3/reference/datamodel.html#preparing-the-class-namespace "Link to this heading")
Once the appropriate metaclass has been identified, then the class namespace is prepared. If the metaclass has a `__prepare__` attribute, it is called as `namespace = metaclass.__prepare__(name, bases, **kwds)` (where the additional keyword arguments, if any, come from the class definition). The `__prepare__` method should be implemented as a [`classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod"). The namespace returned by `__prepare__` is passed in to `__new__`, but when the final class object is created the namespace is copied into a new `dict`.
If the metaclass has no `__prepare__` attribute, then the class namespace is initialised as an empty ordered mapping.
See also
[**PEP 3115**](https://peps.python.org/pep-3115/) - Metaclasses in Python 3000
Introduced the `__prepare__` namespace hook
#### 3\.3.3.5. Executing the class body[¶](https://docs.python.org/3/reference/datamodel.html#executing-the-class-body "Link to this heading")
The class body is executed (approximately) as `exec(body, globals(), namespace)`. The key difference from a normal call to [`exec()`](https://docs.python.org/3/library/functions.html#exec "exec") is that lexical scoping allows the class body (including any methods) to reference names from the current and outer scopes when the class definition occurs inside a function.
However, even when the class definition occurs inside the function, methods defined inside the class still cannot see names defined at the class scope. Class variables must be accessed through the first parameter of instance or class methods, or through the implicit lexically scoped `__class__` reference described in the next section.
#### 3\.3.3.6. Creating the class object[¶](https://docs.python.org/3/reference/datamodel.html#creating-the-class-object "Link to this heading")
Once the class namespace has been populated by executing the class body, the class object is created by calling `metaclass(name, bases, namespace, **kwds)` (the additional keywords passed here are the same as those passed to `__prepare__`).
This class object is the one that will be referenced by the zero-argument form of [`super()`](https://docs.python.org/3/library/functions.html#super "super"). `__class__` is an implicit closure reference created by the compiler if any methods in a class body refer to either `__class__` or `super`. This allows the zero argument form of `super()` to correctly identify the class being defined based on lexical scoping, while the class or instance that was used to make the current call is identified based on the first argument passed to the method.
**CPython implementation detail:** In CPython 3.6 and later, the `__class__` cell is passed to the metaclass as a `__classcell__` entry in the class namespace. If present, this must be propagated up to the `type.__new__` call in order for the class to be initialised correctly. Failing to do so will result in a [`RuntimeError`](https://docs.python.org/3/library/exceptions.html#RuntimeError "RuntimeError") in Python 3.8.
When using the default metaclass [`type`](https://docs.python.org/3/library/functions.html#type "type"), or any metaclass that ultimately calls `type.__new__`, the following additional customization steps are invoked after creating the class object:
1. The `type.__new__` method collects all of the attributes in the class namespace that define a [`__set_name__()`](https://docs.python.org/3/reference/datamodel.html#object.__set_name__ "object.__set_name__") method;
2. Those `__set_name__` methods are called with the class being defined and the assigned name of that particular attribute;
3. The [`__init_subclass__()`](https://docs.python.org/3/reference/datamodel.html#object.__init_subclass__ "object.__init_subclass__") hook is called on the immediate parent of the new class in its method resolution order.
After the class object is created, it is passed to the class decorators included in the class definition (if any) and the resulting object is bound in the local namespace as the defined class.
When a new class is created by `type.__new__`, the object provided as the namespace parameter is copied to a new ordered mapping and the original object is discarded. The new copy is wrapped in a read-only proxy, which becomes the [`__dict__`](https://docs.python.org/3/reference/datamodel.html#type.__dict__ "type.__dict__") attribute of the class object.
See also
[**PEP 3135**](https://peps.python.org/pep-3135/) - New super
Describes the implicit `__class__` closure reference
#### 3\.3.3.7. Uses for metaclasses[¶](https://docs.python.org/3/reference/datamodel.html#uses-for-metaclasses "Link to this heading")
The potential uses for metaclasses are boundless. Some ideas that have been explored include enum, logging, interface checking, automatic delegation, automatic property creation, proxies, frameworks, and automatic resource locking/synchronization.
### 3\.3.4. Customizing instance and subclass checks[¶](https://docs.python.org/3/reference/datamodel.html#customizing-instance-and-subclass-checks "Link to this heading")
The following methods are used to override the default behavior of the [`isinstance()`](https://docs.python.org/3/library/functions.html#isinstance "isinstance") and [`issubclass()`](https://docs.python.org/3/library/functions.html#issubclass "issubclass") built-in functions.
In particular, the metaclass [`abc.ABCMeta`](https://docs.python.org/3/library/abc.html#abc.ABCMeta "abc.ABCMeta") implements these methods in order to allow the addition of Abstract Base Classes (ABCs) as âvirtual base classesâ to any class or type (including built-in types), including other ABCs.
type.\_\_instancecheck\_\_(*self*, *instance*)[¶](https://docs.python.org/3/reference/datamodel.html#type.__instancecheck__ "Link to this definition")
Return true if *instance* should be considered a (direct or indirect) instance of *class*. If defined, called to implement .
type.\_\_subclasscheck\_\_(*self*, *subclass*)[¶](https://docs.python.org/3/reference/datamodel.html#type.__subclasscheck__ "Link to this definition")
Return true if *subclass* should be considered a (direct or indirect) subclass of *class*. If defined, called to implement .
Note that these methods are looked up on the type (metaclass) of a class. They cannot be defined as class methods in the actual class. This is consistent with the lookup of special methods that are called on instances, only in this case the instance is itself a class.
See also
[**PEP 3119**](https://peps.python.org/pep-3119/) - Introducing Abstract Base Classes
Includes the specification for customizing [`isinstance()`](https://docs.python.org/3/library/functions.html#isinstance "isinstance") and [`issubclass()`](https://docs.python.org/3/library/functions.html#issubclass "issubclass") behavior through [`__instancecheck__()`](https://docs.python.org/3/reference/datamodel.html#type.__instancecheck__ "type.__instancecheck__") and [`__subclasscheck__()`](https://docs.python.org/3/reference/datamodel.html#type.__subclasscheck__ "type.__subclasscheck__"), with motivation for this functionality in the context of adding Abstract Base Classes (see the [`abc`](https://docs.python.org/3/library/abc.html#module-abc "abc: Abstract base classes according to :pep:`3119`.") module) to the language.
### 3\.3.5. Emulating generic types[¶](https://docs.python.org/3/reference/datamodel.html#emulating-generic-types "Link to this heading")
When using [type annotations](https://docs.python.org/3/glossary.html#term-annotation), it is often useful to *parameterize* a [generic type](https://docs.python.org/3/glossary.html#term-generic-type) using Pythonâs square-brackets notation. For example, the annotation `list[int]` might be used to signify a [`list`](https://docs.python.org/3/library/stdtypes.html#list "list") in which all the elements are of type [`int`](https://docs.python.org/3/library/functions.html#int "int").
See also
[**PEP 484**](https://peps.python.org/pep-0484/) - Type Hints
Introducing Pythonâs framework for type annotations
[Generic Alias Types](https://docs.python.org/3/library/stdtypes.html#types-genericalias)
Documentation for objects representing parameterized generic classes
[Generics](https://docs.python.org/3/library/typing.html#generics), [user-defined generics](https://docs.python.org/3/library/typing.html#user-defined-generics) and [`typing.Generic`](https://docs.python.org/3/library/typing.html#typing.Generic "typing.Generic")
Documentation on how to implement generic classes that can be parameterized at runtime and understood by static type-checkers.
A class can *generally* only be parameterized if it defines the special class method `__class_getitem__()`.
*classmethod* object.\_\_class\_getitem\_\_(*cls*, *key*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "Link to this definition")
Return an object representing the specialization of a generic class by type arguments found in *key*.
When defined on a class, `__class_getitem__()` is automatically a class method. As such, there is no need for it to be decorated with [`@classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod") when it is defined.
#### 3\.3.5.1. The purpose of *\_\_class\_getitem\_\_*[¶](https://docs.python.org/3/reference/datamodel.html#the-purpose-of-class-getitem "Link to this heading")
The purpose of [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") is to allow runtime parameterization of standard-library generic classes in order to more easily apply [type hints](https://docs.python.org/3/glossary.html#term-type-hint) to these classes.
To implement custom generic classes that can be parameterized at runtime and understood by static type-checkers, users should either inherit from a standard library class that already implements [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__"), or inherit from [`typing.Generic`](https://docs.python.org/3/library/typing.html#typing.Generic "typing.Generic"), which has its own implementation of `__class_getitem__()`.
Custom implementations of [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") on classes defined outside of the standard library may not be understood by third-party type-checkers such as mypy. Using `__class_getitem__()` on any class for purposes other than type hinting is discouraged.
#### 3\.3.5.2. *\_\_class\_getitem\_\_* versus *\_\_getitem\_\_*[¶](https://docs.python.org/3/reference/datamodel.html#class-getitem-versus-getitem "Link to this heading")
Usually, the [subscription](https://docs.python.org/3/reference/expressions.html#subscriptions) of an object using square brackets will call the [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__") instance method defined on the objectâs class. However, if the object being subscribed is itself a class, the class method [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") may be called instead. `__class_getitem__()` should return a [GenericAlias](https://docs.python.org/3/library/stdtypes.html#types-genericalias) object if it is properly defined.
Presented with the [expression](https://docs.python.org/3/glossary.html#term-expression) `obj[x]`, the Python interpreter follows something like the following process to decide whether [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__") or [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") should be called:
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```
from inspect import isclass
def subscribe(obj, x):
"""Return the result of the expression 'obj[x]'"""
class_of_obj = type(obj)
# If the class of obj defines __getitem__,
# call class_of_obj.__getitem__(obj, x)
if hasattr(class_of_obj, '__getitem__'):
return class_of_obj.__getitem__(obj, x)
# Else, if obj is a class and defines __class_getitem__,
# call obj.__class_getitem__(x)
elif isclass(obj) and hasattr(obj, '__class_getitem__'):
return obj.__class_getitem__(x)
# Else, raise an exception
else:
raise TypeError(
f"'{class_of_obj.__name__}' object is not subscriptable"
)
```
In Python, all classes are themselves instances of other classes. The class of a class is known as that classâs [metaclass](https://docs.python.org/3/glossary.html#term-metaclass), and most classes have the [`type`](https://docs.python.org/3/library/functions.html#type "type") class as their metaclass. `type` does not define [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"), meaning that expressions such as `list[int]`, `dict[str, float]` and `tuple[str, bytes]` all result in [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") being called:
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```
>>> # list has class "type" as its metaclass, like most classes:
>>> type(list)
<class 'type'>
>>> type(dict) == type(list) == type(tuple) == type(str) == type(bytes)
True
>>> # "list[int]" calls "list.__class_getitem__(int)"
>>> list[int]
list[int]
>>> # list.__class_getitem__ returns a GenericAlias object:
>>> type(list[int])
<class 'types.GenericAlias'>
```
However, if a class has a custom metaclass that defines [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"), subscribing the class may result in different behaviour. An example of this can be found in the [`enum`](https://docs.python.org/3/library/enum.html#module-enum "enum: Implementation of an enumeration class.") module:
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```
>>> from enum import Enum
>>> class Menu(Enum):
... """A breakfast menu"""
... SPAM = 'spam'
... BACON = 'bacon'
...
>>> # Enum classes have a custom metaclass:
>>> type(Menu)
<class 'enum.EnumMeta'>
>>> # EnumMeta defines __getitem__,
>>> # so __class_getitem__ is not called,
>>> # and the result is not a GenericAlias object:
>>> Menu['SPAM']
<Menu.SPAM: 'spam'>
>>> type(Menu['SPAM'])
<enum 'Menu'>
```
See also
[**PEP 560**](https://peps.python.org/pep-0560/) - Core Support for typing module and generic types
Introducing [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__"), and outlining when a [subscription](https://docs.python.org/3/reference/expressions.html#subscriptions) results in `__class_getitem__()` being called instead of [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__")
### 3\.3.6. Emulating callable objects[¶](https://docs.python.org/3/reference/datamodel.html#emulating-callable-objects "Link to this heading")
object.\_\_call\_\_(*self*\[, *args...*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__call__ "Link to this definition")
Called when the instance is âcalledâ as a function; if this method is defined, `x(arg1, arg2, ...)` roughly translates to `type(x).__call__(x, arg1, ...)`. The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide this method.
### 3\.3.7. Emulating container types[¶](https://docs.python.org/3/reference/datamodel.html#emulating-container-types "Link to this heading")
The following methods can be defined to implement container objects. None of them are provided by the [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself. Containers usually are [sequences](https://docs.python.org/3/glossary.html#term-sequence) (such as [`lists`](https://docs.python.org/3/library/stdtypes.html#list "list") or [`tuples`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple")) or [mappings](https://docs.python.org/3/glossary.html#term-mapping) (like [dictionaries](https://docs.python.org/3/glossary.html#term-dictionary)), but can represent other containers as well. The first set of methods is used either to emulate a sequence or to emulate a mapping; the difference is that for a sequence, the allowable keys should be the integers *k* for which where *N* is the length of the sequence, or [`slice`](https://docs.python.org/3/library/functions.html#slice "slice") objects, which define a range of items. It is also recommended that mappings provide the methods `keys()`, `values()`, `items()`, `get()`, `clear()`, `setdefault()`, `pop()`, `popitem()`, `copy()`, and `update()` behaving similar to those for Pythonâs standard [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") objects. The [`collections.abc`](https://docs.python.org/3/library/collections.abc.html#module-collections.abc "collections.abc: Abstract base classes for containers") module provides a [`MutableMapping`](https://docs.python.org/3/library/collections.abc.html#collections.abc.MutableMapping "collections.abc.MutableMapping") [abstract base class](https://docs.python.org/3/glossary.html#term-abstract-base-class) to help create those methods from a base set of [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"), [`__setitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "object.__setitem__"), [`__delitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__delitem__ "object.__delitem__"), and `keys()`.
Mutable sequences should provide methods [`append()`](https://docs.python.org/3/library/stdtypes.html#sequence.append "sequence.append"), [`clear()`](https://docs.python.org/3/library/stdtypes.html#sequence.clear "sequence.clear"), [`count()`](https://docs.python.org/3/library/stdtypes.html#sequence.count "sequence.count"), [`extend()`](https://docs.python.org/3/library/stdtypes.html#sequence.extend "sequence.extend"), [`index()`](https://docs.python.org/3/library/stdtypes.html#sequence.index "sequence.index"), [`insert()`](https://docs.python.org/3/library/stdtypes.html#sequence.insert "sequence.insert"), [`pop()`](https://docs.python.org/3/library/stdtypes.html#sequence.pop "sequence.pop"), [`remove()`](https://docs.python.org/3/library/stdtypes.html#sequence.remove "sequence.remove"), and [`reverse()`](https://docs.python.org/3/library/stdtypes.html#sequence.reverse "sequence.reverse"), like Python standard [`list`](https://docs.python.org/3/library/stdtypes.html#list "list") objects. Finally, sequence types should implement addition (meaning concatenation) and multiplication (meaning repetition) by defining the methods [`__add__()`](https://docs.python.org/3/reference/datamodel.html#object.__add__ "object.__add__"), [`__radd__()`](https://docs.python.org/3/reference/datamodel.html#object.__radd__ "object.__radd__"), [`__iadd__()`](https://docs.python.org/3/reference/datamodel.html#object.__iadd__ "object.__iadd__"), [`__mul__()`](https://docs.python.org/3/reference/datamodel.html#object.__mul__ "object.__mul__"), [`__rmul__()`](https://docs.python.org/3/reference/datamodel.html#object.__rmul__ "object.__rmul__") and [`__imul__()`](https://docs.python.org/3/reference/datamodel.html#object.__imul__ "object.__imul__") described below; they should not define other numerical operators.
It is recommended that both mappings and sequences implement the [`__contains__()`](https://docs.python.org/3/reference/datamodel.html#object.__contains__ "object.__contains__") method to allow efficient use of the `in` operator; for mappings, `in` should search the mappingâs keys; for sequences, it should search through the values. It is further recommended that both mappings and sequences implement the [`__iter__()`](https://docs.python.org/3/reference/datamodel.html#object.__iter__ "object.__iter__") method to allow efficient iteration through the container; for mappings, `__iter__()` should iterate through the objectâs keys; for sequences, it should iterate through the values.
object.\_\_len\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__len__ "Link to this definition")
Called to implement the built-in function [`len()`](https://docs.python.org/3/library/functions.html#len "len"). Should return the length of the object, an integer `>=` 0. Also, an object that doesnât define a [`__bool__()`](https://docs.python.org/3/reference/datamodel.html#object.__bool__ "object.__bool__") method and whose `__len__()` method returns zero is considered to be false in a Boolean context.
**CPython implementation detail:** In CPython, the length is required to be at most [`sys.maxsize`](https://docs.python.org/3/library/sys.html#sys.maxsize "sys.maxsize"). If the length is larger than `sys.maxsize` some features (such as [`len()`](https://docs.python.org/3/library/functions.html#len "len")) may raise [`OverflowError`](https://docs.python.org/3/library/exceptions.html#OverflowError "OverflowError"). To prevent raising `OverflowError` by truth value testing, an object must define a [`__bool__()`](https://docs.python.org/3/reference/datamodel.html#object.__bool__ "object.__bool__") method.
object.\_\_length\_hint\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__length_hint__ "Link to this definition")
Called to implement [`operator.length_hint()`](https://docs.python.org/3/library/operator.html#operator.length_hint "operator.length_hint"). Should return an estimated length for the object (which may be greater or less than the actual length). The length must be an integer `>=` 0. The return value may also be [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented"), which is treated the same as if the `__length_hint__` method didnât exist at all. This method is purely an optimization and is never required for correctness.
Added in version 3.4.
object.\_\_getitem\_\_(*self*, *subscript*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "Link to this definition")
Called to implement *subscription*, that is, `self[subscript]`. See [Subscriptions and slicings](https://docs.python.org/3/reference/expressions.html#subscriptions) for details on the syntax.
There are two types of built-in objects that support subscription via `__getitem__()`:
- **sequences**, where *subscript* (also called [index](https://docs.python.org/3/glossary.html#term-index)) should be an integer or a [`slice`](https://docs.python.org/3/library/functions.html#slice "slice") object. See the [sequence documentation](https://docs.python.org/3/reference/datamodel.html#datamodel-sequences) for the expected behavior, including handling `slice` objects and negative indices.
- **mappings**, where *subscript* is also called the [key](https://docs.python.org/3/glossary.html#term-key). See [mapping documentation](https://docs.python.org/3/reference/datamodel.html#datamodel-mappings) for the expected behavior.
If *subscript* is of an inappropriate type, `__getitem__()` should raise [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError"). If *subscript* has an inappropriate value, `__getitem__()` should raise an [`LookupError`](https://docs.python.org/3/library/exceptions.html#LookupError "LookupError") or one of its subclasses ([`IndexError`](https://docs.python.org/3/library/exceptions.html#IndexError "IndexError") for sequences; [`KeyError`](https://docs.python.org/3/library/exceptions.html#KeyError "KeyError") for mappings).
Note
Slicing is handled by `__getitem__()`, [`__setitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "object.__setitem__"), and [`__delitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__delitem__ "object.__delitem__"). A call like
Copy
```
a[1:2] = b
```
is translated to
Copy
```
a[slice(1, 2, None)] = b
```
and so forth. Missing slice items are always filled in with `None`.
Note
The sequence iteration protocol (used, for example, in [`for`](https://docs.python.org/3/reference/compound_stmts.html#for) loops), expects that an [`IndexError`](https://docs.python.org/3/library/exceptions.html#IndexError "IndexError") will be raised for illegal indexes to allow proper detection of the end of a sequence.
Note
When [subscripting](https://docs.python.org/3/reference/expressions.html#subscriptions) a *class*, the special class method [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") may be called instead of `__getitem__()`. See [\_\_class\_getitem\_\_ versus \_\_getitem\_\_](https://docs.python.org/3/reference/datamodel.html#classgetitem-versus-getitem) for more details.
object.\_\_setitem\_\_(*self*, *key*, *value*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "Link to this definition")
Called to implement assignment to `self[key]`. Same note as for [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"). This should only be implemented for mappings if the objects support changes to the values for keys, or if new keys can be added, or for sequences if elements can be replaced. The same exceptions should be raised for improper *key* values as for the `__getitem__()` method.
object.\_\_delitem\_\_(*self*, *key*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__delitem__ "Link to this definition")
Called to implement deletion of `self[key]`. Same note as for [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"). This should only be implemented for mappings if the objects support removal of keys, or for sequences if elements can be removed from the sequence. The same exceptions should be raised for improper *key* values as for the `__getitem__()` method.
object.\_\_missing\_\_(*self*, *key*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__missing__ "Link to this definition")
Called by [`dict`](https://docs.python.org/3/library/stdtypes.html#dict "dict").[`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__") to implement `self[key]` for dict subclasses when key is not in the dictionary.
object.\_\_iter\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__iter__ "Link to this definition")
This method is called when an [iterator](https://docs.python.org/3/glossary.html#term-iterator) is required for a container. This method should return a new iterator object that can iterate over all the objects in the container. For mappings, it should iterate over the keys of the container.
object.\_\_reversed\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__reversed__ "Link to this definition")
Called (if present) by the [`reversed()`](https://docs.python.org/3/library/functions.html#reversed "reversed") built-in to implement reverse iteration. It should return a new iterator object that iterates over all the objects in the container in reverse order.
If the `__reversed__()` method is not provided, the [`reversed()`](https://docs.python.org/3/library/functions.html#reversed "reversed") built-in will fall back to using the sequence protocol ([`__len__()`](https://docs.python.org/3/reference/datamodel.html#object.__len__ "object.__len__") and [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__")). Objects that support the sequence protocol should only provide `__reversed__()` if they can provide an implementation that is more efficient than the one provided by `reversed()`.
The membership test operators ([`in`](https://docs.python.org/3/reference/expressions.html#in) and [`not in`](https://docs.python.org/3/reference/expressions.html#not-in)) are normally implemented as an iteration through a container. However, container objects can supply the following special method with a more efficient implementation, which also does not require the object be iterable.
object.\_\_contains\_\_(*self*, *item*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__contains__ "Link to this definition")
Called to implement membership test operators. Should return true if *item* is in *self*, false otherwise. For mapping objects, this should consider the keys of the mapping rather than the values or the key-item pairs.
For objects that donât define `__contains__()`, the membership test first tries iteration via [`__iter__()`](https://docs.python.org/3/reference/datamodel.html#object.__iter__ "object.__iter__"), then the old sequence iteration protocol via [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"), see [this section in the language reference](https://docs.python.org/3/reference/expressions.html#membership-test-details).
### 3\.3.8. Emulating numeric types[¶](https://docs.python.org/3/reference/datamodel.html#emulating-numeric-types "Link to this heading")
The following methods can be defined to emulate numeric objects. Methods corresponding to operations that are not supported by the particular kind of number implemented (e.g., bitwise operations for non-integral numbers) should be left undefined.
object.\_\_add\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__add__ "Link to this definition")
object.\_\_sub\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__sub__ "Link to this definition")
object.\_\_mul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__mul__ "Link to this definition")
object.\_\_matmul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__matmul__ "Link to this definition")
object.\_\_truediv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__truediv__ "Link to this definition")
object.\_\_floordiv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__floordiv__ "Link to this definition")
object.\_\_mod\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__mod__ "Link to this definition")
object.\_\_divmod\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__divmod__ "Link to this definition")
object.\_\_pow\_\_(*self*, *other*\[, *modulo*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__pow__ "Link to this definition")
object.\_\_lshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__lshift__ "Link to this definition")
object.\_\_rshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rshift__ "Link to this definition")
object.\_\_and\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__and__ "Link to this definition")
object.\_\_xor\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__xor__ "Link to this definition")
object.\_\_or\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__or__ "Link to this definition")
These methods are called to implement the binary arithmetic operations (`+`, `-`, `*`, `@`, `/`, `//`, `%`, [`divmod()`](https://docs.python.org/3/library/functions.html#divmod "divmod"), [`pow()`](https://docs.python.org/3/library/functions.html#pow "pow"), `**`, `<<`, `>>`, `&`, `^`, `|`). For instance, to evaluate the expression `x + y`, where *x* is an instance of a class that has an `__add__()` method, `type(x).__add__(x, y)` is called. The `__divmod__()` method should be the equivalent to using `__floordiv__()` and `__mod__()`; it should not be related to `__truediv__()`. Note that `__pow__()` should be defined to accept an optional third argument if the three-argument version of the built-in `pow()` function is to be supported.
If one of those methods does not support the operation with the supplied arguments, it should return [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented").
object.\_\_radd\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__radd__ "Link to this definition")
object.\_\_rsub\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rsub__ "Link to this definition")
object.\_\_rmul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rmul__ "Link to this definition")
object.\_\_rmatmul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rmatmul__ "Link to this definition")
object.\_\_rtruediv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rtruediv__ "Link to this definition")
object.\_\_rfloordiv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rfloordiv__ "Link to this definition")
object.\_\_rmod\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rmod__ "Link to this definition")
object.\_\_rdivmod\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rdivmod__ "Link to this definition")
object.\_\_rpow\_\_(*self*, *other*\[, *modulo*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__rpow__ "Link to this definition")
object.\_\_rlshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rlshift__ "Link to this definition")
object.\_\_rrshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rrshift__ "Link to this definition")
object.\_\_rand\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rand__ "Link to this definition")
object.\_\_rxor\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rxor__ "Link to this definition")
object.\_\_ror\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ror__ "Link to this definition")
These methods are called to implement the binary arithmetic operations (`+`, `-`, `*`, `@`, `/`, `//`, `%`, [`divmod()`](https://docs.python.org/3/library/functions.html#divmod "divmod"), [`pow()`](https://docs.python.org/3/library/functions.html#pow "pow"), `**`, `<<`, `>>`, `&`, `^`, `|`) with reflected (swapped) operands. These functions are only called if the operands are of different types, when the left operand does not support the corresponding operation [\[3\]](https://docs.python.org/3/reference/datamodel.html#id22), or the right operandâs class is derived from the left operandâs class. [\[4\]](https://docs.python.org/3/reference/datamodel.html#id23) For instance, to evaluate the expression `x - y`, where *y* is an instance of a class that has an `__rsub__()` method, `type(y).__rsub__(y, x)` is called if `type(x).__sub__(x, y)` returns [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented") or `type(y)` is a subclass of `type(x)`. [\[5\]](https://docs.python.org/3/reference/datamodel.html#id24)
Note that `__rpow__()` should be defined to accept an optional third argument if the three-argument version of the built-in [`pow()`](https://docs.python.org/3/library/functions.html#pow "pow") function is to be supported.
Changed in version 3.14: Three-argument [`pow()`](https://docs.python.org/3/library/functions.html#pow "pow") now try calling `__rpow__()` if necessary. Previously it was only called in two-argument `pow()` and the binary power operator.
Note
If the right operandâs type is a subclass of the left operandâs type and that subclass provides a different implementation of the reflected method for the operation, this method will be called before the left operandâs non-reflected method. This behavior allows subclasses to override their ancestorsâ operations.
object.\_\_iadd\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__iadd__ "Link to this definition")
object.\_\_isub\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__isub__ "Link to this definition")
object.\_\_imul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__imul__ "Link to this definition")
object.\_\_imatmul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__imatmul__ "Link to this definition")
object.\_\_itruediv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__itruediv__ "Link to this definition")
object.\_\_ifloordiv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ifloordiv__ "Link to this definition")
object.\_\_imod\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__imod__ "Link to this definition")
object.\_\_ipow\_\_(*self*, *other*\[, *modulo*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__ipow__ "Link to this definition")
object.\_\_ilshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ilshift__ "Link to this definition")
object.\_\_irshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__irshift__ "Link to this definition")
object.\_\_iand\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__iand__ "Link to this definition")
object.\_\_ixor\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ixor__ "Link to this definition")
object.\_\_ior\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ior__ "Link to this definition")
These methods are called to implement the augmented arithmetic assignments (`+=`, `-=`, `*=`, `@=`, `/=`, `//=`, `%=`, `**=`, `<<=`, `>>=`, `&=`, `^=`, `|=`). These methods should attempt to do the operation in-place (modifying *self*) and return the result (which could be, but does not have to be, *self*). If a specific method is not defined, or if that method returns [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented"), the augmented assignment falls back to the normal methods. For instance, if *x* is an instance of a class with an `__iadd__()` method, `x += y` is equivalent to `x = x.__iadd__(y)` . If `__iadd__()` does not exist, or if `x.__iadd__(y)` returns `NotImplemented`, `x.__add__(y)` and `y.__radd__(x)` are considered, as with the evaluation of `x + y`. In certain situations, augmented assignment can result in unexpected errors (see [Why does a\_tuple\[i\] += \[âitemâ\] raise an exception when the addition works?](https://docs.python.org/3/faq/programming.html#faq-augmented-assignment-tuple-error)), but this behavior is in fact part of the data model.
object.\_\_neg\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__neg__ "Link to this definition")
object.\_\_pos\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__pos__ "Link to this definition")
object.\_\_abs\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__abs__ "Link to this definition")
object.\_\_invert\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__invert__ "Link to this definition")
Called to implement the unary arithmetic operations (`-`, `+`, [`abs()`](https://docs.python.org/3/library/functions.html#abs "abs") and `~`).
object.\_\_complex\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__complex__ "Link to this definition")
object.\_\_int\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__int__ "Link to this definition")
object.\_\_float\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__float__ "Link to this definition")
Called to implement the built-in functions [`complex()`](https://docs.python.org/3/library/functions.html#complex "complex"), [`int()`](https://docs.python.org/3/library/functions.html#int "int") and [`float()`](https://docs.python.org/3/library/functions.html#float "float"). Should return a value of the appropriate type.
object.\_\_index\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__index__ "Link to this definition")
Called to implement [`operator.index()`](https://docs.python.org/3/library/operator.html#operator.index "operator.index"), and whenever Python needs to losslessly convert the numeric object to an integer object (such as in slicing, or in the built-in [`bin()`](https://docs.python.org/3/library/functions.html#bin "bin"), [`hex()`](https://docs.python.org/3/library/functions.html#hex "hex") and [`oct()`](https://docs.python.org/3/library/functions.html#oct "oct") functions). Presence of this method indicates that the numeric object is an integer type. Must return an integer.
If [`__int__()`](https://docs.python.org/3/reference/datamodel.html#object.__int__ "object.__int__"), [`__float__()`](https://docs.python.org/3/reference/datamodel.html#object.__float__ "object.__float__") and [`__complex__()`](https://docs.python.org/3/reference/datamodel.html#object.__complex__ "object.__complex__") are not defined then corresponding built-in functions [`int()`](https://docs.python.org/3/library/functions.html#int "int"), [`float()`](https://docs.python.org/3/library/functions.html#float "float") and [`complex()`](https://docs.python.org/3/library/functions.html#complex "complex") fall back to `__index__()`.
object.\_\_round\_\_(*self*\[, *ndigits*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__round__ "Link to this definition")
object.\_\_trunc\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__trunc__ "Link to this definition")
object.\_\_floor\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__floor__ "Link to this definition")
object.\_\_ceil\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ceil__ "Link to this definition")
Called to implement the built-in function [`round()`](https://docs.python.org/3/library/functions.html#round "round") and [`math`](https://docs.python.org/3/library/math.html#module-math "math: Mathematical functions (sin() etc.).") functions [`trunc()`](https://docs.python.org/3/library/math.html#math.trunc "math.trunc"), [`floor()`](https://docs.python.org/3/library/math.html#math.floor "math.floor") and [`ceil()`](https://docs.python.org/3/library/math.html#math.ceil "math.ceil"). Unless *ndigits* is passed to `__round__()` all these methods should return the value of the object truncated to an [`Integral`](https://docs.python.org/3/library/numbers.html#numbers.Integral "numbers.Integral") (typically an [`int`](https://docs.python.org/3/library/functions.html#int "int")).
Changed in version 3.14: [`int()`](https://docs.python.org/3/library/functions.html#int "int") no longer delegates to the `__trunc__()` method.
### 3\.3.9. With Statement Context Managers[¶](https://docs.python.org/3/reference/datamodel.html#with-statement-context-managers "Link to this heading")
A *context manager* is an object that defines the runtime context to be established when executing a [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement. The context manager handles the entry into, and the exit from, the desired runtime context for the execution of the block of code. Context managers are normally invoked using the `with` statement (described in section [The with statement](https://docs.python.org/3/reference/compound_stmts.html#with)), but can also be used by directly invoking their methods.
Typical uses of context managers include saving and restoring various kinds of global state, locking and unlocking resources, closing opened files, etc.
For more information on context managers, see [Context Manager Types](https://docs.python.org/3/library/stdtypes.html#typecontextmanager). The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide the context manager methods.
object.\_\_enter\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__enter__ "Link to this definition")
Enter the runtime context related to this object. The [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement will bind this methodâs return value to the target(s) specified in the `as` clause of the statement, if any.
object.\_\_exit\_\_(*self*, *exc\_type*, *exc\_value*, *traceback*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__exit__ "Link to this definition")
Exit the runtime context related to this object. The parameters describe the exception that caused the context to be exited. If the context was exited without an exception, all three arguments will be [`None`](https://docs.python.org/3/library/constants.html#None "None").
If an exception is supplied, and the method wishes to suppress the exception (i.e., prevent it from being propagated), it should return a true value. Otherwise, the exception will be processed normally upon exit from this method.
Note that `__exit__()` methods should not reraise the passed-in exception; this is the callerâs responsibility.
See also
[**PEP 343**](https://peps.python.org/pep-0343/) - The âwithâ statement
The specification, background, and examples for the Python [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement.
### 3\.3.10. Customizing positional arguments in class pattern matching[¶](https://docs.python.org/3/reference/datamodel.html#customizing-positional-arguments-in-class-pattern-matching "Link to this heading")
When using a class name in a pattern, positional arguments in the pattern are not allowed by default, i.e. `case MyClass(x, y)` is typically invalid without special support in `MyClass`. To be able to use that kind of pattern, the class needs to define a *\_\_match\_args\_\_* attribute.
object.\_\_match\_args\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__match_args__ "Link to this definition")
This class variable can be assigned a tuple of strings. When this class is used in a class pattern with positional arguments, each positional argument will be converted into a keyword argument, using the corresponding value in *\_\_match\_args\_\_* as the keyword. The absence of this attribute is equivalent to setting it to `()`.
For example, if `MyClass.__match_args__` is `("left", "center", "right")` that means that `case MyClass(x, y)` is equivalent to `case MyClass(left=x, center=y)`. Note that the number of arguments in the pattern must be smaller than or equal to the number of elements in *\_\_match\_args\_\_*; if it is larger, the pattern match attempt will raise a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError").
Added in version 3.10.
See also
[**PEP 634**](https://peps.python.org/pep-0634/) - Structural Pattern Matching
The specification for the Python `match` statement.
### 3\.3.11. Emulating buffer types[¶](https://docs.python.org/3/reference/datamodel.html#emulating-buffer-types "Link to this heading")
The [buffer protocol](https://docs.python.org/3/c-api/buffer.html#bufferobjects) provides a way for Python objects to expose efficient access to a low-level memory array. This protocol is implemented by builtin types such as [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") and [`memoryview`](https://docs.python.org/3/library/stdtypes.html#memoryview "memoryview"), and third-party libraries may define additional buffer types.
While buffer types are usually implemented in C, it is also possible to implement the protocol in Python.
object.\_\_buffer\_\_(*self*, *flags*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__buffer__ "Link to this definition")
Called when a buffer is requested from *self* (for example, by the [`memoryview`](https://docs.python.org/3/library/stdtypes.html#memoryview "memoryview") constructor). The *flags* argument is an integer representing the kind of buffer requested, affecting for example whether the returned buffer is read-only or writable. [`inspect.BufferFlags`](https://docs.python.org/3/library/inspect.html#inspect.BufferFlags "inspect.BufferFlags") provides a convenient way to interpret the flags. The method must return a `memoryview` object.
**Thread safety:** In [free-threaded](https://docs.python.org/3/glossary.html#term-free-threading) Python, implementations must manage any internal export counter using atomic operations. The method must be safe to call concurrently from multiple threads, and the returned bufferâs underlying data must remain valid until the corresponding [`__release_buffer__()`](https://docs.python.org/3/reference/datamodel.html#object.__release_buffer__ "object.__release_buffer__") call completes. See [Thread safety for memoryview objects](https://docs.python.org/3/library/threadsafety.html#thread-safety-memoryview) for details.
object.\_\_release\_buffer\_\_(*self*, *buffer*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__release_buffer__ "Link to this definition")
Called when a buffer is no longer needed. The *buffer* argument is a [`memoryview`](https://docs.python.org/3/library/stdtypes.html#memoryview "memoryview") object that was previously returned by [`__buffer__()`](https://docs.python.org/3/reference/datamodel.html#object.__buffer__ "object.__buffer__"). The method must release any resources associated with the buffer. This method should return `None`.
**Thread safety:** In [free-threaded](https://docs.python.org/3/glossary.html#term-free-threading) Python, any export counter decrement must use atomic operations. Resource cleanup must be thread-safe, as the final release may race with concurrent releases from other threads.
Buffer objects that do not need to perform any cleanup are not required to implement this method.
Added in version 3.12.
See also
[**PEP 688**](https://peps.python.org/pep-0688/) - Making the buffer protocol accessible in Python
Introduces the Python `__buffer__` and `__release_buffer__` methods.
[`collections.abc.Buffer`](https://docs.python.org/3/library/collections.abc.html#collections.abc.Buffer "collections.abc.Buffer")
ABC for buffer types.
### 3\.3.12. Annotations[¶](https://docs.python.org/3/reference/datamodel.html#annotations "Link to this heading")
Functions, classes, and modules may contain [annotations](https://docs.python.org/3/glossary.html#term-annotation), which are a way to associate information (usually [type hints](https://docs.python.org/3/glossary.html#term-type-hint)) with a symbol.
object.\_\_annotations\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__annotations__ "Link to this definition")
This attribute contains the annotations for an object. It is [lazily evaluated](https://docs.python.org/3/reference/executionmodel.html#lazy-evaluation), so accessing the attribute may execute arbitrary code and raise exceptions. If evaluation is successful, the attribute is set to a dictionary mapping from variable names to annotations.
Changed in version 3.14: Annotations are now lazily evaluated.
object.\_\_annotate\_\_(*format*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__annotate__ "Link to this definition")
An [annotate function](https://docs.python.org/3/glossary.html#term-annotate-function). Returns a new dictionary object mapping attribute/parameter names to their annotation values.
Takes a format parameter specifying the format in which annotations values should be provided. It must be a member of the [`annotationlib.Format`](https://docs.python.org/3/library/annotationlib.html#annotationlib.Format "annotationlib.Format") enum, or an integer with a value corresponding to a member of the enum.
If an annotate function doesnât support the requested format, it must raise [`NotImplementedError`](https://docs.python.org/3/library/exceptions.html#NotImplementedError "NotImplementedError"). Annotate functions must always support [`VALUE`](https://docs.python.org/3/library/annotationlib.html#annotationlib.Format.VALUE "annotationlib.Format.VALUE") format; they must not raise [`NotImplementedError()`](https://docs.python.org/3/library/exceptions.html#NotImplementedError "NotImplementedError") when called with this format.
When called with [`VALUE`](https://docs.python.org/3/library/annotationlib.html#annotationlib.Format.VALUE "annotationlib.Format.VALUE") format, an annotate function may raise [`NameError`](https://docs.python.org/3/library/exceptions.html#NameError "NameError"); it must not raise `NameError` when called requesting any other format.
If an object does not have any annotations, [`__annotate__`](https://docs.python.org/3/reference/datamodel.html#object.__annotate__ "object.__annotate__") should preferably be set to `None` (it canât be deleted), rather than set to a function that returns an empty dict.
Added in version 3.14.
See also
[**PEP 649**](https://peps.python.org/pep-0649/) â Deferred evaluation of annotation using descriptors
Introduces lazy evaluation of annotations and the `__annotate__` function.
### 3\.3.13. Special method lookup[¶](https://docs.python.org/3/reference/datamodel.html#special-method-lookup "Link to this heading")
For custom classes, implicit invocations of special methods are only guaranteed to work correctly if defined on an objectâs type, not in the objectâs instance dictionary. That behaviour is the reason why the following code raises an exception:
Copy
```
>>> class C:
... pass
...
>>> c = C()
>>> c.__len__ = lambda: 5
>>> len(c)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type 'C' has no len()
```
The rationale behind this behaviour lies with a number of special methods such as [`__hash__()`](https://docs.python.org/3/reference/datamodel.html#object.__hash__ "object.__hash__") and [`__repr__()`](https://docs.python.org/3/reference/datamodel.html#object.__repr__ "object.__repr__") that are implemented by all objects, including type objects. If the implicit lookup of these methods used the conventional lookup process, they would fail when invoked on the type object itself:
Copy
```
>>> 1 .__hash__() == hash(1)
True
>>> int.__hash__() == hash(int)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: descriptor '__hash__' of 'int' object needs an argument
```
Incorrectly attempting to invoke an unbound method of a class in this way is sometimes referred to as âmetaclass confusionâ, and is avoided by bypassing the instance when looking up special methods:
Copy
```
>>> type(1).__hash__(1) == hash(1)
True
>>> type(int).__hash__(int) == hash(int)
True
```
In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the [`__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__") method even of the objectâs metaclass:
Copy
```
>>> class Meta(type):
... def __getattribute__(*args):
... print("Metaclass getattribute invoked")
... return type.__getattribute__(*args)
...
>>> class C(object, metaclass=Meta):
... def __len__(self):
... return 10
... def __getattribute__(*args):
... print("Class getattribute invoked")
... return object.__getattribute__(*args)
...
>>> c = C()
>>> c.__len__() # Explicit lookup via instance
Class getattribute invoked
10
>>> type(c).__len__(c) # Explicit lookup via type
Metaclass getattribute invoked
10
>>> len(c) # Implicit lookup
10
```
Bypassing the [`__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__") machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method *must* be set on the class object itself in order to be consistently invoked by the interpreter).
## 3\.4. Coroutines[¶](https://docs.python.org/3/reference/datamodel.html#coroutines "Link to this heading")
### 3\.4.1. Awaitable Objects[¶](https://docs.python.org/3/reference/datamodel.html#awaitable-objects "Link to this heading")
An [awaitable](https://docs.python.org/3/glossary.html#term-awaitable) object generally implements an [`__await__()`](https://docs.python.org/3/reference/datamodel.html#object.__await__ "object.__await__") method. [Coroutine objects](https://docs.python.org/3/glossary.html#term-coroutine) returned from [`async def`](https://docs.python.org/3/reference/compound_stmts.html#async-def) functions are awaitable.
Note
The [generator iterator](https://docs.python.org/3/glossary.html#term-generator-iterator) objects returned from generators decorated with [`types.coroutine()`](https://docs.python.org/3/library/types.html#types.coroutine "types.coroutine") are also awaitable, but they do not implement [`__await__()`](https://docs.python.org/3/reference/datamodel.html#object.__await__ "object.__await__").
object.\_\_await\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__await__ "Link to this definition")
Must return an [iterator](https://docs.python.org/3/glossary.html#term-iterator). Should be used to implement [awaitable](https://docs.python.org/3/glossary.html#term-awaitable) objects. For instance, [`asyncio.Future`](https://docs.python.org/3/library/asyncio-future.html#asyncio.Future "asyncio.Future") implements this method to be compatible with the [`await`](https://docs.python.org/3/reference/expressions.html#await) expression. The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself is not awaitable and does not provide this method.
Note
The language doesnât place any restriction on the type or value of the objects yielded by the iterator returned by `__await__`, as this is specific to the implementation of the asynchronous execution framework (e.g. [`asyncio`](https://docs.python.org/3/library/asyncio.html#module-asyncio "asyncio: Asynchronous I/O.")) that will be managing the [awaitable](https://docs.python.org/3/glossary.html#term-awaitable) object.
Added in version 3.5.
See also
[**PEP 492**](https://peps.python.org/pep-0492/) for additional information about awaitable objects.
### 3\.4.2. Coroutine Objects[¶](https://docs.python.org/3/reference/datamodel.html#coroutine-objects "Link to this heading")
[Coroutine objects](https://docs.python.org/3/glossary.html#term-coroutine) are [awaitable](https://docs.python.org/3/glossary.html#term-awaitable) objects. A coroutineâs execution can be controlled by calling [`__await__()`](https://docs.python.org/3/reference/datamodel.html#object.__await__ "object.__await__") and iterating over the result. When the coroutine has finished executing and returns, the iterator raises [`StopIteration`](https://docs.python.org/3/library/exceptions.html#StopIteration "StopIteration"), and the exceptionâs [`value`](https://docs.python.org/3/library/exceptions.html#StopIteration.value "StopIteration.value") attribute holds the return value. If the coroutine raises an exception, it is propagated by the iterator. Coroutines should not directly raise unhandled `StopIteration` exceptions.
Coroutines also have the methods listed below, which are analogous to those of generators (see [Generator-iterator methods](https://docs.python.org/3/reference/expressions.html#generator-methods)). However, unlike generators, coroutines do not directly support iteration.
Changed in version 3.5.2: It is a [`RuntimeError`](https://docs.python.org/3/library/exceptions.html#RuntimeError "RuntimeError") to await on a coroutine more than once.
coroutine.send(*value*)[¶](https://docs.python.org/3/reference/datamodel.html#coroutine.send "Link to this definition")
Starts or resumes execution of the coroutine. If *value* is `None`, this is equivalent to advancing the iterator returned by [`__await__()`](https://docs.python.org/3/reference/datamodel.html#object.__await__ "object.__await__"). If *value* is not `None`, this method delegates to the [`send()`](https://docs.python.org/3/reference/expressions.html#generator.send "generator.send") method of the iterator that caused the coroutine to suspend. The result (return value, [`StopIteration`](https://docs.python.org/3/library/exceptions.html#StopIteration "StopIteration"), or other exception) is the same as when iterating over the `__await__()` return value, described above.
coroutine.throw(*value*)[¶](https://docs.python.org/3/reference/datamodel.html#coroutine.throw "Link to this definition")
coroutine.throw(*type*\[, *value*\[, *traceback*\]\])
Raises the specified exception in the coroutine. This method delegates to the [`throw()`](https://docs.python.org/3/reference/expressions.html#generator.throw "generator.throw") method of the iterator that caused the coroutine to suspend, if it has such a method. Otherwise, the exception is raised at the suspension point. The result (return value, [`StopIteration`](https://docs.python.org/3/library/exceptions.html#StopIteration "StopIteration"), or other exception) is the same as when iterating over the [`__await__()`](https://docs.python.org/3/reference/datamodel.html#object.__await__ "object.__await__") return value, described above. If the exception is not caught in the coroutine, it propagates back to the caller.
Changed in version 3.12: The second signature (type\[, value\[, traceback\]\]) is deprecated and may be removed in a future version of Python.
coroutine.close()[¶](https://docs.python.org/3/reference/datamodel.html#coroutine.close "Link to this definition")
Causes the coroutine to clean itself up and exit. If the coroutine is suspended, this method first delegates to the [`close()`](https://docs.python.org/3/reference/expressions.html#generator.close "generator.close") method of the iterator that caused the coroutine to suspend, if it has such a method. Then it raises [`GeneratorExit`](https://docs.python.org/3/library/exceptions.html#GeneratorExit "GeneratorExit") at the suspension point, causing the coroutine to immediately clean itself up. Finally, the coroutine is marked as having finished executing, even if it was never started.
Coroutine objects are automatically closed using the above process when they are about to be destroyed.
### 3\.4.3. Asynchronous Iterators[¶](https://docs.python.org/3/reference/datamodel.html#asynchronous-iterators "Link to this heading")
An *asynchronous iterator* can call asynchronous code in its `__anext__` method.
Asynchronous iterators can be used in an [`async for`](https://docs.python.org/3/reference/compound_stmts.html#async-for) statement.
The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide these methods.
object.\_\_aiter\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__aiter__ "Link to this definition")
Must return an *asynchronous iterator* object.
object.\_\_anext\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__anext__ "Link to this definition")
Must return an *awaitable* resulting in a next value of the iterator. Should raise a [`StopAsyncIteration`](https://docs.python.org/3/library/exceptions.html#StopAsyncIteration "StopAsyncIteration") error when the iteration is over.
An example of an asynchronous iterable object:
Copy
```
class Reader:
async def readline(self):
...
def __aiter__(self):
return self
async def __anext__(self):
val = await self.readline()
if val == b'':
raise StopAsyncIteration
return val
```
Added in version 3.5.
Changed in version 3.7: Prior to Python 3.7, [`__aiter__()`](https://docs.python.org/3/reference/datamodel.html#object.__aiter__ "object.__aiter__") could return an *awaitable* that would resolve to an [asynchronous iterator](https://docs.python.org/3/glossary.html#term-asynchronous-iterator).
Starting with Python 3.7, [`__aiter__()`](https://docs.python.org/3/reference/datamodel.html#object.__aiter__ "object.__aiter__") must return an asynchronous iterator object. Returning anything else will result in a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") error.
### 3\.4.4. Asynchronous Context Managers[¶](https://docs.python.org/3/reference/datamodel.html#asynchronous-context-managers "Link to this heading")
An *asynchronous context manager* is a *context manager* that is able to suspend execution in its `__aenter__` and `__aexit__` methods.
Asynchronous context managers can be used in an [`async with`](https://docs.python.org/3/reference/compound_stmts.html#async-with) statement.
The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide these methods.
object.\_\_aenter\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__aenter__ "Link to this definition")
Semantically similar to [`__enter__()`](https://docs.python.org/3/reference/datamodel.html#object.__enter__ "object.__enter__"), the only difference being that it must return an *awaitable*.
object.\_\_aexit\_\_(*self*, *exc\_type*, *exc\_value*, *traceback*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__aexit__ "Link to this definition")
Semantically similar to [`__exit__()`](https://docs.python.org/3/reference/datamodel.html#object.__exit__ "object.__exit__"), the only difference being that it must return an *awaitable*.
An example of an asynchronous context manager class:
Copy
```
class AsyncContextManager:
async def __aenter__(self):
await log('entering context')
async def __aexit__(self, exc_type, exc, tb):
await log('exiting context')
```
Added in version 3.5.
Footnotes
\[[1](https://docs.python.org/3/reference/datamodel.html#id1)\]
It *is* possible in some cases to change an objectâs type, under certain controlled conditions. It generally isnât a good idea though, since it can lead to some very strange behaviour if it is handled incorrectly.
\[[2](https://docs.python.org/3/reference/datamodel.html#id12)\]
The [`__hash__()`](https://docs.python.org/3/reference/datamodel.html#object.__hash__ "object.__hash__"), [`__iter__()`](https://docs.python.org/3/reference/datamodel.html#object.__iter__ "object.__iter__"), [`__reversed__()`](https://docs.python.org/3/reference/datamodel.html#object.__reversed__ "object.__reversed__"), [`__contains__()`](https://docs.python.org/3/reference/datamodel.html#object.__contains__ "object.__contains__"), [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") and [`__fspath__()`](https://docs.python.org/3/library/os.html#os.PathLike.__fspath__ "os.PathLike.__fspath__") methods have special handling for this. Others will still raise a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError"), but may do so by relying on the behavior that `None` is not callable.
\[[3](https://docs.python.org/3/reference/datamodel.html#id16)\]
âDoes not supportâ here means that the class has no such method, or the method returns [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented"). Do not set the method to `None` if you want to force fallback to the right operandâs reflected methodâthat will instead have the opposite effect of explicitly *blocking* such fallback.
\[[4](https://docs.python.org/3/reference/datamodel.html#id17)\]
For operands of the same type, it is assumed that if the non-reflected method (such as [`__add__()`](https://docs.python.org/3/reference/datamodel.html#object.__add__ "object.__add__")) fails then the operation is not supported, which is why the reflected method is not called.
\[[5](https://docs.python.org/3/reference/datamodel.html#id18)\]
If the right operandâs type is a subclass of the left operandâs type, the reflected method having precedence allows subclasses to override their ancestorsâ operations.
### [Table of Contents](https://docs.python.org/3/contents.html)
- [3\. Data model](https://docs.python.org/3/reference/datamodel.html)
- [3\.1. Objects, values and types](https://docs.python.org/3/reference/datamodel.html#objects-values-and-types)
- [3\.2. The standard type hierarchy](https://docs.python.org/3/reference/datamodel.html#the-standard-type-hierarchy)
- [3\.2.1. None](https://docs.python.org/3/reference/datamodel.html#none)
- [3\.2.2. NotImplemented](https://docs.python.org/3/reference/datamodel.html#notimplemented)
- [3\.2.3. Ellipsis](https://docs.python.org/3/reference/datamodel.html#ellipsis)
- [3\.2.4. `numbers.Number`](https://docs.python.org/3/reference/datamodel.html#numbers-number)
- [3\.2.4.1. `numbers.Integral`](https://docs.python.org/3/reference/datamodel.html#numbers-integral)
- [3\.2.4.2. `numbers.Real` (`float`)](https://docs.python.org/3/reference/datamodel.html#numbers-real-float)
- [3\.2.4.3. `numbers.Complex` (`complex`)](https://docs.python.org/3/reference/datamodel.html#numbers-complex-complex)
- [3\.2.5. Sequences](https://docs.python.org/3/reference/datamodel.html#sequences)
- [3\.2.5.1. Immutable sequences](https://docs.python.org/3/reference/datamodel.html#immutable-sequences)
- [3\.2.5.2. Mutable sequences](https://docs.python.org/3/reference/datamodel.html#mutable-sequences)
- [3\.2.6. Set types](https://docs.python.org/3/reference/datamodel.html#set-types)
- [3\.2.7. Mappings](https://docs.python.org/3/reference/datamodel.html#mappings)
- [3\.2.7.1. Dictionaries](https://docs.python.org/3/reference/datamodel.html#dictionaries)
- [3\.2.8. Callable types](https://docs.python.org/3/reference/datamodel.html#callable-types)
- [3\.2.8.1. User-defined functions](https://docs.python.org/3/reference/datamodel.html#user-defined-functions)
- [3\.2.8.1.1. Special read-only attributes](https://docs.python.org/3/reference/datamodel.html#special-read-only-attributes)
- [3\.2.8.1.2. Special writable attributes](https://docs.python.org/3/reference/datamodel.html#special-writable-attributes)
- [3\.2.8.2. Instance methods](https://docs.python.org/3/reference/datamodel.html#instance-methods)
- [3\.2.8.3. Generator functions](https://docs.python.org/3/reference/datamodel.html#generator-functions)
- [3\.2.8.4. Coroutine functions](https://docs.python.org/3/reference/datamodel.html#coroutine-functions)
- [3\.2.8.5. Asynchronous generator functions](https://docs.python.org/3/reference/datamodel.html#asynchronous-generator-functions)
- [3\.2.8.6. Built-in functions](https://docs.python.org/3/reference/datamodel.html#built-in-functions)
- [3\.2.8.7. Built-in methods](https://docs.python.org/3/reference/datamodel.html#built-in-methods)
- [3\.2.8.8. Classes](https://docs.python.org/3/reference/datamodel.html#classes)
- [3\.2.8.9. Class Instances](https://docs.python.org/3/reference/datamodel.html#class-instances)
- [3\.2.9. Modules](https://docs.python.org/3/reference/datamodel.html#modules)
- [3\.2.9.1. Import-related attributes on module objects](https://docs.python.org/3/reference/datamodel.html#import-related-attributes-on-module-objects)
- [3\.2.9.2. Other writable attributes on module objects](https://docs.python.org/3/reference/datamodel.html#other-writable-attributes-on-module-objects)
- [3\.2.9.3. Module dictionaries](https://docs.python.org/3/reference/datamodel.html#module-dictionaries)
- [3\.2.10. Custom classes](https://docs.python.org/3/reference/datamodel.html#custom-classes)
- [3\.2.10.1. Special attributes](https://docs.python.org/3/reference/datamodel.html#special-attributes)
- [3\.2.10.2. Special methods](https://docs.python.org/3/reference/datamodel.html#special-methods)
- [3\.2.11. Class instances](https://docs.python.org/3/reference/datamodel.html#id4)
- [3\.2.11.1. Special attributes](https://docs.python.org/3/reference/datamodel.html#id5)
- [3\.2.12. I/O objects (also known as file objects)](https://docs.python.org/3/reference/datamodel.html#i-o-objects-also-known-as-file-objects)
- [3\.2.13. Internal types](https://docs.python.org/3/reference/datamodel.html#internal-types)
- [3\.2.13.1. Code objects](https://docs.python.org/3/reference/datamodel.html#code-objects)
- [3\.2.13.1.1. Special read-only attributes](https://docs.python.org/3/reference/datamodel.html#index-64)
- [3\.2.13.1.2. Methods on code objects](https://docs.python.org/3/reference/datamodel.html#methods-on-code-objects)
- [3\.2.13.2. Frame objects](https://docs.python.org/3/reference/datamodel.html#frame-objects)
- [3\.2.13.2.1. Special read-only attributes](https://docs.python.org/3/reference/datamodel.html#index-70)
- [3\.2.13.2.2. Special writable attributes](https://docs.python.org/3/reference/datamodel.html#index-71)
- [3\.2.13.2.3. Frame object methods](https://docs.python.org/3/reference/datamodel.html#frame-object-methods)
- [3\.2.13.3. Traceback objects](https://docs.python.org/3/reference/datamodel.html#traceback-objects)
- [3\.2.13.4. Slice objects](https://docs.python.org/3/reference/datamodel.html#slice-objects)
- [3\.2.13.5. Static method objects](https://docs.python.org/3/reference/datamodel.html#static-method-objects)
- [3\.2.13.6. Class method objects](https://docs.python.org/3/reference/datamodel.html#class-method-objects)
- [3\.3. Special method names](https://docs.python.org/3/reference/datamodel.html#special-method-names)
- [3\.3.1. Basic customization](https://docs.python.org/3/reference/datamodel.html#basic-customization)
- [3\.3.2. Customizing attribute access](https://docs.python.org/3/reference/datamodel.html#customizing-attribute-access)
- [3\.3.2.1. Customizing module attribute access](https://docs.python.org/3/reference/datamodel.html#customizing-module-attribute-access)
- [3\.3.2.2. Implementing Descriptors](https://docs.python.org/3/reference/datamodel.html#implementing-descriptors)
- [3\.3.2.3. Invoking Descriptors](https://docs.python.org/3/reference/datamodel.html#invoking-descriptors)
- [3\.3.2.4. \_\_slots\_\_](https://docs.python.org/3/reference/datamodel.html#slots)
- [3\.3.3. Customizing class creation](https://docs.python.org/3/reference/datamodel.html#customizing-class-creation)
- [3\.3.3.1. Metaclasses](https://docs.python.org/3/reference/datamodel.html#metaclasses)
- [3\.3.3.2. Resolving MRO entries](https://docs.python.org/3/reference/datamodel.html#resolving-mro-entries)
- [3\.3.3.3. Determining the appropriate metaclass](https://docs.python.org/3/reference/datamodel.html#determining-the-appropriate-metaclass)
- [3\.3.3.4. Preparing the class namespace](https://docs.python.org/3/reference/datamodel.html#preparing-the-class-namespace)
- [3\.3.3.5. Executing the class body](https://docs.python.org/3/reference/datamodel.html#executing-the-class-body)
- [3\.3.3.6. Creating the class object](https://docs.python.org/3/reference/datamodel.html#creating-the-class-object)
- [3\.3.3.7. Uses for metaclasses](https://docs.python.org/3/reference/datamodel.html#uses-for-metaclasses)
- [3\.3.4. Customizing instance and subclass checks](https://docs.python.org/3/reference/datamodel.html#customizing-instance-and-subclass-checks)
- [3\.3.5. Emulating generic types](https://docs.python.org/3/reference/datamodel.html#emulating-generic-types)
- [3\.3.5.1. The purpose of *\_\_class\_getitem\_\_*](https://docs.python.org/3/reference/datamodel.html#the-purpose-of-class-getitem)
- [3\.3.5.2. *\_\_class\_getitem\_\_* versus *\_\_getitem\_\_*](https://docs.python.org/3/reference/datamodel.html#class-getitem-versus-getitem)
- [3\.3.6. Emulating callable objects](https://docs.python.org/3/reference/datamodel.html#emulating-callable-objects)
- [3\.3.7. Emulating container types](https://docs.python.org/3/reference/datamodel.html#emulating-container-types)
- [3\.3.8. Emulating numeric types](https://docs.python.org/3/reference/datamodel.html#emulating-numeric-types)
- [3\.3.9. With Statement Context Managers](https://docs.python.org/3/reference/datamodel.html#with-statement-context-managers)
- [3\.3.10. Customizing positional arguments in class pattern matching](https://docs.python.org/3/reference/datamodel.html#customizing-positional-arguments-in-class-pattern-matching)
- [3\.3.11. Emulating buffer types](https://docs.python.org/3/reference/datamodel.html#emulating-buffer-types)
- [3\.3.12. Annotations](https://docs.python.org/3/reference/datamodel.html#annotations)
- [3\.3.13. Special method lookup](https://docs.python.org/3/reference/datamodel.html#special-method-lookup)
- [3\.4. Coroutines](https://docs.python.org/3/reference/datamodel.html#coroutines)
- [3\.4.1. Awaitable Objects](https://docs.python.org/3/reference/datamodel.html#awaitable-objects)
- [3\.4.2. Coroutine Objects](https://docs.python.org/3/reference/datamodel.html#coroutine-objects)
- [3\.4.3. Asynchronous Iterators](https://docs.python.org/3/reference/datamodel.html#asynchronous-iterators)
- [3\.4.4. Asynchronous Context Managers](https://docs.python.org/3/reference/datamodel.html#asynchronous-context-managers)
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| Readable Markdown | ## 3\.1. Objects, values and types[¶](https://docs.python.org/3/reference/datamodel.html#objects-values-and-types "Link to this heading")
*Objects* are Pythonâs abstraction for data. All data in a Python program is represented by objects or by relations between objects. Even code is represented by objects.
Every object has an identity, a type and a value. An objectâs *identity* never changes once it has been created; you may think of it as the objectâs address in memory. The [`is`](https://docs.python.org/3/reference/expressions.html#is) operator compares the identity of two objects; the [`id()`](https://docs.python.org/3/library/functions.html#id "id") function returns an integer representing its identity.
**CPython implementation detail:** For CPython, `id(x)` is the memory address where `x` is stored.
An objectâs type determines the operations that the object supports (e.g., âdoes it have a length?â) and also defines the possible values for objects of that type. The [`type()`](https://docs.python.org/3/library/functions.html#type "type") function returns an objectâs type (which is an object itself). Like its identity, an objectâs *type* is also unchangeable. [\[1\]](https://docs.python.org/3/reference/datamodel.html#id20)
The *value* of some objects can change. Objects whose value can change are said to be *mutable*; objects whose value is unchangeable once they are created are called *immutable*. (The value of an immutable container object that contains a reference to a mutable object can change when the latterâs value is changed; however the container is still considered immutable, because the collection of objects it contains cannot be changed. So, immutability is not strictly the same as having an unchangeable value, it is more subtle.) An objectâs mutability is determined by its type; for instance, numbers, strings and tuples are immutable, while dictionaries and lists are mutable.
Objects are never explicitly destroyed; however, when they become unreachable they may be garbage-collected. An implementation is allowed to postpone garbage collection or omit it altogether â it is a matter of implementation quality how garbage collection is implemented, as long as no objects are collected that are still reachable.
**CPython implementation detail:** CPython currently uses a reference-counting scheme with (optional) delayed detection of cyclically linked garbage, which collects most objects as soon as they become unreachable, but is not guaranteed to collect garbage containing circular references. See the documentation of the [`gc`](https://docs.python.org/3/library/gc.html#module-gc "gc: Interface to the cycle-detecting garbage collector.") module for information on controlling the collection of cyclic garbage. Other implementations act differently and CPython may change. Do not depend on immediate finalization of objects when they become unreachable (so you should always close files explicitly).
Note that the use of the implementationâs tracing or debugging facilities may keep objects alive that would normally be collectable. Also note that catching an exception with a [`try`](https://docs.python.org/3/reference/compound_stmts.html#try)âŠ[`except`](https://docs.python.org/3/reference/compound_stmts.html#except) statement may keep objects alive.
Some objects contain references to âexternalâ resources such as open files or windows. It is understood that these resources are freed when the object is garbage-collected, but since garbage collection is not guaranteed to happen, such objects also provide an explicit way to release the external resource, usually a `close()` method. Programs are strongly recommended to explicitly close such objects. The [`try`](https://docs.python.org/3/reference/compound_stmts.html#try)âŠ[`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) statement and the [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement provide convenient ways to do this.
Some objects contain references to other objects; these are called *containers*. Examples of containers are tuples, lists and dictionaries. The references are part of a containerâs value. In most cases, when we talk about the value of a container, we imply the values, not the identities of the contained objects; however, when we talk about the mutability of a container, only the identities of the immediately contained objects are implied. So, if an immutable container (like a tuple) contains a reference to a mutable object, its value changes if that mutable object is changed.
Types affect almost all aspects of object behavior. Even the importance of object identity is affected in some sense: for immutable types, operations that compute new values may actually return a reference to any existing object with the same type and value, while for mutable objects this is not allowed. For example, after `a = 1; b = 1`, *a* and *b* may or may not refer to the same object with the value one, depending on the implementation. This is because [`int`](https://docs.python.org/3/library/functions.html#int "int") is an immutable type, so the reference to `1` can be reused. This behaviour depends on the implementation used, so should not be relied upon, but is something to be aware of when making use of object identity tests. However, after `c = []; d = []`, *c* and *d* are guaranteed to refer to two different, unique, newly created empty lists. (Note that `e = f = []` assigns the *same* object to both *e* and *f*.)
## 3\.2. The standard type hierarchy[¶](https://docs.python.org/3/reference/datamodel.html#the-standard-type-hierarchy "Link to this heading")
Below is a list of the types that are built into Python. Extension modules (written in C, Java, or other languages, depending on the implementation) can define additional types. Future versions of Python may add types to the type hierarchy (e.g., rational numbers, efficiently stored arrays of integers, etc.), although such additions will often be provided via the standard library instead.
Some of the type descriptions below contain a paragraph listing âspecial attributes.â These are attributes that provide access to the implementation and are not intended for general use. Their definition may change in the future.
### 3\.2.1. None[¶](https://docs.python.org/3/reference/datamodel.html#none "Link to this heading")
This type has a single value. There is a single object with this value. This object is accessed through the built-in name `None`. It is used to signify the absence of a value in many situations, e.g., it is returned from functions that donât explicitly return anything. Its truth value is false.
### 3\.2.2. NotImplemented[¶](https://docs.python.org/3/reference/datamodel.html#notimplemented "Link to this heading")
This type has a single value. There is a single object with this value. This object is accessed through the built-in name [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented"). Numeric methods and rich comparison methods should return this value if they do not implement the operation for the operands provided. (The interpreter will then try the reflected operation, or some other fallback, depending on the operator.) It should not be evaluated in a boolean context.
See [Implementing the arithmetic operations](https://docs.python.org/3/library/numbers.html#implementing-the-arithmetic-operations) for more details.
Changed in version 3.9: Evaluating [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented") in a boolean context was deprecated.
### 3\.2.3. Ellipsis[¶](https://docs.python.org/3/reference/datamodel.html#ellipsis "Link to this heading")
This type has a single value. There is a single object with this value. This object is accessed through the literal `...` or the built-in name `Ellipsis`. Its truth value is true.
### 3\.2.4. [`numbers.Number`](https://docs.python.org/3/library/numbers.html#numbers.Number "numbers.Number")[¶](https://docs.python.org/3/reference/datamodel.html#numbers-number "Link to this heading")
These are created by numeric literals and returned as results by arithmetic operators and arithmetic built-in functions. Numeric objects are immutable; once created their value never changes. Python numbers are of course strongly related to mathematical numbers, but subject to the limitations of numerical representation in computers.
The string representations of the numeric classes, computed by [`__repr__()`](https://docs.python.org/3/reference/datamodel.html#object.__repr__ "object.__repr__") and [`__str__()`](https://docs.python.org/3/reference/datamodel.html#object.__str__ "object.__str__"), have the following properties:
- They are valid numeric literals which, when passed to their class constructor, produce an object having the value of the original numeric.
- The representation is in base 10, when possible.
- Leading zeros, possibly excepting a single zero before a decimal point, are not shown.
- Trailing zeros, possibly excepting a single zero after a decimal point, are not shown.
- A sign is shown only when the number is negative.
Python distinguishes between integers, floating-point numbers, and complex numbers:
#### 3\.2.4.1. [`numbers.Integral`](https://docs.python.org/3/library/numbers.html#numbers.Integral "numbers.Integral")[¶](https://docs.python.org/3/reference/datamodel.html#numbers-integral "Link to this heading")
These represent elements from the mathematical set of integers (positive and negative).
Note
The rules for integer representation are intended to give the most meaningful interpretation of shift and mask operations involving negative integers.
There are two types of integers:
Integers ([`int`](https://docs.python.org/3/library/functions.html#int "int"))
These represent numbers in an unlimited range, subject to available (virtual) memory only. For the purpose of shift and mask operations, a binary representation is assumed, and negative numbers are represented in a variant of 2âs complement which gives the illusion of an infinite string of sign bits extending to the left.
Booleans ([`bool`](https://docs.python.org/3/library/functions.html#bool "bool"))
These represent the truth values False and True. The two objects representing the values `False` and `True` are the only Boolean objects. The Boolean type is a subtype of the integer type, and Boolean values behave like the values 0 and 1, respectively, in almost all contexts, the exception being that when converted to a string, the strings `"False"` or `"True"` are returned, respectively.
#### 3\.2.4.2. [`numbers.Real`](https://docs.python.org/3/library/numbers.html#numbers.Real "numbers.Real") ([`float`](https://docs.python.org/3/library/functions.html#float "float"))[¶](https://docs.python.org/3/reference/datamodel.html#numbers-real-float "Link to this heading")
These represent machine-level double precision floating-point numbers. You are at the mercy of the underlying machine architecture (and C or Java implementation) for the accepted range and handling of overflow. Python does not support single-precision floating-point numbers; the savings in processor and memory usage that are usually the reason for using these are dwarfed by the overhead of using objects in Python, so there is no reason to complicate the language with two kinds of floating-point numbers.
#### 3\.2.4.3. [`numbers.Complex`](https://docs.python.org/3/library/numbers.html#numbers.Complex "numbers.Complex") ([`complex`](https://docs.python.org/3/library/functions.html#complex "complex"))[¶](https://docs.python.org/3/reference/datamodel.html#numbers-complex-complex "Link to this heading")
These represent complex numbers as a pair of machine-level double precision floating-point numbers. The same caveats apply as for floating-point numbers. The real and imaginary parts of a complex number `z` can be retrieved through the read-only attributes `z.real` and `z.imag`.
### 3\.2.5. Sequences[¶](https://docs.python.org/3/reference/datamodel.html#sequences "Link to this heading")
These represent finite ordered sets indexed by non-negative numbers. The built-in function [`len()`](https://docs.python.org/3/library/functions.html#len "len") returns the number of items of a sequence. When the length of a sequence is *n*, the index set contains the numbers 0, 1, âŠ, *n*\-1. Item *i* of sequence *a* is selected by `a[i]`. Some sequences, including built-in sequences, interpret negative subscripts by adding the sequence length. For example, `a[-2]` equals `a[n-2]`, the second to last item of sequence a with length `n`.
The resulting value must be a nonnegative integer less than the number of items in the sequence. If it is not, an [`IndexError`](https://docs.python.org/3/library/exceptions.html#IndexError "IndexError") is raised.
Sequences also support slicing: `a[start:stop]` selects all items with index *k* such that *start* `<=` *k* `<` *stop*. When used as an expression, a slice is a sequence of the same type. The comment above about negative subscripts also applies to negative slice positions. Note that no error is raised if a slice position is less than zero or larger than the length of the sequence.
If *start* is missing or [`None`](https://docs.python.org/3/library/constants.html#None "None"), slicing behaves as if *start* was zero. If *stop* is missing or `None`, slicing behaves as if *stop* was equal to the length of the sequence.
Some sequences also support âextended slicingâ with a third âstepâ parameter: `a[i:j:k]` selects all items of *a* with index *x* where `x = i + n*k`, *n* `>=` `0` and *i* `<=` *x* `<` *j*.
Sequences are distinguished according to their mutability:
#### 3\.2.5.1. Immutable sequences[¶](https://docs.python.org/3/reference/datamodel.html#immutable-sequences "Link to this heading")
An object of an immutable sequence type cannot change once it is created. (If the object contains references to other objects, these other objects may be mutable and may be changed; however, the collection of objects directly referenced by an immutable object cannot change.)
The following types are immutable sequences:
Strings
A string ([`str`](https://docs.python.org/3/library/stdtypes.html#str "str")) is a sequence of values that represent *characters*, or more formally, *Unicode code points*. All the code points in the range `0` to `0x10FFFF` can be represented in a string.
Python doesnât have a dedicated *character* type. Instead, every code point in the string is represented as a string object with length `1`.
The built-in function [`ord()`](https://docs.python.org/3/library/functions.html#ord "ord") converts a code point from its string form to an integer in the range `0` to `0x10FFFF`; [`chr()`](https://docs.python.org/3/library/functions.html#chr "chr") converts an integer in the range `0` to `0x10FFFF` to the corresponding length `1` string object. [`str.encode()`](https://docs.python.org/3/library/stdtypes.html#str.encode "str.encode") can be used to convert a [`str`](https://docs.python.org/3/library/stdtypes.html#str "str") to [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") using the given text encoding, and [`bytes.decode()`](https://docs.python.org/3/library/stdtypes.html#bytes.decode "bytes.decode") can be used to achieve the opposite.
Tuples
The items of a [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") are arbitrary Python objects. Tuples of two or more items are formed by comma-separated lists of expressions. A tuple of one item (a âsingletonâ) can be formed by affixing a comma to an expression (an expression by itself does not create a tuple, since parentheses must be usable for grouping of expressions). An empty tuple can be formed by an empty pair of parentheses.
Bytes
A [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") object is an immutable array. The items are 8-bit bytes, represented by integers in the range 0 \<= x \< 256. Bytes literals (like `b'abc'`) and the built-in [`bytes()`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") constructor can be used to create bytes objects. Also, bytes objects can be decoded to strings via the [`decode()`](https://docs.python.org/3/library/stdtypes.html#bytes.decode "bytes.decode") method.
#### 3\.2.5.2. Mutable sequences[¶](https://docs.python.org/3/reference/datamodel.html#mutable-sequences "Link to this heading")
Mutable sequences can be changed after they are created. The subscription and slicing notations can be used as the target of assignment and [`del`](https://docs.python.org/3/reference/simple_stmts.html#del) (delete) statements.
Note
The [`collections`](https://docs.python.org/3/library/collections.html#module-collections "collections: Container datatypes") and [`array`](https://docs.python.org/3/library/array.html#module-array "array: Space efficient arrays of uniformly typed numeric values.") module provide additional examples of mutable sequence types.
There are currently two intrinsic mutable sequence types:
Lists
The items of a list are arbitrary Python objects. Lists are formed by placing a comma-separated list of expressions in square brackets. (Note that there are no special cases needed to form lists of length 0 or 1.)
Byte Arrays
A bytearray object is a mutable array. They are created by the built-in [`bytearray()`](https://docs.python.org/3/library/stdtypes.html#bytearray "bytearray") constructor. Aside from being mutable (and hence unhashable), byte arrays otherwise provide the same interface and functionality as immutable [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") objects.
### 3\.2.6. Set types[¶](https://docs.python.org/3/reference/datamodel.html#set-types "Link to this heading")
These represent unordered, finite sets of unique, immutable objects. As such, they cannot be indexed by any subscript. However, they can be iterated over, and the built-in function [`len()`](https://docs.python.org/3/library/functions.html#len "len") returns the number of items in a set. Common uses for sets are fast membership testing, removing duplicates from a sequence, and computing mathematical operations such as intersection, union, difference, and symmetric difference.
For set elements, the same immutability rules apply as for dictionary keys. Note that numeric types obey the normal rules for numeric comparison: if two numbers compare equal (e.g., `1` and `1.0`), only one of them can be contained in a set.
There are currently two intrinsic set types:
Sets
These represent a mutable set. They are created by the built-in [`set()`](https://docs.python.org/3/library/stdtypes.html#set "set") constructor and can be modified afterwards by several methods, such as [`add()`](https://docs.python.org/3/library/stdtypes.html#set.add "set.add").
Frozen sets
These represent an immutable set. They are created by the built-in [`frozenset()`](https://docs.python.org/3/library/stdtypes.html#frozenset "frozenset") constructor. As a frozenset is immutable and [hashable](https://docs.python.org/3/glossary.html#term-hashable), it can be used again as an element of another set, or as a dictionary key.
### 3\.2.7. Mappings[¶](https://docs.python.org/3/reference/datamodel.html#mappings "Link to this heading")
These represent finite sets of objects indexed by arbitrary index sets. The subscript notation `a[k]` selects the item indexed by `k` from the mapping `a`; this can be used in expressions and as the target of assignments or [`del`](https://docs.python.org/3/reference/simple_stmts.html#del) statements. The built-in function [`len()`](https://docs.python.org/3/library/functions.html#len "len") returns the number of items in a mapping.
There is currently a single intrinsic mapping type:
#### 3\.2.7.1. Dictionaries[¶](https://docs.python.org/3/reference/datamodel.html#dictionaries "Link to this heading")
These represent finite sets of objects indexed by nearly arbitrary values. The only types of values not acceptable as keys are values containing lists or dictionaries or other mutable types that are compared by value rather than by object identity, the reason being that the efficient implementation of dictionaries requires a keyâs hash value to remain constant. Numeric types used for keys obey the normal rules for numeric comparison: if two numbers compare equal (e.g., `1` and `1.0`) then they can be used interchangeably to index the same dictionary entry.
Dictionaries preserve insertion order, meaning that keys will be produced in the same order they were added sequentially over the dictionary. Replacing an existing key does not change the order, however removing a key and re-inserting it will add it to the end instead of keeping its old place.
Dictionaries are mutable; they can be created by the `{}` notation (see section [Dictionary displays](https://docs.python.org/3/reference/expressions.html#dict)).
The extension modules [`dbm.ndbm`](https://docs.python.org/3/library/dbm.html#module-dbm.ndbm "dbm.ndbm: The New Database Manager") and [`dbm.gnu`](https://docs.python.org/3/library/dbm.html#module-dbm.gnu "dbm.gnu: GNU database manager") provide additional examples of mapping types, as does the [`collections`](https://docs.python.org/3/library/collections.html#module-collections "collections: Container datatypes") module.
Changed in version 3.7: Dictionaries did not preserve insertion order in versions of Python before 3.6. In CPython 3.6, insertion order was preserved, but it was considered an implementation detail at that time rather than a language guarantee.
### 3\.2.8. Callable types[¶](https://docs.python.org/3/reference/datamodel.html#callable-types "Link to this heading")
These are the types to which the function call operation (see section [Calls](https://docs.python.org/3/reference/expressions.html#calls)) can be applied:
#### 3\.2.8.1. User-defined functions[¶](https://docs.python.org/3/reference/datamodel.html#user-defined-functions "Link to this heading")
A user-defined function object is created by a function definition (see section [Function definitions](https://docs.python.org/3/reference/compound_stmts.html#function)). It should be called with an argument list containing the same number of items as the functionâs formal parameter list.
##### 3\.2.8.1.1. Special read-only attributes[¶](https://docs.python.org/3/reference/datamodel.html#special-read-only-attributes "Link to this heading")
| Attribute | Meaning |
|---|---|
| function.\_\_builtins\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__builtins__ "Link to this definition") | A reference to the [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") that holds the functionâs builtins namespace. Added in version 3.10. |
| function.\_\_globals\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__globals__ "Link to this definition") | A reference to the [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") that holds the functionâs [global variables](https://docs.python.org/3/reference/executionmodel.html#naming) â the global namespace of the module in which the function was defined. |
| function.\_\_closure\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__closure__ "Link to this definition") | `None` or a [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") of cells that contain bindings for the names specified in the [`co_freevars`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_freevars "codeobject.co_freevars") attribute of the functionâs [`code object`](https://docs.python.org/3/reference/datamodel.html#function.__code__ "function.__code__"). A cell object has the attribute `cell_contents`. This can be used to get the value of the cell, as well as set the value. |
##### 3\.2.8.1.2. Special writable attributes[¶](https://docs.python.org/3/reference/datamodel.html#special-writable-attributes "Link to this heading")
Most of these attributes check the type of the assigned value:
| Attribute | Meaning |
|---|---|
| function.\_\_doc\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__doc__ "Link to this definition") | The functionâs documentation string, or `None` if unavailable. |
| function.\_\_name\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__name__ "Link to this definition") | The functionâs name. See also: [`__name__ attributes`](https://docs.python.org/3/library/stdtypes.html#definition.__name__ "definition.__name__"). |
| function.\_\_qualname\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__qualname__ "Link to this definition") | The functionâs [qualified name](https://docs.python.org/3/glossary.html#term-qualified-name). See also: [`__qualname__ attributes`](https://docs.python.org/3/library/stdtypes.html#definition.__qualname__ "definition.__qualname__"). Added in version 3.3. |
| function.\_\_module\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__module__ "Link to this definition") | The name of the module the function was defined in, or `None` if unavailable. |
| function.\_\_defaults\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__defaults__ "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing default [parameter](https://docs.python.org/3/glossary.html#term-parameter) values for those parameters that have defaults, or `None` if no parameters have a default value. |
| function.\_\_code\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__code__ "Link to this definition") | The [code object](https://docs.python.org/3/reference/datamodel.html#code-objects) representing the compiled function body. |
| function.\_\_dict\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__dict__ "Link to this definition") | The namespace supporting arbitrary function attributes. See also: [`__dict__ attributes`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__"). |
| function.\_\_annotations\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__annotations__ "Link to this definition") | A [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") containing annotations of [parameters](https://docs.python.org/3/glossary.html#term-parameter). The keys of the dictionary are the parameter names, and `'return'` for the return annotation, if provided. See also: [`object.__annotations__`](https://docs.python.org/3/reference/datamodel.html#object.__annotations__ "object.__annotations__"). Changed in version 3.14: Annotations are now [lazily evaluated](https://docs.python.org/3/reference/executionmodel.html#lazy-evaluation). See [**PEP 649**](https://peps.python.org/pep-0649/). |
| function.\_\_annotate\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__annotate__ "Link to this definition") | The [annotate function](https://docs.python.org/3/glossary.html#term-annotate-function) for this function, or `None` if the function has no annotations. See [`object.__annotate__`](https://docs.python.org/3/reference/datamodel.html#object.__annotate__ "object.__annotate__"). Added in version 3.14. |
| function.\_\_kwdefaults\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__kwdefaults__ "Link to this definition") | A [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") containing defaults for keyword-only [parameters](https://docs.python.org/3/glossary.html#term-parameter). |
| function.\_\_type\_params\_\_[¶](https://docs.python.org/3/reference/datamodel.html#function.__type_params__ "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the [type parameters](https://docs.python.org/3/reference/compound_stmts.html#type-params) of a [generic function](https://docs.python.org/3/reference/compound_stmts.html#generic-functions). Added in version 3.12. |
Function objects also support getting and setting arbitrary attributes, which can be used, for example, to attach metadata to functions. Regular attribute dot-notation is used to get and set such attributes.
**CPython implementation detail:** CPythonâs current implementation only supports function attributes on user-defined functions. Function attributes on [built-in functions](https://docs.python.org/3/reference/datamodel.html#builtin-functions) may be supported in the future.
Additional information about a functionâs definition can be retrieved from its [code object](https://docs.python.org/3/reference/datamodel.html#code-objects) (accessible via the [`__code__`](https://docs.python.org/3/reference/datamodel.html#function.__code__ "function.__code__") attribute).
#### 3\.2.8.2. Instance methods[¶](https://docs.python.org/3/reference/datamodel.html#instance-methods "Link to this heading")
An instance method object combines a class, a class instance and any callable object (normally a user-defined function).
Special read-only attributes:
| | |
|---|---|
| method.\_\_self\_\_[¶](https://docs.python.org/3/reference/datamodel.html#method.__self__ "Link to this definition") | Refers to the class instance object to which the method is [bound](https://docs.python.org/3/reference/datamodel.html#method-binding) |
| method.\_\_func\_\_[¶](https://docs.python.org/3/reference/datamodel.html#method.__func__ "Link to this definition") | Refers to the original [function object](https://docs.python.org/3/reference/datamodel.html#user-defined-funcs) |
| method.\_\_doc\_\_[¶](https://docs.python.org/3/reference/datamodel.html#method.__doc__ "Link to this definition") | The methodâs documentation (same as [`method.__func__.__doc__`](https://docs.python.org/3/reference/datamodel.html#function.__doc__ "function.__doc__")). A [`string`](https://docs.python.org/3/library/stdtypes.html#str "str") if the original function had a docstring, else `None`. |
| method.\_\_name\_\_[¶](https://docs.python.org/3/reference/datamodel.html#method.__name__ "Link to this definition") | The name of the method (same as [`method.__func__.__name__`](https://docs.python.org/3/reference/datamodel.html#function.__name__ "function.__name__")) |
| method.\_\_module\_\_[¶](https://docs.python.org/3/reference/datamodel.html#method.__module__ "Link to this definition") | The name of the module the method was defined in, or `None` if unavailable. |
Methods also support accessing (but not setting) the arbitrary function attributes on the underlying [function object](https://docs.python.org/3/reference/datamodel.html#user-defined-funcs).
User-defined method objects may be created when getting an attribute of a class (perhaps via an instance of that class), if that attribute is a user-defined [function object](https://docs.python.org/3/reference/datamodel.html#user-defined-funcs) or a [`classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod") object.
When an instance method object is created by retrieving a user-defined [function object](https://docs.python.org/3/reference/datamodel.html#user-defined-funcs) from a class via one of its instances, its [`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__") attribute is the instance, and the method object is said to be *bound*. The new methodâs [`__func__`](https://docs.python.org/3/reference/datamodel.html#method.__func__ "method.__func__") attribute is the original function object.
When an instance method object is created by retrieving a [`classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod") object from a class or instance, its [`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__") attribute is the class itself, and its [`__func__`](https://docs.python.org/3/reference/datamodel.html#method.__func__ "method.__func__") attribute is the function object underlying the class method.
When an instance method object is called, the underlying function ([`__func__`](https://docs.python.org/3/reference/datamodel.html#method.__func__ "method.__func__")) is called, inserting the class instance ([`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__")) in front of the argument list. For instance, when `C` is a class which contains a definition for a function `f()`, and `x` is an instance of `C`, calling `x.f(1)` is equivalent to calling `C.f(x, 1)`.
When an instance method object is derived from a [`classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod") object, the âclass instanceâ stored in [`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__") will actually be the class itself, so that calling either `x.f(1)` or `C.f(1)` is equivalent to calling `f(C,1)` where `f` is the underlying function.
It is important to note that user-defined functions which are attributes of a class instance are not converted to bound methods; this *only* happens when the function is an attribute of the class.
#### 3\.2.8.3. Generator functions[¶](https://docs.python.org/3/reference/datamodel.html#generator-functions "Link to this heading")
A function or method which uses the [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) statement (see section [The yield statement](https://docs.python.org/3/reference/simple_stmts.html#yield)) is called a *generator function*. Such a function, when called, always returns an [iterator](https://docs.python.org/3/glossary.html#term-iterator) object which can be used to execute the body of the function: calling the iteratorâs [`iterator.__next__()`](https://docs.python.org/3/library/stdtypes.html#iterator.__next__ "iterator.__next__") method will cause the function to execute until it provides a value using the `yield` statement. When the function executes a [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement or falls off the end, a [`StopIteration`](https://docs.python.org/3/library/exceptions.html#StopIteration "StopIteration") exception is raised and the iterator will have reached the end of the set of values to be returned.
#### 3\.2.8.4. Coroutine functions[¶](https://docs.python.org/3/reference/datamodel.html#coroutine-functions "Link to this heading")
A function or method which is defined using [`async def`](https://docs.python.org/3/reference/compound_stmts.html#async-def) is called a *coroutine function*. Such a function, when called, returns a [coroutine](https://docs.python.org/3/glossary.html#term-coroutine) object. It may contain [`await`](https://docs.python.org/3/reference/expressions.html#await) expressions, as well as [`async with`](https://docs.python.org/3/reference/compound_stmts.html#async-with) and [`async for`](https://docs.python.org/3/reference/compound_stmts.html#async-for) statements. See also the [Coroutine Objects](https://docs.python.org/3/reference/datamodel.html#coroutine-objects) section.
#### 3\.2.8.5. Asynchronous generator functions[¶](https://docs.python.org/3/reference/datamodel.html#asynchronous-generator-functions "Link to this heading")
A function or method which is defined using [`async def`](https://docs.python.org/3/reference/compound_stmts.html#async-def) and which uses the [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) statement is called a *asynchronous generator function*. Such a function, when called, returns an [asynchronous iterator](https://docs.python.org/3/glossary.html#term-asynchronous-iterator) object which can be used in an [`async for`](https://docs.python.org/3/reference/compound_stmts.html#async-for) statement to execute the body of the function.
Calling the asynchronous iteratorâs [`aiterator.__anext__`](https://docs.python.org/3/reference/datamodel.html#object.__anext__ "object.__anext__") method will return an [awaitable](https://docs.python.org/3/glossary.html#term-awaitable) which when awaited will execute until it provides a value using the [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) expression. When the function executes an empty [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement or falls off the end, a [`StopAsyncIteration`](https://docs.python.org/3/library/exceptions.html#StopAsyncIteration "StopAsyncIteration") exception is raised and the asynchronous iterator will have reached the end of the set of values to be yielded.
#### 3\.2.8.6. Built-in functions[¶](https://docs.python.org/3/reference/datamodel.html#built-in-functions "Link to this heading")
A built-in function object is a wrapper around a C function. Examples of built-in functions are [`len()`](https://docs.python.org/3/library/functions.html#len "len") and [`math.sin()`](https://docs.python.org/3/library/math.html#math.sin "math.sin") ([`math`](https://docs.python.org/3/library/math.html#module-math "math: Mathematical functions (sin() etc.).") is a standard built-in module). The number and type of the arguments are determined by the C function. Special read-only attributes:
- `__doc__` is the functionâs documentation string, or `None` if unavailable. See [`function.__doc__`](https://docs.python.org/3/reference/datamodel.html#function.__doc__ "function.__doc__").
- `__name__` is the functionâs name. See [`function.__name__`](https://docs.python.org/3/reference/datamodel.html#function.__name__ "function.__name__").
- `__self__` is set to `None` (but see the next item).
- `__module__` is the name of the module the function was defined in or `None` if unavailable. See [`function.__module__`](https://docs.python.org/3/reference/datamodel.html#function.__module__ "function.__module__").
#### 3\.2.8.7. Built-in methods[¶](https://docs.python.org/3/reference/datamodel.html#built-in-methods "Link to this heading")
This is really a different disguise of a built-in function, this time containing an object passed to the C function as an implicit extra argument. An example of a built-in method is `alist.append()`, assuming *alist* is a list object. In this case, the special read-only attribute `__self__` is set to the object denoted by *alist*. (The attribute has the same semantics as it does with [`other instance methods`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__").)
#### 3\.2.8.8. Classes[¶](https://docs.python.org/3/reference/datamodel.html#classes "Link to this heading")
Classes are callable. These objects normally act as factories for new instances of themselves, but variations are possible for class types that override [`__new__()`](https://docs.python.org/3/reference/datamodel.html#object.__new__ "object.__new__"). The arguments of the call are passed to `__new__()` and, in the typical case, to [`__init__()`](https://docs.python.org/3/reference/datamodel.html#object.__init__ "object.__init__") to initialize the new instance.
#### 3\.2.8.9. Class Instances[¶](https://docs.python.org/3/reference/datamodel.html#class-instances "Link to this heading")
Instances of arbitrary classes can be made callable by defining a [`__call__()`](https://docs.python.org/3/reference/datamodel.html#object.__call__ "object.__call__") method in their class.
### 3\.2.9. Modules[¶](https://docs.python.org/3/reference/datamodel.html#modules "Link to this heading")
Modules are a basic organizational unit of Python code, and are created by the [import system](https://docs.python.org/3/reference/import.html#importsystem) as invoked either by the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement, or by calling functions such as [`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module") and built-in [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__"). A module object has a namespace implemented by a [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") object (this is the dictionary referenced by the [`__globals__`](https://docs.python.org/3/reference/datamodel.html#function.__globals__ "function.__globals__") attribute of functions defined in the module). Attribute references are translated to lookups in this dictionary, e.g., `m.x` is equivalent to `m.__dict__["x"]`. A module object does not contain the code object used to initialize the module (since it isnât needed once the initialization is done).
Attribute assignment updates the moduleâs namespace dictionary, e.g., `m.x = 1` is equivalent to `m.__dict__["x"] = 1`.
#### 3\.2.9.2. Other writable attributes on module objects[¶](https://docs.python.org/3/reference/datamodel.html#other-writable-attributes-on-module-objects "Link to this heading")
As well as the import-related attributes listed above, module objects also have the following writable attributes:
module.\_\_doc\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__doc__ "Link to this definition")
The moduleâs documentation string, or `None` if unavailable. See also: [`__doc__ attributes`](https://docs.python.org/3/library/stdtypes.html#definition.__doc__ "definition.__doc__").
module.\_\_annotations\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__annotations__ "Link to this definition")
A dictionary containing [variable annotations](https://docs.python.org/3/glossary.html#term-variable-annotation) collected during module body execution. For best practices on working with `__annotations__`, see [`annotationlib`](https://docs.python.org/3/library/annotationlib.html#module-annotationlib "annotationlib: Functionality for introspecting annotations").
module.\_\_annotate\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__annotate__ "Link to this definition")
The [annotate function](https://docs.python.org/3/glossary.html#term-annotate-function) for this module, or `None` if the module has no annotations. See also: [`__annotate__`](https://docs.python.org/3/reference/datamodel.html#object.__annotate__ "object.__annotate__") attributes.
Added in version 3.14.
#### 3\.2.9.3. Module dictionaries[¶](https://docs.python.org/3/reference/datamodel.html#module-dictionaries "Link to this heading")
Module objects also have the following special read-only attribute:
module.\_\_dict\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__dict__ "Link to this definition")
The moduleâs namespace as a dictionary object. Uniquely among the attributes listed here, `__dict__` cannot be accessed as a global variable from within a module; it can only be accessed as an attribute on module objects.
**CPython implementation detail:** Because of the way CPython clears module dictionaries, the module dictionary will be cleared when the module falls out of scope even if the dictionary still has live references. To avoid this, copy the dictionary or keep the module around while using its dictionary directly.
### 3\.2.10. Custom classes[¶](https://docs.python.org/3/reference/datamodel.html#custom-classes "Link to this heading")
Custom class types are typically created by class definitions (see section [Class definitions](https://docs.python.org/3/reference/compound_stmts.html#class)). A class has a namespace implemented by a dictionary object. Class attribute references are translated to lookups in this dictionary, e.g., `C.x` is translated to `C.__dict__["x"]` (although there are a number of hooks which allow for other means of locating attributes). When the attribute name is not found there, the attribute search continues in the base classes. This search of the base classes uses the C3 method resolution order which behaves correctly even in the presence of âdiamondâ inheritance structures where there are multiple inheritance paths leading back to a common ancestor. Additional details on the C3 MRO used by Python can be found at [The Python 2.3 Method Resolution Order](https://docs.python.org/3/howto/mro.html#python-2-3-mro).
When a class attribute reference (for class `C`, say) would yield a class method object, it is transformed into an instance method object whose [`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__") attribute is `C`. When it would yield a [`staticmethod`](https://docs.python.org/3/library/functions.html#staticmethod "staticmethod") object, it is transformed into the object wrapped by the static method object. See section [Implementing Descriptors](https://docs.python.org/3/reference/datamodel.html#descriptors) for another way in which attributes retrieved from a class may differ from those actually contained in its [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__").
Class attribute assignments update the classâs dictionary, never the dictionary of a base class.
A class object can be called (see above) to yield a class instance (see below).
#### 3\.2.10.1. Special attributes[¶](https://docs.python.org/3/reference/datamodel.html#special-attributes "Link to this heading")
| Attribute | Meaning |
|---|---|
| type.\_\_name\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__name__ "Link to this definition") | The classâs name. See also: [`__name__ attributes`](https://docs.python.org/3/library/stdtypes.html#definition.__name__ "definition.__name__"). |
| type.\_\_qualname\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__qualname__ "Link to this definition") | The classâs [qualified name](https://docs.python.org/3/glossary.html#term-qualified-name). See also: [`__qualname__ attributes`](https://docs.python.org/3/library/stdtypes.html#definition.__qualname__ "definition.__qualname__"). |
| type.\_\_module\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__module__ "Link to this definition") | The name of the module in which the class was defined. |
| type.\_\_dict\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__dict__ "Link to this definition") | A [`mapping proxy`](https://docs.python.org/3/library/types.html#types.MappingProxyType "types.MappingProxyType") providing a read-only view of the classâs namespace. See also: [`__dict__ attributes`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__"). |
| type.\_\_bases\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__bases__ "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the classâs bases. In most cases, for a class defined as `class X(A, B, C)`, `X.__bases__` will be exactly equal to `(A, B, C)`. |
| type.\_\_base\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__base__ "Link to this definition") | **CPython implementation detail:** The single base class in the inheritance chain that is responsible for the memory layout of instances. This attribute corresponds to [`tp_base`](https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_base "PyTypeObject.tp_base") at the C level. |
| type.\_\_doc\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__doc__ "Link to this definition") | The classâs documentation string, or `None` if undefined. Not inherited by subclasses. |
| type.\_\_annotations\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__annotations__ "Link to this definition") | A dictionary containing [variable annotations](https://docs.python.org/3/glossary.html#term-variable-annotation) collected during class body execution. See also: [`__annotations__ attributes`](https://docs.python.org/3/reference/datamodel.html#object.__annotations__ "object.__annotations__"). For best practices on working with [`__annotations__`](https://docs.python.org/3/reference/datamodel.html#object.__annotations__ "object.__annotations__"), please see [`annotationlib`](https://docs.python.org/3/library/annotationlib.html#module-annotationlib "annotationlib: Functionality for introspecting annotations"). Use [`annotationlib.get_annotations()`](https://docs.python.org/3/library/annotationlib.html#annotationlib.get_annotations "annotationlib.get_annotations") instead of accessing this attribute directly. Warning Accessing the `__annotations__` attribute directly on a class object may return annotations for the wrong class, specifically in certain cases where the class, its base class, or a metaclass is defined under `from __future__ import annotations`. See [**749**](https://peps.python.org/pep-0749/#pep749-metaclasses) for details. This attribute does not exist on certain builtin classes. On user-defined classes without `__annotations__`, it is an empty dictionary. Changed in version 3.14: Annotations are now [lazily evaluated](https://docs.python.org/3/reference/executionmodel.html#lazy-evaluation). See [**PEP 649**](https://peps.python.org/pep-0649/). |
| type.\_\_annotate\_\_()[¶](https://docs.python.org/3/reference/datamodel.html#type.__annotate__ "Link to this definition") | The [annotate function](https://docs.python.org/3/glossary.html#term-annotate-function) for this class, or `None` if the class has no annotations. See also: [`__annotate__ attributes`](https://docs.python.org/3/reference/datamodel.html#object.__annotate__ "object.__annotate__"). Added in version 3.14. |
| type.\_\_type\_params\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__type_params__ "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the [type parameters](https://docs.python.org/3/reference/compound_stmts.html#type-params) of a [generic class](https://docs.python.org/3/reference/compound_stmts.html#generic-classes). Added in version 3.12. |
| type.\_\_static\_attributes\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__static_attributes__ "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing names of attributes of this class which are assigned through `self.X` from any function in its body. Added in version 3.13. |
| type.\_\_firstlineno\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__firstlineno__ "Link to this definition") | The line number of the first line of the class definition, including decorators. Setting the [`__module__`](https://docs.python.org/3/reference/datamodel.html#type.__module__ "type.__module__") attribute removes the `__firstlineno__` item from the typeâs dictionary. Added in version 3.13. |
| type.\_\_mro\_\_[¶](https://docs.python.org/3/reference/datamodel.html#type.__mro__ "Link to this definition") | The [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") of classes that are considered when looking for base classes during method resolution. |
#### 3\.2.10.2. Special methods[¶](https://docs.python.org/3/reference/datamodel.html#special-methods "Link to this heading")
In addition to the special attributes described above, all Python classes also have the following two methods available:
type.mro()[¶](https://docs.python.org/3/reference/datamodel.html#type.mro "Link to this definition")
This method can be overridden by a metaclass to customize the method resolution order for its instances. It is called at class instantiation, and its result is stored in [`__mro__`](https://docs.python.org/3/reference/datamodel.html#type.__mro__ "type.__mro__").
type.\_\_subclasses\_\_()[¶](https://docs.python.org/3/reference/datamodel.html#type.__subclasses__ "Link to this definition")
Each class keeps a list of weak references to its immediate subclasses. This method returns a list of all those references still alive. The list is in definition order. Example:
```
>>> class A: pass
>>> class B(A): pass
>>> A.__subclasses__()
[<class 'B'>]
```
### 3\.2.11. Class instances[¶](https://docs.python.org/3/reference/datamodel.html#id4 "Link to this heading")
A class instance is created by calling a class object (see above). A class instance has a namespace implemented as a dictionary which is the first place in which attribute references are searched. When an attribute is not found there, and the instanceâs class has an attribute by that name, the search continues with the class attributes. If a class attribute is found that is a user-defined function object, it is transformed into an instance method object whose [`__self__`](https://docs.python.org/3/reference/datamodel.html#method.__self__ "method.__self__") attribute is the instance. Static method and class method objects are also transformed; see above under âClassesâ. See section [Implementing Descriptors](https://docs.python.org/3/reference/datamodel.html#descriptors) for another way in which attributes of a class retrieved via its instances may differ from the objects actually stored in the classâs [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__"). If no class attribute is found, and the objectâs class has a [`__getattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattr__ "object.__getattr__") method, that is called to satisfy the lookup.
Attribute assignments and deletions update the instanceâs dictionary, never a classâs dictionary. If the class has a [`__setattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__setattr__ "object.__setattr__") or [`__delattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__delattr__ "object.__delattr__") method, this is called instead of updating the instance dictionary directly.
Class instances can pretend to be numbers, sequences, or mappings if they have methods with certain special names. See section [Special method names](https://docs.python.org/3/reference/datamodel.html#specialnames).
#### 3\.2.11.1. Special attributes[¶](https://docs.python.org/3/reference/datamodel.html#id5 "Link to this heading")
object.\_\_class\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__class__ "Link to this definition")
The class to which a class instance belongs.
object.\_\_dict\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "Link to this definition")
A dictionary or other mapping object used to store an objectâs (writable) attributes. Not all instances have a `__dict__` attribute; see the section on [\_\_slots\_\_](https://docs.python.org/3/reference/datamodel.html#slots) for more details.
### 3\.2.12. I/O objects (also known as file objects)[¶](https://docs.python.org/3/reference/datamodel.html#i-o-objects-also-known-as-file-objects "Link to this heading")
A [file object](https://docs.python.org/3/glossary.html#term-file-object) represents an open file. Various shortcuts are available to create file objects: the [`open()`](https://docs.python.org/3/library/functions.html#open "open") built-in function, and also [`os.popen()`](https://docs.python.org/3/library/os.html#os.popen "os.popen"), [`os.fdopen()`](https://docs.python.org/3/library/os.html#os.fdopen "os.fdopen"), and the [`makefile()`](https://docs.python.org/3/library/socket.html#socket.socket.makefile "socket.socket.makefile") method of socket objects (and perhaps by other functions or methods provided by extension modules).
File objects implement common methods, listed below, to simplify usage in generic code. They are expected to be [With Statement Context Managers](https://docs.python.org/3/reference/datamodel.html#context-managers).
The objects `sys.stdin`, `sys.stdout` and `sys.stderr` are initialized to file objects corresponding to the interpreterâs standard input, output and error streams; they are all open in text mode and therefore follow the interface defined by the [`io.TextIOBase`](https://docs.python.org/3/library/io.html#io.TextIOBase "io.TextIOBase") abstract class.
file.read(*size\=\-1*, */*)[¶](https://docs.python.org/3/reference/datamodel.html#file.read "Link to this definition")
Retrieve up to *size* data from the file. As a convenience if *size* is unspecified or -1 retrieve all data available.
file.write(*data*, */*)[¶](https://docs.python.org/3/reference/datamodel.html#file.write "Link to this definition")
Store *data* to the file.
file.close()[¶](https://docs.python.org/3/reference/datamodel.html#file.close "Link to this definition")
Flush any buffers and close the underlying file.
### 3\.2.13. Internal types[¶](https://docs.python.org/3/reference/datamodel.html#internal-types "Link to this heading")
A few types used internally by the interpreter are exposed to the user. Their definitions may change with future versions of the interpreter, but they are mentioned here for completeness.
#### 3\.2.13.1. Code objects[¶](https://docs.python.org/3/reference/datamodel.html#code-objects "Link to this heading")
Code objects represent *byte-compiled* executable Python code, or [bytecode](https://docs.python.org/3/glossary.html#term-bytecode). The difference between a code object and a function object is that the function object contains an explicit reference to the functionâs globals (the module in which it was defined), while a code object contains no context; also the default argument values are stored in the function object, not in the code object (because they represent values calculated at run-time). Unlike function objects, code objects are immutable and contain no references (directly or indirectly) to mutable objects.
##### 3\.2.13.1.1. Special read-only attributes[¶](https://docs.python.org/3/reference/datamodel.html#index-64 "Link to this heading")
| | |
|---|---|
| codeobject.co\_name[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_name "Link to this definition") | The function name |
| codeobject.co\_qualname[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_qualname "Link to this definition") | The fully qualified function name Added in version 3.11. |
| codeobject.co\_argcount[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_argcount "Link to this definition") | The total number of positional [parameters](https://docs.python.org/3/glossary.html#term-parameter) (including positional-only parameters and parameters with default values) that the function has |
| codeobject.co\_posonlyargcount[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_posonlyargcount "Link to this definition") | The number of positional-only [parameters](https://docs.python.org/3/glossary.html#term-parameter) (including arguments with default values) that the function has |
| codeobject.co\_kwonlyargcount[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_kwonlyargcount "Link to this definition") | The number of keyword-only [parameters](https://docs.python.org/3/glossary.html#term-parameter) (including arguments with default values) that the function has |
| codeobject.co\_nlocals[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_nlocals "Link to this definition") | The number of [local variables](https://docs.python.org/3/reference/executionmodel.html#naming) used by the function (including parameters) |
| codeobject.co\_varnames[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_varnames "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the names of the local variables in the function (starting with the parameter names) |
| codeobject.co\_cellvars[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_cellvars "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the names of [local variables](https://docs.python.org/3/reference/executionmodel.html#naming) that are referenced from at least one [nested scope](https://docs.python.org/3/glossary.html#term-nested-scope) inside the function |
| codeobject.co\_freevars[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_freevars "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the names of [free (closure) variables](https://docs.python.org/3/glossary.html#term-closure-variable) that a [nested scope](https://docs.python.org/3/glossary.html#term-nested-scope) references in an outer scope. See also [`function.__closure__`](https://docs.python.org/3/reference/datamodel.html#function.__closure__ "function.__closure__"). Note: references to global and builtin names are *not* included. |
| codeobject.co\_code[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_code "Link to this definition") | A string representing the sequence of [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) instructions in the function |
| codeobject.co\_consts[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_consts "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the literals used by the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) in the function |
| codeobject.co\_names[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_names "Link to this definition") | A [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") containing the names used by the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) in the function |
| codeobject.co\_filename[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_filename "Link to this definition") | The name of the file from which the code was compiled |
| codeobject.co\_firstlineno[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_firstlineno "Link to this definition") | The line number of the first line of the function |
| codeobject.co\_lnotab[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_lnotab "Link to this definition") | A string encoding the mapping from [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) offsets to line numbers. For details, see the source code of the interpreter. Deprecated since version 3.12: This attribute of code objects is deprecated, and may be removed in Python 3.15. |
| codeobject.co\_stacksize[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_stacksize "Link to this definition") | The required stack size of the code object |
| codeobject.co\_flags[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_flags "Link to this definition") | An [`integer`](https://docs.python.org/3/library/functions.html#int "int") encoding a number of flags for the interpreter. |
The following flag bits are defined for [`co_flags`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_flags "codeobject.co_flags"): bit `0x04` is set if the function uses the `*arguments` syntax to accept an arbitrary number of positional arguments; bit `0x08` is set if the function uses the `**keywords` syntax to accept arbitrary keyword arguments; bit `0x20` is set if the function is a generator. See [Code Objects Bit Flags](https://docs.python.org/3/library/inspect.html#inspect-module-co-flags) for details on the semantics of each flags that might be present.
Future feature declarations (for example, `from __future__ import division`) also use bits in [`co_flags`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_flags "codeobject.co_flags") to indicate whether a code object was compiled with a particular feature enabled. See [`compiler_flag`](https://docs.python.org/3/library/__future__.html#future__._Feature.compiler_flag "__future__._Feature.compiler_flag").
Other bits in [`co_flags`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_flags "codeobject.co_flags") are reserved for internal use.
If a code object represents a function and has a docstring, the [`CO_HAS_DOCSTRING`](https://docs.python.org/3/library/inspect.html#inspect.CO_HAS_DOCSTRING "inspect.CO_HAS_DOCSTRING") bit is set in [`co_flags`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_flags "codeobject.co_flags") and the first item in [`co_consts`](https://docs.python.org/3/reference/datamodel.html#codeobject.co_consts "codeobject.co_consts") is the docstring of the function.
##### 3\.2.13.1.2. Methods on code objects[¶](https://docs.python.org/3/reference/datamodel.html#methods-on-code-objects "Link to this heading")
codeobject.co\_positions()[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_positions "Link to this definition")
Returns an iterable over the source code positions of each [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) instruction in the code object.
The iterator returns [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple")s containing the . The *i-th* tuple corresponds to the position of the source code that compiled to the *i-th* code unit. Column information is 0-indexed utf-8 byte offsets on the given source line.
This positional information can be missing. A non-exhaustive lists of cases where this may happen:
- Running the interpreter with [`-X`](https://docs.python.org/3/using/cmdline.html#cmdoption-X) `no_debug_ranges`.
- Loading a pyc file compiled while using [`-X`](https://docs.python.org/3/using/cmdline.html#cmdoption-X) `no_debug_ranges`.
- Position tuples corresponding to artificial instructions.
- Line and column numbers that canât be represented due to implementation specific limitations.
When this occurs, some or all of the tuple elements can be [`None`](https://docs.python.org/3/library/constants.html#None "None").
Added in version 3.11.
Note
This feature requires storing column positions in code objects which may result in a small increase of disk usage of compiled Python files or interpreter memory usage. To avoid storing the extra information and/or deactivate printing the extra traceback information, the [`-X`](https://docs.python.org/3/using/cmdline.html#cmdoption-X) `no_debug_ranges` command line flag or the [`PYTHONNODEBUGRANGES`](https://docs.python.org/3/using/cmdline.html#envvar-PYTHONNODEBUGRANGES) environment variable can be used.
codeobject.co\_lines()[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.co_lines "Link to this definition")
Returns an iterator that yields information about successive ranges of [bytecode](https://docs.python.org/3/glossary.html#term-bytecode)s. Each item yielded is a `(start, end, lineno)` [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple"):
- `start` (an [`int`](https://docs.python.org/3/library/functions.html#int "int")) represents the offset (inclusive) of the start of the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) range
- `end` (an [`int`](https://docs.python.org/3/library/functions.html#int "int")) represents the offset (exclusive) of the end of the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) range
- `lineno` is an [`int`](https://docs.python.org/3/library/functions.html#int "int") representing the line number of the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) range, or `None` if the bytecodes in the given range have no line number
The items yielded will have the following properties:
- The first range yielded will have a `start` of 0.
- The `(start, end)` ranges will be non-decreasing and consecutive. That is, for any pair of [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple")s, the `start` of the second will be equal to the `end` of the first.
- No range will be backwards: `end >= start` for all triples.
- The last [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple") yielded will have `end` equal to the size of the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode).
Zero-width ranges, where `start == end`, are allowed. Zero-width ranges are used for lines that are present in the source code, but have been eliminated by the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) compiler.
Added in version 3.10.
See also
[**PEP 626**](https://peps.python.org/pep-0626/) - Precise line numbers for debugging and other tools.
The PEP that introduced the `co_lines()` method.
codeobject.replace(*\*\*kwargs*)[¶](https://docs.python.org/3/reference/datamodel.html#codeobject.replace "Link to this definition")
Return a copy of the code object with new values for the specified fields.
Code objects are also supported by the generic function [`copy.replace()`](https://docs.python.org/3/library/copy.html#copy.replace "copy.replace").
Added in version 3.8.
#### 3\.2.13.2. Frame objects[¶](https://docs.python.org/3/reference/datamodel.html#frame-objects "Link to this heading")
Frame objects represent execution frames. They may occur in [traceback objects](https://docs.python.org/3/reference/datamodel.html#traceback-objects), and are also passed to registered trace functions.
##### 3\.2.13.2.1. Special read-only attributes[¶](https://docs.python.org/3/reference/datamodel.html#index-70 "Link to this heading")
| | |
|---|---|
| frame.f\_back[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_back "Link to this definition") | Points to the previous stack frame (towards the caller), or `None` if this is the bottom stack frame |
| frame.f\_code[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_code "Link to this definition") | The [code object](https://docs.python.org/3/reference/datamodel.html#code-objects) being executed in this frame. Accessing this attribute raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `object.__getattr__` with arguments `obj` and `"f_code"`. |
| frame.f\_locals[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_locals "Link to this definition") | The mapping used by the frame to look up [local variables](https://docs.python.org/3/reference/executionmodel.html#naming). If the frame refers to an [optimized scope](https://docs.python.org/3/glossary.html#term-optimized-scope), this may return a write-through proxy object. Changed in version 3.13: Return a proxy for optimized scopes. |
| frame.f\_globals[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_globals "Link to this definition") | The dictionary used by the frame to look up [global variables](https://docs.python.org/3/reference/executionmodel.html#naming) |
| frame.f\_builtins[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_builtins "Link to this definition") | The dictionary used by the frame to look up [built-in (intrinsic) names](https://docs.python.org/3/reference/executionmodel.html#naming) |
| frame.f\_lasti[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_lasti "Link to this definition") | The âprecise instructionâ of the frame object (this is an index into the [bytecode](https://docs.python.org/3/glossary.html#term-bytecode) string of the [code object](https://docs.python.org/3/reference/datamodel.html#code-objects)) |
| frame.f\_generator[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_generator "Link to this definition") | The [generator](https://docs.python.org/3/glossary.html#term-generator) or [coroutine](https://docs.python.org/3/glossary.html#term-coroutine) object that owns this frame, or `None` if the frame is a normal function. Added in version 3.14. |
##### 3\.2.13.2.2. Special writable attributes[¶](https://docs.python.org/3/reference/datamodel.html#index-71 "Link to this heading")
| | |
|---|---|
| frame.f\_trace[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_trace "Link to this definition") | If not `None`, this is a function called for various events during code execution (this is used by debuggers). Normally an event is triggered for each new source line (see [`f_trace_lines`](https://docs.python.org/3/reference/datamodel.html#frame.f_trace_lines "frame.f_trace_lines")). |
| frame.f\_trace\_lines[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_trace_lines "Link to this definition") | Set this attribute to [`False`](https://docs.python.org/3/library/constants.html#False "False") to disable triggering a tracing event for each source line. |
| frame.f\_trace\_opcodes[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_trace_opcodes "Link to this definition") | Set this attribute to [`True`](https://docs.python.org/3/library/constants.html#True "True") to allow per-opcode events to be requested. Note that this may lead to undefined interpreter behaviour if exceptions raised by the trace function escape to the function being traced. |
| frame.f\_lineno[¶](https://docs.python.org/3/reference/datamodel.html#frame.f_lineno "Link to this definition") | The current line number of the frame â writing to this from within a trace function jumps to the given line (only for the bottom-most frame). A debugger can implement a Jump command (aka Set Next Statement) by writing to this attribute. |
##### 3\.2.13.2.3. Frame object methods[¶](https://docs.python.org/3/reference/datamodel.html#frame-object-methods "Link to this heading")
Frame objects support one method:
frame.clear()[¶](https://docs.python.org/3/reference/datamodel.html#frame.clear "Link to this definition")
This method clears all references to [local variables](https://docs.python.org/3/reference/executionmodel.html#naming) held by the frame. Also, if the frame belonged to a [generator](https://docs.python.org/3/glossary.html#term-generator), the generator is finalized. This helps break reference cycles involving frame objects (for example when catching an [exception](https://docs.python.org/3/library/exceptions.html#bltin-exceptions) and storing its [traceback](https://docs.python.org/3/reference/datamodel.html#traceback-objects) for later use).
[`RuntimeError`](https://docs.python.org/3/library/exceptions.html#RuntimeError "RuntimeError") is raised if the frame is currently executing or suspended.
Added in version 3.4.
Changed in version 3.13: Attempting to clear a suspended frame raises [`RuntimeError`](https://docs.python.org/3/library/exceptions.html#RuntimeError "RuntimeError") (as has always been the case for executing frames).
#### 3\.2.13.3. Traceback objects[¶](https://docs.python.org/3/reference/datamodel.html#traceback-objects "Link to this heading")
Traceback objects represent the stack trace of an [exception](https://docs.python.org/3/tutorial/errors.html#tut-errors). A traceback object is implicitly created when an exception occurs, and may also be explicitly created by calling [`types.TracebackType`](https://docs.python.org/3/library/types.html#types.TracebackType "types.TracebackType").
Changed in version 3.7: Traceback objects can now be explicitly instantiated from Python code.
For implicitly created tracebacks, when the search for an exception handler unwinds the execution stack, at each unwound level a traceback object is inserted in front of the current traceback. When an exception handler is entered, the stack trace is made available to the program. (See section [The try statement](https://docs.python.org/3/reference/compound_stmts.html#try).) It is accessible as the third item of the tuple returned by [`sys.exc_info()`](https://docs.python.org/3/library/sys.html#sys.exc_info "sys.exc_info"), and as the [`__traceback__`](https://docs.python.org/3/library/exceptions.html#BaseException.__traceback__ "BaseException.__traceback__") attribute of the caught exception.
When the program contains no suitable handler, the stack trace is written (nicely formatted) to the standard error stream; if the interpreter is interactive, it is also made available to the user as [`sys.last_traceback`](https://docs.python.org/3/library/sys.html#sys.last_traceback "sys.last_traceback").
For explicitly created tracebacks, it is up to the creator of the traceback to determine how the [`tb_next`](https://docs.python.org/3/reference/datamodel.html#traceback.tb_next "traceback.tb_next") attributes should be linked to form a full stack trace.
Special read-only attributes:
| | |
|---|---|
| traceback.tb\_frame[¶](https://docs.python.org/3/reference/datamodel.html#traceback.tb_frame "Link to this definition") | Points to the execution [frame](https://docs.python.org/3/reference/datamodel.html#frame-objects) of the current level. Accessing this attribute raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `object.__getattr__` with arguments `obj` and `"tb_frame"`. |
| traceback.tb\_lineno[¶](https://docs.python.org/3/reference/datamodel.html#traceback.tb_lineno "Link to this definition") | Gives the line number where the exception occurred |
| traceback.tb\_lasti[¶](https://docs.python.org/3/reference/datamodel.html#traceback.tb_lasti "Link to this definition") | Indicates the âprecise instructionâ. |
The line number and last instruction in the traceback may differ from the line number of its [frame object](https://docs.python.org/3/reference/datamodel.html#frame-objects) if the exception occurred in a [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement with no matching except clause or with a [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause.
traceback.tb\_next[¶](https://docs.python.org/3/reference/datamodel.html#traceback.tb_next "Link to this definition")
The special writable attribute `tb_next` is the next level in the stack trace (towards the frame where the exception occurred), or `None` if there is no next level.
Changed in version 3.7: This attribute is now writable
#### 3\.2.13.4. Slice objects[¶](https://docs.python.org/3/reference/datamodel.html#slice-objects "Link to this heading")
Slice objects are used to represent slices for [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__") methods. They are also created by the built-in [`slice()`](https://docs.python.org/3/library/functions.html#slice "slice") function.
Special read-only attributes: [`start`](https://docs.python.org/3/library/functions.html#slice.start "slice.start") is the lower bound; [`stop`](https://docs.python.org/3/library/functions.html#slice.stop "slice.stop") is the upper bound; [`step`](https://docs.python.org/3/library/functions.html#slice.step "slice.step") is the step value; each is `None` if omitted. These attributes can have any type.
Slice objects support one method:
slice.indices(*self*, *length*)[¶](https://docs.python.org/3/reference/datamodel.html#slice.indices "Link to this definition")
This method takes a single integer argument *length* and computes information about the slice that the slice object would describe if applied to a sequence of *length* items. It returns a tuple of three integers; respectively these are the *start* and *stop* indices and the *step* or stride length of the slice. Missing or out-of-bounds indices are handled in a manner consistent with regular slices.
#### 3\.2.13.5. Static method objects[¶](https://docs.python.org/3/reference/datamodel.html#static-method-objects "Link to this heading")
Static method objects provide a way of defeating the transformation of function objects to method objects described above. A static method object is a wrapper around any other object, usually a user-defined method object. When a static method object is retrieved from a class or a class instance, the object actually returned is the wrapped object, which is not subject to any further transformation. Static method objects are also callable. Static method objects are created by the built-in [`staticmethod()`](https://docs.python.org/3/library/functions.html#staticmethod "staticmethod") constructor.
#### 3\.2.13.6. Class method objects[¶](https://docs.python.org/3/reference/datamodel.html#class-method-objects "Link to this heading")
A class method object, like a static method object, is a wrapper around another object that alters the way in which that object is retrieved from classes and class instances. The behaviour of class method objects upon such retrieval is described above, under [âinstance methodsâ](https://docs.python.org/3/reference/datamodel.html#instance-methods). Class method objects are created by the built-in [`classmethod()`](https://docs.python.org/3/library/functions.html#classmethod "classmethod") constructor.
## 3\.3. Special method names[¶](https://docs.python.org/3/reference/datamodel.html#special-method-names "Link to this heading")
A class can implement certain operations that are invoked by special syntax (such as arithmetic operations or subscripting and slicing) by defining methods with special names. This is Pythonâs approach to *operator overloading*, allowing classes to define their own behavior with respect to language operators. For instance, if a class defines a method named [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"), and `x` is an instance of this class, then `x[i]` is roughly equivalent to `type(x).__getitem__(x, i)`. Except where mentioned, attempts to execute an operation raise an exception when no appropriate method is defined (typically [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError") or [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError")).
Setting a special method to `None` indicates that the corresponding operation is not available. For example, if a class sets [`__iter__()`](https://docs.python.org/3/reference/datamodel.html#object.__iter__ "object.__iter__") to `None`, the class is not iterable, so calling [`iter()`](https://docs.python.org/3/library/functions.html#iter "iter") on its instances will raise a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") (without falling back to [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__")). [\[2\]](https://docs.python.org/3/reference/datamodel.html#id21)
When implementing a class that emulates any built-in type, it is important that the emulation only be implemented to the degree that it makes sense for the object being modelled. For example, some sequences may work well with retrieval of individual elements, but extracting a slice may not make sense. (One example of this is the [NodeList](https://docs.python.org/3/library/xml.dom.html#dom-nodelist-objects) interface in the W3Câs Document Object Model.)
### 3\.3.1. Basic customization[¶](https://docs.python.org/3/reference/datamodel.html#basic-customization "Link to this heading")
object.\_\_new\_\_(*cls*\[, *...*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__new__ "Link to this definition")
Called to create a new instance of class *cls*. `__new__()` is a static method (special-cased so you need not declare it as such) that takes the class of which an instance was requested as its first argument. The remaining arguments are those passed to the object constructor expression (the call to the class). The return value of `__new__()` should be the new object instance (usually an instance of *cls*).
Typical implementations create a new instance of the class by invoking the superclassâs `__new__()` method using `super().__new__(cls[, ...])` with appropriate arguments and then modifying the newly created instance as necessary before returning it.
If `__new__()` is invoked during object construction and it returns an instance of *cls*, then the new instanceâs [`__init__()`](https://docs.python.org/3/reference/datamodel.html#object.__init__ "object.__init__") method will be invoked like `__init__(self[, ...])`, where *self* is the new instance and the remaining arguments are the same as were passed to the object constructor.
If `__new__()` does not return an instance of *cls*, then the new instanceâs [`__init__()`](https://docs.python.org/3/reference/datamodel.html#object.__init__ "object.__init__") method will not be invoked.
`__new__()` is intended mainly to allow subclasses of immutable types (like int, str, or tuple) to customize instance creation. It is also commonly overridden in custom metaclasses in order to customize class creation.
object.\_\_init\_\_(*self*\[, *...*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__init__ "Link to this definition")
Called after the instance has been created (by [`__new__()`](https://docs.python.org/3/reference/datamodel.html#object.__new__ "object.__new__")), but before it is returned to the caller. The arguments are those passed to the class constructor expression. If a base class has an `__init__()` method, the derived classâs `__init__()` method, if any, must explicitly call it to ensure proper initialization of the base class part of the instance; for example: `super().__init__([args...])`.
Because [`__new__()`](https://docs.python.org/3/reference/datamodel.html#object.__new__ "object.__new__") and `__init__()` work together in constructing objects (`__new__()` to create it, and `__init__()` to customize it), no non-`None` value may be returned by `__init__()`; doing so will cause a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") to be raised at runtime.
object.\_\_del\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__del__ "Link to this definition")
Called when the instance is about to be destroyed. This is also called a finalizer or (improperly) a destructor. If a base class has a `__del__()` method, the derived classâs `__del__()` method, if any, must explicitly call it to ensure proper deletion of the base class part of the instance.
It is possible (though not recommended!) for the `__del__()` method to postpone destruction of the instance by creating a new reference to it. This is called object *resurrection*. It is implementation-dependent whether `__del__()` is called a second time when a resurrected object is about to be destroyed; the current [CPython](https://docs.python.org/3/glossary.html#term-CPython) implementation only calls it once.
It is not guaranteed that `__del__()` methods are called for objects that still exist when the interpreter exits. [`weakref.finalize`](https://docs.python.org/3/library/weakref.html#weakref.finalize "weakref.finalize") provides a straightforward way to register a cleanup function to be called when an object is garbage collected.
Note
`del x` doesnât directly call `x.__del__()` â the former decrements the reference count for `x` by one, and the latter is only called when `x`âs reference count reaches zero.
**CPython implementation detail:** It is possible for a reference cycle to prevent the reference count of an object from going to zero. In this case, the cycle will be later detected and deleted by the [cyclic garbage collector](https://docs.python.org/3/glossary.html#term-garbage-collection). A common cause of reference cycles is when an exception has been caught in a local variable. The frameâs locals then reference the exception, which references its own traceback, which references the locals of all frames caught in the traceback.
See also
Documentation for the [`gc`](https://docs.python.org/3/library/gc.html#module-gc "gc: Interface to the cycle-detecting garbage collector.") module.
Warning
Due to the precarious circumstances under which `__del__()` methods are invoked, exceptions that occur during their execution are ignored, and a warning is printed to `sys.stderr` instead. In particular:
- `__del__()` can be invoked when arbitrary code is being executed, including from any arbitrary thread. If `__del__()` needs to take a lock or invoke any other blocking resource, it may deadlock as the resource may already be taken by the code that gets interrupted to execute `__del__()`.
- `__del__()` can be executed during interpreter shutdown. As a consequence, the global variables it needs to access (including other modules) may already have been deleted or set to `None`. Python guarantees that globals whose name begins with a single underscore are deleted from their module before other globals are deleted; if no other references to such globals exist, this may help in assuring that imported modules are still available at the time when the `__del__()` method is called.
object.\_\_repr\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__repr__ "Link to this definition")
Called by the [`repr()`](https://docs.python.org/3/library/functions.html#repr "repr") built-in function to compute the âofficialâ string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form `<...some useful description...>` should be returned. The return value must be a string object. If a class defines `__repr__()` but not [`__str__()`](https://docs.python.org/3/reference/datamodel.html#object.__str__ "object.__str__"), then `__repr__()` is also used when an âinformalâ string representation of instances of that class is required.
This is typically used for debugging, so it is important that the representation is information-rich and unambiguous. A default implementation is provided by the [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself.
object.\_\_str\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__str__ "Link to this definition")
Called by [`str(object)`](https://docs.python.org/3/library/stdtypes.html#str "str"), the default [`__format__()`](https://docs.python.org/3/reference/datamodel.html#object.__format__ "object.__format__") implementation, and the built-in function [`print()`](https://docs.python.org/3/library/functions.html#print "print"), to compute the âinformalâ or nicely printable string representation of an object. The return value must be a [str](https://docs.python.org/3/library/stdtypes.html#textseq) object.
This method differs from [`object.__repr__()`](https://docs.python.org/3/reference/datamodel.html#object.__repr__ "object.__repr__") in that there is no expectation that `__str__()` return a valid Python expression: a more convenient or concise representation can be used.
The default implementation defined by the built-in type [`object`](https://docs.python.org/3/library/functions.html#object "object") calls [`object.__repr__()`](https://docs.python.org/3/reference/datamodel.html#object.__repr__ "object.__repr__").
object.\_\_bytes\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__bytes__ "Link to this definition")
Called by [bytes](https://docs.python.org/3/library/functions.html#func-bytes) to compute a byte-string representation of an object. This should return a [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") object. The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide this method.
object.\_\_format\_\_(*self*, *format\_spec*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__format__ "Link to this definition")
Called by the [`format()`](https://docs.python.org/3/library/functions.html#format "format") built-in function, and by extension, evaluation of [formatted string literals](https://docs.python.org/3/reference/lexical_analysis.html#f-strings) and the [`str.format()`](https://docs.python.org/3/library/stdtypes.html#str.format "str.format") method, to produce a âformattedâ string representation of an object. The *format\_spec* argument is a string that contains a description of the formatting options desired. The interpretation of the *format\_spec* argument is up to the type implementing `__format__()`, however most classes will either delegate formatting to one of the built-in types, or use a similar formatting option syntax.
See [Format specification mini-language](https://docs.python.org/3/library/string.html#formatspec) for a description of the standard formatting syntax.
The return value must be a string object.
The default implementation by the [`object`](https://docs.python.org/3/library/functions.html#object "object") class should be given an empty *format\_spec* string. It delegates to [`__str__()`](https://docs.python.org/3/reference/datamodel.html#object.__str__ "object.__str__").
Changed in version 3.4: The \_\_format\_\_ method of `object` itself raises a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") if passed any non-empty string.
Changed in version 3.7: `object.__format__(x, '')` is now equivalent to `str(x)` rather than `format(str(x), '')`.
object.\_\_lt\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__lt__ "Link to this definition")
object.\_\_le\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__le__ "Link to this definition")
object.\_\_eq\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "Link to this definition")
object.\_\_ne\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ne__ "Link to this definition")
object.\_\_gt\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__gt__ "Link to this definition")
object.\_\_ge\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ge__ "Link to this definition")
These are the so-called ârich comparisonâ methods. The correspondence between operator symbols and method names is as follows: `x<y` calls `x.__lt__(y)`, `x<=y` calls `x.__le__(y)`, `x==y` calls `x.__eq__(y)`, `x!=y` calls `x.__ne__(y)`, `x>y` calls `x.__gt__(y)`, and `x>=y` calls `x.__ge__(y)`.
A rich comparison method may return the singleton [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented") if it does not implement the operation for a given pair of arguments. By convention, `False` and `True` are returned for a successful comparison. However, these methods can return any value, so if the comparison operator is used in a Boolean context (e.g., in the condition of an `if` statement), Python will call [`bool()`](https://docs.python.org/3/library/functions.html#bool "bool") on the value to determine if the result is true or false.
By default, `object` implements `__eq__()` by using `is`, returning [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented") in the case of a false comparison: `True if x is y else NotImplemented`. For `__ne__()`, by default it delegates to `__eq__()` and inverts the result unless it is `NotImplemented`. There are no other implied relationships among the comparison operators or default implementations; for example, the truth of `(x<y or x==y)` does not imply `x<=y`. To automatically generate ordering operations from a single root operation, see [`functools.total_ordering()`](https://docs.python.org/3/library/functools.html#functools.total_ordering "functools.total_ordering").
By default, the [`object`](https://docs.python.org/3/library/functions.html#object "object") class provides implementations consistent with [Value comparisons](https://docs.python.org/3/reference/expressions.html#expressions-value-comparisons): equality compares according to object identity, and order comparisons raise [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError"). Each default method may generate these results directly, but may also return [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented").
See the paragraph on [`__hash__()`](https://docs.python.org/3/reference/datamodel.html#object.__hash__ "object.__hash__") for some important notes on creating [hashable](https://docs.python.org/3/glossary.html#term-hashable) objects which support custom comparison operations and are usable as dictionary keys.
There are no swapped-argument versions of these methods (to be used when the left argument does not support the operation but the right argument does); rather, `__lt__()` and `__gt__()` are each otherâs reflection, `__le__()` and `__ge__()` are each otherâs reflection, and `__eq__()` and `__ne__()` are their own reflection. If the operands are of different types, and the right operandâs type is a direct or indirect subclass of the left operandâs type, the reflected method of the right operand has priority, otherwise the left operandâs method has priority. Virtual subclassing is not considered.
When no appropriate method returns any value other than [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented"), the `==` and `!=` operators will fall back to `is` and `is not`, respectively.
object.\_\_hash\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__hash__ "Link to this definition")
Called by built-in function [`hash()`](https://docs.python.org/3/library/functions.html#hash "hash") and for operations on members of hashed collections including [`set`](https://docs.python.org/3/library/stdtypes.html#set "set"), [`frozenset`](https://docs.python.org/3/library/stdtypes.html#frozenset "frozenset"), and [`dict`](https://docs.python.org/3/library/stdtypes.html#dict "dict"). The `__hash__()` method should return an integer. The only required property is that objects which compare equal have the same hash value; it is advised to mix together the hash values of the components of the object that also play a part in comparison of objects by packing them into a tuple and hashing the tuple. Example:
```
def __hash__(self):
return hash((self.name, self.nick, self.color))
```
Note
[`hash()`](https://docs.python.org/3/library/functions.html#hash "hash") truncates the value returned from an objectâs custom `__hash__()` method to the size of a [`Py_ssize_t`](https://docs.python.org/3/c-api/intro.html#c.Py_ssize_t "Py_ssize_t"). This is typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds. If an objectâs `__hash__()` must interoperate on builds of different bit sizes, be sure to check the width on all supported builds. An easy way to do this is with `python -c "import sys; print(sys.hash_info.width)"`.
If a class does not define an [`__eq__()`](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "object.__eq__") method it should not define a `__hash__()` operation either; if it defines `__eq__()` but not `__hash__()`, its instances will not be usable as items in hashable collections. If a class defines mutable objects and implements an `__eq__()` method, it should not implement `__hash__()`, since the implementation of [hashable](https://docs.python.org/3/glossary.html#term-hashable) collections requires that a keyâs hash value is immutable (if the objectâs hash value changes, it will be in the wrong hash bucket).
User-defined classes have [`__eq__()`](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "object.__eq__") and `__hash__()` methods by default (inherited from the [`object`](https://docs.python.org/3/library/functions.html#object "object") class); with them, all objects compare unequal (except with themselves) and `x.__hash__()` returns an appropriate value such that `x == y` implies both that `x is y` and `hash(x) == hash(y)`.
A class that overrides [`__eq__()`](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "object.__eq__") and does not define `__hash__()` will have its `__hash__()` implicitly set to `None`. When the `__hash__()` method of a class is `None`, instances of the class will raise an appropriate [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") when a program attempts to retrieve their hash value, and will also be correctly identified as unhashable when checking `isinstance(obj, collections.abc.Hashable)`.
If a class that overrides [`__eq__()`](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "object.__eq__") needs to retain the implementation of `__hash__()` from a parent class, the interpreter must be told this explicitly by setting `__hash__ = <ParentClass>.__hash__`.
If a class that does not override [`__eq__()`](https://docs.python.org/3/reference/datamodel.html#object.__eq__ "object.__eq__") wishes to suppress hash support, it should include `__hash__ = None` in the class definition. A class which defines its own `__hash__()` that explicitly raises a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") would be incorrectly identified as hashable by an `isinstance(obj, collections.abc.Hashable)` call.
Note
By default, the `__hash__()` values of str and bytes objects are âsaltedâ with an unpredictable random value. Although they remain constant within an individual Python process, they are not predictable between repeated invocations of Python.
This is intended to provide protection against a denial-of-service caused by carefully chosen inputs that exploit the worst case performance of a dict insertion, *O*(*n*2) complexity. See <https://ocert.org/advisories/ocert-2011-003.html> for details.
Changing hash values affects the iteration order of sets. Python has never made guarantees about this ordering (and it typically varies between 32-bit and 64-bit builds).
See also [`PYTHONHASHSEED`](https://docs.python.org/3/using/cmdline.html#envvar-PYTHONHASHSEED).
Changed in version 3.3: Hash randomization is enabled by default.
object.\_\_bool\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__bool__ "Link to this definition")
Called to implement truth value testing and the built-in operation `bool()`; should return `False` or `True`. When this method is not defined, [`__len__()`](https://docs.python.org/3/reference/datamodel.html#object.__len__ "object.__len__") is called, if it is defined, and the object is considered true if its result is nonzero. If a class defines neither `__len__()` nor `__bool__()` (which is true of the [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself), all its instances are considered true.
### 3\.3.2. Customizing attribute access[¶](https://docs.python.org/3/reference/datamodel.html#customizing-attribute-access "Link to this heading")
The following methods can be defined to customize the meaning of attribute access (use of, assignment to, or deletion of `x.name`) for class instances.
object.\_\_getattr\_\_(*self*, *name*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__getattr__ "Link to this definition")
Called when the default attribute access fails with an [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError") (either [`__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__") raises an `AttributeError` because *name* is not an instance attribute or an attribute in the class tree for `self`; or [`__get__()`](https://docs.python.org/3/reference/datamodel.html#object.__get__ "object.__get__") of a *name* property raises `AttributeError`). This method should either return the (computed) attribute value or raise an `AttributeError` exception. The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide this method.
Note that if the attribute is found through the normal mechanism, `__getattr__()` is not called. (This is an intentional asymmetry between `__getattr__()` and [`__setattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__setattr__ "object.__setattr__").) This is done both for efficiency reasons and because otherwise `__getattr__()` would have no way to access other attributes of the instance. Note that at least for instance variables, you can take total control by not inserting any values in the instance attribute dictionary (but instead inserting them in another object). See the [`__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__") method below for a way to actually get total control over attribute access.
object.\_\_getattribute\_\_(*self*, *name*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "Link to this definition")
Called unconditionally to implement attribute accesses for instances of the class. If the class also defines [`__getattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattr__ "object.__getattr__"), the latter will not be called unless `__getattribute__()` either calls it explicitly or raises an [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError"). This method should return the (computed) attribute value or raise an `AttributeError` exception. In order to avoid infinite recursion in this method, its implementation should always call the base class method with the same name to access any attributes it needs, for example, `object.__getattribute__(self, name)`.
Note
This method may still be bypassed when looking up special methods as the result of implicit invocation via language syntax or [built-in functions](https://docs.python.org/3/reference/datamodel.html#builtin-functions). See [Special method lookup](https://docs.python.org/3/reference/datamodel.html#special-lookup).
For certain sensitive attribute accesses, raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `object.__getattr__` with arguments `obj` and `name`.
object.\_\_setattr\_\_(*self*, *name*, *value*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__setattr__ "Link to this definition")
Called when an attribute assignment is attempted. This is called instead of the normal mechanism (i.e. store the value in the instance dictionary). *name* is the attribute name, *value* is the value to be assigned to it.
If `__setattr__()` wants to assign to an instance attribute, it should call the base class method with the same name, for example, `object.__setattr__(self, name, value)`.
For certain sensitive attribute assignments, raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `object.__setattr__` with arguments `obj`, `name`, `value`.
object.\_\_delattr\_\_(*self*, *name*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__delattr__ "Link to this definition")
Like [`__setattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__setattr__ "object.__setattr__") but for attribute deletion instead of assignment. This should only be implemented if `del obj.name` is meaningful for the object.
For certain sensitive attribute deletions, raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `object.__delattr__` with arguments `obj` and `name`.
object.\_\_dir\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__dir__ "Link to this definition")
Called when [`dir()`](https://docs.python.org/3/library/functions.html#dir "dir") is called on the object. An iterable must be returned. `dir()` converts the returned iterable to a list and sorts it.
#### 3\.3.2.1. Customizing module attribute access[¶](https://docs.python.org/3/reference/datamodel.html#customizing-module-attribute-access "Link to this heading")
module.\_\_getattr\_\_()[¶](https://docs.python.org/3/reference/datamodel.html#module.__getattr__ "Link to this definition")
module.\_\_dir\_\_()[¶](https://docs.python.org/3/reference/datamodel.html#module.__dir__ "Link to this definition")
Special names `__getattr__` and `__dir__` can be also used to customize access to module attributes. The `__getattr__` function at the module level should accept one argument which is the name of an attribute and return the computed value or raise an [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError"). If an attribute is not found on a module object through the normal lookup, i.e. [`object.__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__"), then `__getattr__` is searched in the module `__dict__` before raising an `AttributeError`. If found, it is called with the attribute name and the result is returned.
The `__dir__` function should accept no arguments, and return an iterable of strings that represents the names accessible on module. If present, this function overrides the standard [`dir()`](https://docs.python.org/3/library/functions.html#dir "dir") search on a module.
module.\_\_class\_\_[¶](https://docs.python.org/3/reference/datamodel.html#module.__class__ "Link to this definition")
For a more fine grained customization of the module behavior (setting attributes, properties, etc.), one can set the `__class__` attribute of a module object to a subclass of [`types.ModuleType`](https://docs.python.org/3/library/types.html#types.ModuleType "types.ModuleType"). For example:
```
import sys
from types import ModuleType
class VerboseModule(ModuleType):
def __repr__(self):
return f'Verbose {self.__name__}'
def __setattr__(self, attr, value):
print(f'Setting {attr}...')
super().__setattr__(attr, value)
sys.modules[__name__].__class__ = VerboseModule
```
Note
Defining module `__getattr__` and setting module `__class__` only affect lookups made using the attribute access syntax â directly accessing the module globals (whether by code within the module, or via a reference to the moduleâs globals dictionary) is unaffected.
Changed in version 3.5: `__class__` module attribute is now writable.
Added in version 3.7: `__getattr__` and `__dir__` module attributes.
See also
[**PEP 562**](https://peps.python.org/pep-0562/) - Module \_\_getattr\_\_ and \_\_dir\_\_
Describes the `__getattr__` and `__dir__` functions on modules.
#### 3\.3.2.2. Implementing Descriptors[¶](https://docs.python.org/3/reference/datamodel.html#implementing-descriptors "Link to this heading")
The following methods only apply when an instance of the class containing the method (a so-called *descriptor* class) appears in an *owner* class (the descriptor must be in either the ownerâs class dictionary or in the class dictionary for one of its parents). In the examples below, âthe attributeâ refers to the attribute whose name is the key of the property in the owner classâ [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__"). The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not implement any of these protocols.
object.\_\_get\_\_(*self*, *instance*, *owner\=None*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__get__ "Link to this definition")
Called to get the attribute of the owner class (class attribute access) or of an instance of that class (instance attribute access). The optional *owner* argument is the owner class, while *instance* is the instance that the attribute was accessed through, or `None` when the attribute is accessed through the *owner*.
This method should return the computed attribute value or raise an [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError") exception.
[**PEP 252**](https://peps.python.org/pep-0252/) specifies that `__get__()` is callable with one or two arguments. Pythonâs own built-in descriptors support this specification; however, it is likely that some third-party tools have descriptors that require both arguments. Pythonâs own [`__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__") implementation always passes in both arguments whether they are required or not.
object.\_\_set\_\_(*self*, *instance*, *value*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__set__ "Link to this definition")
Called to set the attribute on an instance *instance* of the owner class to a new value, *value*.
Note, adding `__set__()` or [`__delete__()`](https://docs.python.org/3/reference/datamodel.html#object.__delete__ "object.__delete__") changes the kind of descriptor to a âdata descriptorâ. See [Invoking Descriptors](https://docs.python.org/3/reference/datamodel.html#descriptor-invocation) for more details.
object.\_\_delete\_\_(*self*, *instance*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__delete__ "Link to this definition")
Called to delete the attribute on an instance *instance* of the owner class.
Instances of descriptors may also have the `__objclass__` attribute present:
object.\_\_objclass\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__objclass__ "Link to this definition")
The attribute `__objclass__` is interpreted by the [`inspect`](https://docs.python.org/3/library/inspect.html#module-inspect "inspect: Extract information and source code from live objects.") module as specifying the class where this object was defined (setting this appropriately can assist in runtime introspection of dynamic class attributes). For callables, it may indicate that an instance of the given type (or a subclass) is expected or required as the first positional argument (for example, CPython sets this attribute for unbound methods that are implemented in C).
#### 3\.3.2.3. Invoking Descriptors[¶](https://docs.python.org/3/reference/datamodel.html#invoking-descriptors "Link to this heading")
In general, a descriptor is an object attribute with âbinding behaviorâ, one whose attribute access has been overridden by methods in the descriptor protocol: [`__get__()`](https://docs.python.org/3/reference/datamodel.html#object.__get__ "object.__get__"), [`__set__()`](https://docs.python.org/3/reference/datamodel.html#object.__set__ "object.__set__"), and [`__delete__()`](https://docs.python.org/3/reference/datamodel.html#object.__delete__ "object.__delete__"). If any of those methods are defined for an object, it is said to be a descriptor.
The default behavior for attribute access is to get, set, or delete the attribute from an objectâs dictionary. For instance, `a.x` has a lookup chain starting with `a.__dict__['x']`, then `type(a).__dict__['x']`, and continuing through the base classes of `type(a)` excluding metaclasses.
However, if the looked-up value is an object defining one of the descriptor methods, then Python may override the default behavior and invoke the descriptor method instead. Where this occurs in the precedence chain depends on which descriptor methods were defined and how they were called.
The starting point for descriptor invocation is a binding, `a.x`. How the arguments are assembled depends on `a`:
Direct Call
The simplest and least common call is when user code directly invokes a descriptor method: `x.__get__(a)`.
Instance Binding
If binding to an object instance, `a.x` is transformed into the call: `type(a).__dict__['x'].__get__(a, type(a))`.
Class Binding
If binding to a class, `A.x` is transformed into the call: `A.__dict__['x'].__get__(None, A)`.
Super Binding
A dotted lookup such as `super(A, a).x` searches `a.__class__.__mro__` for a base class `B` following `A` and then returns `B.__dict__['x'].__get__(a, A)`. If not a descriptor, `x` is returned unchanged.
For instance bindings, the precedence of descriptor invocation depends on which descriptor methods are defined. A descriptor can define any combination of [`__get__()`](https://docs.python.org/3/reference/datamodel.html#object.__get__ "object.__get__"), [`__set__()`](https://docs.python.org/3/reference/datamodel.html#object.__set__ "object.__set__") and [`__delete__()`](https://docs.python.org/3/reference/datamodel.html#object.__delete__ "object.__delete__"). If it does not define `__get__()`, then accessing the attribute will return the descriptor object itself unless there is a value in the objectâs instance dictionary. If the descriptor defines `__set__()` and/or `__delete__()`, it is a data descriptor; if it defines neither, it is a non-data descriptor. Normally, data descriptors define both `__get__()` and `__set__()`, while non-data descriptors have just the `__get__()` method. Data descriptors with `__get__()` and `__set__()` (and/or `__delete__()`) defined always override a redefinition in an instance dictionary. In contrast, non-data descriptors can be overridden by instances.
Python methods (including those decorated with [`@staticmethod`](https://docs.python.org/3/library/functions.html#staticmethod "staticmethod") and [`@classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod")) are implemented as non-data descriptors. Accordingly, instances can redefine and override methods. This allows individual instances to acquire behaviors that differ from other instances of the same class.
The [`property()`](https://docs.python.org/3/library/functions.html#property "property") function is implemented as a data descriptor. Accordingly, instances cannot override the behavior of a property.
#### 3\.3.2.4. \_\_slots\_\_[¶](https://docs.python.org/3/reference/datamodel.html#slots "Link to this heading")
*\_\_slots\_\_* allow us to explicitly declare data members (like properties) and deny the creation of [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") and *\_\_weakref\_\_* (unless explicitly declared in *\_\_slots\_\_* or available in a parent.)
The space saved over using [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") can be significant. Attribute lookup speed can be significantly improved as well.
object.\_\_slots\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__slots__ "Link to this definition")
This class variable can be assigned a string, iterable, or sequence of strings with variable names used by instances. *\_\_slots\_\_* reserves space for the declared variables and prevents the automatic creation of [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") and *\_\_weakref\_\_* for each instance.
Notes on using *\_\_slots\_\_*:
- When inheriting from a class without *\_\_slots\_\_*, the [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") and *\_\_weakref\_\_* attribute of the instances will always be accessible.
- Without a [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") variable, instances cannot be assigned new variables not listed in the *\_\_slots\_\_* definition. Attempts to assign to an unlisted variable name raises [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError"). If dynamic assignment of new variables is desired, then add `'__dict__'` to the sequence of strings in the *\_\_slots\_\_* declaration.
- Without a *\_\_weakref\_\_* variable for each instance, classes defining *\_\_slots\_\_* do not support [`weak references`](https://docs.python.org/3/library/weakref.html#module-weakref "weakref: Support for weak references and weak dictionaries.") to its instances. If weak reference support is needed, then add `'__weakref__'` to the sequence of strings in the *\_\_slots\_\_* declaration.
- *\_\_slots\_\_* are implemented at the class level by creating [descriptors](https://docs.python.org/3/reference/datamodel.html#descriptors) for each variable name. As a result, class attributes cannot be used to set default values for instance variables defined by *\_\_slots\_\_*; otherwise, the class attribute would overwrite the descriptor assignment.
- The action of a *\_\_slots\_\_* declaration is not limited to the class where it is defined. *\_\_slots\_\_* declared in parents are available in child classes. However, instances of a child subclass will get a [`__dict__`](https://docs.python.org/3/reference/datamodel.html#object.__dict__ "object.__dict__") and *\_\_weakref\_\_* unless the subclass also defines *\_\_slots\_\_* (which should only contain names of any *additional* slots).
- If a class defines a slot also defined in a base class, the instance variable defined by the base class slot is inaccessible (except by retrieving its descriptor directly from the base class). This renders the meaning of the program undefined. In the future, a check may be added to prevent this.
- [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") will be raised if nonempty *\_\_slots\_\_* are defined for a class derived from a [`"variable-length" built-in type`](https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_itemsize "PyTypeObject.tp_itemsize") such as [`int`](https://docs.python.org/3/library/functions.html#int "int"), [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes"), and [`tuple`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple").
- Any non-string [iterable](https://docs.python.org/3/glossary.html#term-iterable) may be assigned to *\_\_slots\_\_*.
- If a [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") is used to assign *\_\_slots\_\_*, the dictionary keys will be used as the slot names. The values of the dictionary can be used to provide per-attribute docstrings that will be recognised by [`inspect.getdoc()`](https://docs.python.org/3/library/inspect.html#inspect.getdoc "inspect.getdoc") and displayed in the output of [`help()`](https://docs.python.org/3/library/functions.html#help "help").
- [`__class__`](https://docs.python.org/3/reference/datamodel.html#object.__class__ "object.__class__") assignment works only if both classes have the same *\_\_slots\_\_*.
- [Multiple inheritance](https://docs.python.org/3/tutorial/classes.html#tut-multiple) with multiple slotted parent classes can be used, but only one parent is allowed to have attributes created by slots (the other bases must have empty slot layouts) - violations raise [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError").
- If an [iterator](https://docs.python.org/3/glossary.html#term-iterator) is used for *\_\_slots\_\_* then a [descriptor](https://docs.python.org/3/glossary.html#term-descriptor) is created for each of the iteratorâs values. However, the *\_\_slots\_\_* attribute will be an empty iterator.
### 3\.3.3. Customizing class creation[¶](https://docs.python.org/3/reference/datamodel.html#customizing-class-creation "Link to this heading")
Whenever a class inherits from another class, [`__init_subclass__()`](https://docs.python.org/3/reference/datamodel.html#object.__init_subclass__ "object.__init_subclass__") is called on the parent class. This way, it is possible to write classes which change the behavior of subclasses. This is closely related to class decorators, but where class decorators only affect the specific class theyâre applied to, `__init_subclass__` solely applies to future subclasses of the class defining the method.
*classmethod* object.\_\_init\_subclass\_\_(*cls*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__init_subclass__ "Link to this definition")
This method is called whenever the containing class is subclassed. *cls* is then the new subclass. If defined as a normal instance method, this method is implicitly converted to a class method.
Keyword arguments which are given to a new class are passed to the parent classâs `__init_subclass__`. For compatibility with other classes using `__init_subclass__`, one should take out the needed keyword arguments and pass the others over to the base class, as in:
```
class Philosopher:
def __init_subclass__(cls, /, default_name, **kwargs):
super().__init_subclass__(**kwargs)
cls.default_name = default_name
class AustralianPhilosopher(Philosopher, default_name="Bruce"):
pass
```
The default implementation `object.__init_subclass__` does nothing, but raises an error if it is called with any arguments.
Note
The metaclass hint `metaclass` is consumed by the rest of the type machinery, and is never passed to `__init_subclass__` implementations. The actual metaclass (rather than the explicit hint) can be accessed as `type(cls)`.
Added in version 3.6.
When a class is created, `type.__new__()` scans the class variables and makes callbacks to those with a [`__set_name__()`](https://docs.python.org/3/reference/datamodel.html#object.__set_name__ "object.__set_name__") hook.
object.\_\_set\_name\_\_(*self*, *owner*, *name*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__set_name__ "Link to this definition")
Automatically called at the time the owning class *owner* is created. The object has been assigned to *name* in that class:
```
class A:
x = C() # Automatically calls: x.__set_name__(A, 'x')
```
If the class variable is assigned after the class is created, `__set_name__()` will not be called automatically. If needed, `__set_name__()` can be called directly:
```
class A:
pass
c = C()
A.x = c # The hook is not called
c.__set_name__(A, 'x') # Manually invoke the hook
```
See [Creating the class object](https://docs.python.org/3/reference/datamodel.html#class-object-creation) for more details.
Added in version 3.6.
#### 3\.3.3.1. Metaclasses[¶](https://docs.python.org/3/reference/datamodel.html#metaclasses "Link to this heading")
By default, classes are constructed using [`type()`](https://docs.python.org/3/library/functions.html#type "type"). The class body is executed in a new namespace and the class name is bound locally to the result of `type(name, bases, namespace)`.
The class creation process can be customized by passing the `metaclass` keyword argument in the class definition line, or by inheriting from an existing class that included such an argument. In the following example, both `MyClass` and `MySubclass` are instances of `Meta`:
```
class Meta(type):
pass
class MyClass(metaclass=Meta):
pass
class MySubclass(MyClass):
pass
```
Any other keyword arguments that are specified in the class definition are passed through to all metaclass operations described below.
When a class definition is executed, the following steps occur:
- MRO entries are resolved;
- the appropriate metaclass is determined;
- the class namespace is prepared;
- the class body is executed;
- the class object is created.
#### 3\.3.3.2. Resolving MRO entries[¶](https://docs.python.org/3/reference/datamodel.html#resolving-mro-entries "Link to this heading")
object.\_\_mro\_entries\_\_(*self*, *bases*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__mro_entries__ "Link to this definition")
If a base that appears in a class definition is not an instance of [`type`](https://docs.python.org/3/library/functions.html#type "type"), then an `__mro_entries__()` method is searched on the base. If an `__mro_entries__()` method is found, the base is substituted with the result of a call to `__mro_entries__()` when creating the class. The method is called with the original bases tuple passed to the *bases* parameter, and must return a tuple of classes that will be used instead of the base. The returned tuple may be empty: in these cases, the original base is ignored.
#### 3\.3.3.3. Determining the appropriate metaclass[¶](https://docs.python.org/3/reference/datamodel.html#determining-the-appropriate-metaclass "Link to this heading")
The appropriate metaclass for a class definition is determined as follows:
- if no bases and no explicit metaclass are given, then [`type()`](https://docs.python.org/3/library/functions.html#type "type") is used;
- if an explicit metaclass is given and it is *not* an instance of [`type()`](https://docs.python.org/3/library/functions.html#type "type"), then it is used directly as the metaclass;
- if an instance of [`type()`](https://docs.python.org/3/library/functions.html#type "type") is given as the explicit metaclass, or bases are defined, then the most derived metaclass is used.
The most derived metaclass is selected from the explicitly specified metaclass (if any) and the metaclasses (i.e. `type(cls)`) of all specified base classes. The most derived metaclass is one which is a subtype of *all* of these candidate metaclasses. If none of the candidate metaclasses meets that criterion, then the class definition will fail with `TypeError`.
#### 3\.3.3.4. Preparing the class namespace[¶](https://docs.python.org/3/reference/datamodel.html#preparing-the-class-namespace "Link to this heading")
Once the appropriate metaclass has been identified, then the class namespace is prepared. If the metaclass has a `__prepare__` attribute, it is called as `namespace = metaclass.__prepare__(name, bases, **kwds)` (where the additional keyword arguments, if any, come from the class definition). The `__prepare__` method should be implemented as a [`classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod"). The namespace returned by `__prepare__` is passed in to `__new__`, but when the final class object is created the namespace is copied into a new `dict`.
If the metaclass has no `__prepare__` attribute, then the class namespace is initialised as an empty ordered mapping.
See also
[**PEP 3115**](https://peps.python.org/pep-3115/) - Metaclasses in Python 3000
Introduced the `__prepare__` namespace hook
#### 3\.3.3.5. Executing the class body[¶](https://docs.python.org/3/reference/datamodel.html#executing-the-class-body "Link to this heading")
The class body is executed (approximately) as `exec(body, globals(), namespace)`. The key difference from a normal call to [`exec()`](https://docs.python.org/3/library/functions.html#exec "exec") is that lexical scoping allows the class body (including any methods) to reference names from the current and outer scopes when the class definition occurs inside a function.
However, even when the class definition occurs inside the function, methods defined inside the class still cannot see names defined at the class scope. Class variables must be accessed through the first parameter of instance or class methods, or through the implicit lexically scoped `__class__` reference described in the next section.
#### 3\.3.3.6. Creating the class object[¶](https://docs.python.org/3/reference/datamodel.html#creating-the-class-object "Link to this heading")
Once the class namespace has been populated by executing the class body, the class object is created by calling `metaclass(name, bases, namespace, **kwds)` (the additional keywords passed here are the same as those passed to `__prepare__`).
This class object is the one that will be referenced by the zero-argument form of [`super()`](https://docs.python.org/3/library/functions.html#super "super"). `__class__` is an implicit closure reference created by the compiler if any methods in a class body refer to either `__class__` or `super`. This allows the zero argument form of `super()` to correctly identify the class being defined based on lexical scoping, while the class or instance that was used to make the current call is identified based on the first argument passed to the method.
**CPython implementation detail:** In CPython 3.6 and later, the `__class__` cell is passed to the metaclass as a `__classcell__` entry in the class namespace. If present, this must be propagated up to the `type.__new__` call in order for the class to be initialised correctly. Failing to do so will result in a [`RuntimeError`](https://docs.python.org/3/library/exceptions.html#RuntimeError "RuntimeError") in Python 3.8.
When using the default metaclass [`type`](https://docs.python.org/3/library/functions.html#type "type"), or any metaclass that ultimately calls `type.__new__`, the following additional customization steps are invoked after creating the class object:
1. The `type.__new__` method collects all of the attributes in the class namespace that define a [`__set_name__()`](https://docs.python.org/3/reference/datamodel.html#object.__set_name__ "object.__set_name__") method;
2. Those `__set_name__` methods are called with the class being defined and the assigned name of that particular attribute;
3. The [`__init_subclass__()`](https://docs.python.org/3/reference/datamodel.html#object.__init_subclass__ "object.__init_subclass__") hook is called on the immediate parent of the new class in its method resolution order.
After the class object is created, it is passed to the class decorators included in the class definition (if any) and the resulting object is bound in the local namespace as the defined class.
When a new class is created by `type.__new__`, the object provided as the namespace parameter is copied to a new ordered mapping and the original object is discarded. The new copy is wrapped in a read-only proxy, which becomes the [`__dict__`](https://docs.python.org/3/reference/datamodel.html#type.__dict__ "type.__dict__") attribute of the class object.
See also
[**PEP 3135**](https://peps.python.org/pep-3135/) - New super
Describes the implicit `__class__` closure reference
#### 3\.3.3.7. Uses for metaclasses[¶](https://docs.python.org/3/reference/datamodel.html#uses-for-metaclasses "Link to this heading")
The potential uses for metaclasses are boundless. Some ideas that have been explored include enum, logging, interface checking, automatic delegation, automatic property creation, proxies, frameworks, and automatic resource locking/synchronization.
### 3\.3.4. Customizing instance and subclass checks[¶](https://docs.python.org/3/reference/datamodel.html#customizing-instance-and-subclass-checks "Link to this heading")
The following methods are used to override the default behavior of the [`isinstance()`](https://docs.python.org/3/library/functions.html#isinstance "isinstance") and [`issubclass()`](https://docs.python.org/3/library/functions.html#issubclass "issubclass") built-in functions.
In particular, the metaclass [`abc.ABCMeta`](https://docs.python.org/3/library/abc.html#abc.ABCMeta "abc.ABCMeta") implements these methods in order to allow the addition of Abstract Base Classes (ABCs) as âvirtual base classesâ to any class or type (including built-in types), including other ABCs.
type.\_\_instancecheck\_\_(*self*, *instance*)[¶](https://docs.python.org/3/reference/datamodel.html#type.__instancecheck__ "Link to this definition")
Return true if *instance* should be considered a (direct or indirect) instance of *class*. If defined, called to implement .
type.\_\_subclasscheck\_\_(*self*, *subclass*)[¶](https://docs.python.org/3/reference/datamodel.html#type.__subclasscheck__ "Link to this definition")
Return true if *subclass* should be considered a (direct or indirect) subclass of *class*. If defined, called to implement .
Note that these methods are looked up on the type (metaclass) of a class. They cannot be defined as class methods in the actual class. This is consistent with the lookup of special methods that are called on instances, only in this case the instance is itself a class.
See also
[**PEP 3119**](https://peps.python.org/pep-3119/) - Introducing Abstract Base Classes
Includes the specification for customizing [`isinstance()`](https://docs.python.org/3/library/functions.html#isinstance "isinstance") and [`issubclass()`](https://docs.python.org/3/library/functions.html#issubclass "issubclass") behavior through [`__instancecheck__()`](https://docs.python.org/3/reference/datamodel.html#type.__instancecheck__ "type.__instancecheck__") and [`__subclasscheck__()`](https://docs.python.org/3/reference/datamodel.html#type.__subclasscheck__ "type.__subclasscheck__"), with motivation for this functionality in the context of adding Abstract Base Classes (see the [`abc`](https://docs.python.org/3/library/abc.html#module-abc "abc: Abstract base classes according to :pep:`3119`.") module) to the language.
### 3\.3.5. Emulating generic types[¶](https://docs.python.org/3/reference/datamodel.html#emulating-generic-types "Link to this heading")
When using [type annotations](https://docs.python.org/3/glossary.html#term-annotation), it is often useful to *parameterize* a [generic type](https://docs.python.org/3/glossary.html#term-generic-type) using Pythonâs square-brackets notation. For example, the annotation `list[int]` might be used to signify a [`list`](https://docs.python.org/3/library/stdtypes.html#list "list") in which all the elements are of type [`int`](https://docs.python.org/3/library/functions.html#int "int").
See also
[**PEP 484**](https://peps.python.org/pep-0484/) - Type Hints
Introducing Pythonâs framework for type annotations
[Generic Alias Types](https://docs.python.org/3/library/stdtypes.html#types-genericalias)
Documentation for objects representing parameterized generic classes
[Generics](https://docs.python.org/3/library/typing.html#generics), [user-defined generics](https://docs.python.org/3/library/typing.html#user-defined-generics) and [`typing.Generic`](https://docs.python.org/3/library/typing.html#typing.Generic "typing.Generic")
Documentation on how to implement generic classes that can be parameterized at runtime and understood by static type-checkers.
A class can *generally* only be parameterized if it defines the special class method `__class_getitem__()`.
*classmethod* object.\_\_class\_getitem\_\_(*cls*, *key*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "Link to this definition")
Return an object representing the specialization of a generic class by type arguments found in *key*.
When defined on a class, `__class_getitem__()` is automatically a class method. As such, there is no need for it to be decorated with [`@classmethod`](https://docs.python.org/3/library/functions.html#classmethod "classmethod") when it is defined.
#### 3\.3.5.1. The purpose of *\_\_class\_getitem\_\_*[¶](https://docs.python.org/3/reference/datamodel.html#the-purpose-of-class-getitem "Link to this heading")
The purpose of [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") is to allow runtime parameterization of standard-library generic classes in order to more easily apply [type hints](https://docs.python.org/3/glossary.html#term-type-hint) to these classes.
To implement custom generic classes that can be parameterized at runtime and understood by static type-checkers, users should either inherit from a standard library class that already implements [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__"), or inherit from [`typing.Generic`](https://docs.python.org/3/library/typing.html#typing.Generic "typing.Generic"), which has its own implementation of `__class_getitem__()`.
Custom implementations of [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") on classes defined outside of the standard library may not be understood by third-party type-checkers such as mypy. Using `__class_getitem__()` on any class for purposes other than type hinting is discouraged.
#### 3\.3.5.2. *\_\_class\_getitem\_\_* versus *\_\_getitem\_\_*[¶](https://docs.python.org/3/reference/datamodel.html#class-getitem-versus-getitem "Link to this heading")
Usually, the [subscription](https://docs.python.org/3/reference/expressions.html#subscriptions) of an object using square brackets will call the [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__") instance method defined on the objectâs class. However, if the object being subscribed is itself a class, the class method [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") may be called instead. `__class_getitem__()` should return a [GenericAlias](https://docs.python.org/3/library/stdtypes.html#types-genericalias) object if it is properly defined.
Presented with the [expression](https://docs.python.org/3/glossary.html#term-expression) `obj[x]`, the Python interpreter follows something like the following process to decide whether [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__") or [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") should be called:
```
from inspect import isclass
def subscribe(obj, x):
"""Return the result of the expression 'obj[x]'"""
class_of_obj = type(obj)
# If the class of obj defines __getitem__,
# call class_of_obj.__getitem__(obj, x)
if hasattr(class_of_obj, '__getitem__'):
return class_of_obj.__getitem__(obj, x)
# Else, if obj is a class and defines __class_getitem__,
# call obj.__class_getitem__(x)
elif isclass(obj) and hasattr(obj, '__class_getitem__'):
return obj.__class_getitem__(x)
# Else, raise an exception
else:
raise TypeError(
f"'{class_of_obj.__name__}' object is not subscriptable"
)
```
In Python, all classes are themselves instances of other classes. The class of a class is known as that classâs [metaclass](https://docs.python.org/3/glossary.html#term-metaclass), and most classes have the [`type`](https://docs.python.org/3/library/functions.html#type "type") class as their metaclass. `type` does not define [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"), meaning that expressions such as `list[int]`, `dict[str, float]` and `tuple[str, bytes]` all result in [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") being called:
```
>>> # list has class "type" as its metaclass, like most classes:
>>> type(list)
<class 'type'>
>>> type(dict) == type(list) == type(tuple) == type(str) == type(bytes)
True
>>> # "list[int]" calls "list.__class_getitem__(int)"
>>> list[int]
list[int]
>>> # list.__class_getitem__ returns a GenericAlias object:
>>> type(list[int])
<class 'types.GenericAlias'>
```
However, if a class has a custom metaclass that defines [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"), subscribing the class may result in different behaviour. An example of this can be found in the [`enum`](https://docs.python.org/3/library/enum.html#module-enum "enum: Implementation of an enumeration class.") module:
```
>>> from enum import Enum
>>> class Menu(Enum):
... """A breakfast menu"""
... SPAM = 'spam'
... BACON = 'bacon'
...
>>> # Enum classes have a custom metaclass:
>>> type(Menu)
<class 'enum.EnumMeta'>
>>> # EnumMeta defines __getitem__,
>>> # so __class_getitem__ is not called,
>>> # and the result is not a GenericAlias object:
>>> Menu['SPAM']
<Menu.SPAM: 'spam'>
>>> type(Menu['SPAM'])
<enum 'Menu'>
```
See also
[**PEP 560**](https://peps.python.org/pep-0560/) - Core Support for typing module and generic types
Introducing [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__"), and outlining when a [subscription](https://docs.python.org/3/reference/expressions.html#subscriptions) results in `__class_getitem__()` being called instead of [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__")
### 3\.3.6. Emulating callable objects[¶](https://docs.python.org/3/reference/datamodel.html#emulating-callable-objects "Link to this heading")
object.\_\_call\_\_(*self*\[, *args...*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__call__ "Link to this definition")
Called when the instance is âcalledâ as a function; if this method is defined, `x(arg1, arg2, ...)` roughly translates to `type(x).__call__(x, arg1, ...)`. The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide this method.
### 3\.3.7. Emulating container types[¶](https://docs.python.org/3/reference/datamodel.html#emulating-container-types "Link to this heading")
The following methods can be defined to implement container objects. None of them are provided by the [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself. Containers usually are [sequences](https://docs.python.org/3/glossary.html#term-sequence) (such as [`lists`](https://docs.python.org/3/library/stdtypes.html#list "list") or [`tuples`](https://docs.python.org/3/library/stdtypes.html#tuple "tuple")) or [mappings](https://docs.python.org/3/glossary.html#term-mapping) (like [dictionaries](https://docs.python.org/3/glossary.html#term-dictionary)), but can represent other containers as well. The first set of methods is used either to emulate a sequence or to emulate a mapping; the difference is that for a sequence, the allowable keys should be the integers *k* for which where *N* is the length of the sequence, or [`slice`](https://docs.python.org/3/library/functions.html#slice "slice") objects, which define a range of items. It is also recommended that mappings provide the methods `keys()`, `values()`, `items()`, `get()`, `clear()`, `setdefault()`, `pop()`, `popitem()`, `copy()`, and `update()` behaving similar to those for Pythonâs standard [`dictionary`](https://docs.python.org/3/library/stdtypes.html#dict "dict") objects. The [`collections.abc`](https://docs.python.org/3/library/collections.abc.html#module-collections.abc "collections.abc: Abstract base classes for containers") module provides a [`MutableMapping`](https://docs.python.org/3/library/collections.abc.html#collections.abc.MutableMapping "collections.abc.MutableMapping") [abstract base class](https://docs.python.org/3/glossary.html#term-abstract-base-class) to help create those methods from a base set of [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"), [`__setitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "object.__setitem__"), [`__delitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__delitem__ "object.__delitem__"), and `keys()`.
Mutable sequences should provide methods [`append()`](https://docs.python.org/3/library/stdtypes.html#sequence.append "sequence.append"), [`clear()`](https://docs.python.org/3/library/stdtypes.html#sequence.clear "sequence.clear"), [`count()`](https://docs.python.org/3/library/stdtypes.html#sequence.count "sequence.count"), [`extend()`](https://docs.python.org/3/library/stdtypes.html#sequence.extend "sequence.extend"), [`index()`](https://docs.python.org/3/library/stdtypes.html#sequence.index "sequence.index"), [`insert()`](https://docs.python.org/3/library/stdtypes.html#sequence.insert "sequence.insert"), [`pop()`](https://docs.python.org/3/library/stdtypes.html#sequence.pop "sequence.pop"), [`remove()`](https://docs.python.org/3/library/stdtypes.html#sequence.remove "sequence.remove"), and [`reverse()`](https://docs.python.org/3/library/stdtypes.html#sequence.reverse "sequence.reverse"), like Python standard [`list`](https://docs.python.org/3/library/stdtypes.html#list "list") objects. Finally, sequence types should implement addition (meaning concatenation) and multiplication (meaning repetition) by defining the methods [`__add__()`](https://docs.python.org/3/reference/datamodel.html#object.__add__ "object.__add__"), [`__radd__()`](https://docs.python.org/3/reference/datamodel.html#object.__radd__ "object.__radd__"), [`__iadd__()`](https://docs.python.org/3/reference/datamodel.html#object.__iadd__ "object.__iadd__"), [`__mul__()`](https://docs.python.org/3/reference/datamodel.html#object.__mul__ "object.__mul__"), [`__rmul__()`](https://docs.python.org/3/reference/datamodel.html#object.__rmul__ "object.__rmul__") and [`__imul__()`](https://docs.python.org/3/reference/datamodel.html#object.__imul__ "object.__imul__") described below; they should not define other numerical operators.
It is recommended that both mappings and sequences implement the [`__contains__()`](https://docs.python.org/3/reference/datamodel.html#object.__contains__ "object.__contains__") method to allow efficient use of the `in` operator; for mappings, `in` should search the mappingâs keys; for sequences, it should search through the values. It is further recommended that both mappings and sequences implement the [`__iter__()`](https://docs.python.org/3/reference/datamodel.html#object.__iter__ "object.__iter__") method to allow efficient iteration through the container; for mappings, `__iter__()` should iterate through the objectâs keys; for sequences, it should iterate through the values.
object.\_\_len\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__len__ "Link to this definition")
Called to implement the built-in function [`len()`](https://docs.python.org/3/library/functions.html#len "len"). Should return the length of the object, an integer `>=` 0. Also, an object that doesnât define a [`__bool__()`](https://docs.python.org/3/reference/datamodel.html#object.__bool__ "object.__bool__") method and whose `__len__()` method returns zero is considered to be false in a Boolean context.
**CPython implementation detail:** In CPython, the length is required to be at most [`sys.maxsize`](https://docs.python.org/3/library/sys.html#sys.maxsize "sys.maxsize"). If the length is larger than `sys.maxsize` some features (such as [`len()`](https://docs.python.org/3/library/functions.html#len "len")) may raise [`OverflowError`](https://docs.python.org/3/library/exceptions.html#OverflowError "OverflowError"). To prevent raising `OverflowError` by truth value testing, an object must define a [`__bool__()`](https://docs.python.org/3/reference/datamodel.html#object.__bool__ "object.__bool__") method.
object.\_\_length\_hint\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__length_hint__ "Link to this definition")
Called to implement [`operator.length_hint()`](https://docs.python.org/3/library/operator.html#operator.length_hint "operator.length_hint"). Should return an estimated length for the object (which may be greater or less than the actual length). The length must be an integer `>=` 0. The return value may also be [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented"), which is treated the same as if the `__length_hint__` method didnât exist at all. This method is purely an optimization and is never required for correctness.
Added in version 3.4.
object.\_\_getitem\_\_(*self*, *subscript*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "Link to this definition")
Called to implement *subscription*, that is, `self[subscript]`. See [Subscriptions and slicings](https://docs.python.org/3/reference/expressions.html#subscriptions) for details on the syntax.
There are two types of built-in objects that support subscription via `__getitem__()`:
- **sequences**, where *subscript* (also called [index](https://docs.python.org/3/glossary.html#term-index)) should be an integer or a [`slice`](https://docs.python.org/3/library/functions.html#slice "slice") object. See the [sequence documentation](https://docs.python.org/3/reference/datamodel.html#datamodel-sequences) for the expected behavior, including handling `slice` objects and negative indices.
- **mappings**, where *subscript* is also called the [key](https://docs.python.org/3/glossary.html#term-key). See [mapping documentation](https://docs.python.org/3/reference/datamodel.html#datamodel-mappings) for the expected behavior.
If *subscript* is of an inappropriate type, `__getitem__()` should raise [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError"). If *subscript* has an inappropriate value, `__getitem__()` should raise an [`LookupError`](https://docs.python.org/3/library/exceptions.html#LookupError "LookupError") or one of its subclasses ([`IndexError`](https://docs.python.org/3/library/exceptions.html#IndexError "IndexError") for sequences; [`KeyError`](https://docs.python.org/3/library/exceptions.html#KeyError "KeyError") for mappings).
Note
Slicing is handled by `__getitem__()`, [`__setitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "object.__setitem__"), and [`__delitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__delitem__ "object.__delitem__"). A call like
```
a[1:2] = b
```
is translated to
```
a[slice(1, 2, None)] = b
```
and so forth. Missing slice items are always filled in with `None`.
Note
The sequence iteration protocol (used, for example, in [`for`](https://docs.python.org/3/reference/compound_stmts.html#for) loops), expects that an [`IndexError`](https://docs.python.org/3/library/exceptions.html#IndexError "IndexError") will be raised for illegal indexes to allow proper detection of the end of a sequence.
Note
When [subscripting](https://docs.python.org/3/reference/expressions.html#subscriptions) a *class*, the special class method [`__class_getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__class_getitem__ "object.__class_getitem__") may be called instead of `__getitem__()`. See [\_\_class\_getitem\_\_ versus \_\_getitem\_\_](https://docs.python.org/3/reference/datamodel.html#classgetitem-versus-getitem) for more details.
object.\_\_setitem\_\_(*self*, *key*, *value*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "Link to this definition")
Called to implement assignment to `self[key]`. Same note as for [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"). This should only be implemented for mappings if the objects support changes to the values for keys, or if new keys can be added, or for sequences if elements can be replaced. The same exceptions should be raised for improper *key* values as for the `__getitem__()` method.
object.\_\_delitem\_\_(*self*, *key*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__delitem__ "Link to this definition")
Called to implement deletion of `self[key]`. Same note as for [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"). This should only be implemented for mappings if the objects support removal of keys, or for sequences if elements can be removed from the sequence. The same exceptions should be raised for improper *key* values as for the `__getitem__()` method.
object.\_\_missing\_\_(*self*, *key*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__missing__ "Link to this definition")
Called by [`dict`](https://docs.python.org/3/library/stdtypes.html#dict "dict").[`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__") to implement `self[key]` for dict subclasses when key is not in the dictionary.
object.\_\_iter\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__iter__ "Link to this definition")
This method is called when an [iterator](https://docs.python.org/3/glossary.html#term-iterator) is required for a container. This method should return a new iterator object that can iterate over all the objects in the container. For mappings, it should iterate over the keys of the container.
object.\_\_reversed\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__reversed__ "Link to this definition")
Called (if present) by the [`reversed()`](https://docs.python.org/3/library/functions.html#reversed "reversed") built-in to implement reverse iteration. It should return a new iterator object that iterates over all the objects in the container in reverse order.
If the `__reversed__()` method is not provided, the [`reversed()`](https://docs.python.org/3/library/functions.html#reversed "reversed") built-in will fall back to using the sequence protocol ([`__len__()`](https://docs.python.org/3/reference/datamodel.html#object.__len__ "object.__len__") and [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__")). Objects that support the sequence protocol should only provide `__reversed__()` if they can provide an implementation that is more efficient than the one provided by `reversed()`.
The membership test operators ([`in`](https://docs.python.org/3/reference/expressions.html#in) and [`not in`](https://docs.python.org/3/reference/expressions.html#not-in)) are normally implemented as an iteration through a container. However, container objects can supply the following special method with a more efficient implementation, which also does not require the object be iterable.
object.\_\_contains\_\_(*self*, *item*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__contains__ "Link to this definition")
Called to implement membership test operators. Should return true if *item* is in *self*, false otherwise. For mapping objects, this should consider the keys of the mapping rather than the values or the key-item pairs.
For objects that donât define `__contains__()`, the membership test first tries iteration via [`__iter__()`](https://docs.python.org/3/reference/datamodel.html#object.__iter__ "object.__iter__"), then the old sequence iteration protocol via [`__getitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__getitem__ "object.__getitem__"), see [this section in the language reference](https://docs.python.org/3/reference/expressions.html#membership-test-details).
### 3\.3.8. Emulating numeric types[¶](https://docs.python.org/3/reference/datamodel.html#emulating-numeric-types "Link to this heading")
The following methods can be defined to emulate numeric objects. Methods corresponding to operations that are not supported by the particular kind of number implemented (e.g., bitwise operations for non-integral numbers) should be left undefined.
object.\_\_add\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__add__ "Link to this definition")
object.\_\_sub\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__sub__ "Link to this definition")
object.\_\_mul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__mul__ "Link to this definition")
object.\_\_matmul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__matmul__ "Link to this definition")
object.\_\_truediv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__truediv__ "Link to this definition")
object.\_\_floordiv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__floordiv__ "Link to this definition")
object.\_\_mod\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__mod__ "Link to this definition")
object.\_\_divmod\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__divmod__ "Link to this definition")
object.\_\_pow\_\_(*self*, *other*\[, *modulo*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__pow__ "Link to this definition")
object.\_\_lshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__lshift__ "Link to this definition")
object.\_\_rshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rshift__ "Link to this definition")
object.\_\_and\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__and__ "Link to this definition")
object.\_\_xor\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__xor__ "Link to this definition")
object.\_\_or\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__or__ "Link to this definition")
These methods are called to implement the binary arithmetic operations (`+`, `-`, `*`, `@`, `/`, `//`, `%`, [`divmod()`](https://docs.python.org/3/library/functions.html#divmod "divmod"), [`pow()`](https://docs.python.org/3/library/functions.html#pow "pow"), `**`, `<<`, `>>`, `&`, `^`, `|`). For instance, to evaluate the expression `x + y`, where *x* is an instance of a class that has an `__add__()` method, `type(x).__add__(x, y)` is called. The `__divmod__()` method should be the equivalent to using `__floordiv__()` and `__mod__()`; it should not be related to `__truediv__()`. Note that `__pow__()` should be defined to accept an optional third argument if the three-argument version of the built-in `pow()` function is to be supported.
If one of those methods does not support the operation with the supplied arguments, it should return [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented").
object.\_\_radd\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__radd__ "Link to this definition")
object.\_\_rsub\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rsub__ "Link to this definition")
object.\_\_rmul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rmul__ "Link to this definition")
object.\_\_rmatmul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rmatmul__ "Link to this definition")
object.\_\_rtruediv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rtruediv__ "Link to this definition")
object.\_\_rfloordiv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rfloordiv__ "Link to this definition")
object.\_\_rmod\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rmod__ "Link to this definition")
object.\_\_rdivmod\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rdivmod__ "Link to this definition")
object.\_\_rpow\_\_(*self*, *other*\[, *modulo*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__rpow__ "Link to this definition")
object.\_\_rlshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rlshift__ "Link to this definition")
object.\_\_rrshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rrshift__ "Link to this definition")
object.\_\_rand\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rand__ "Link to this definition")
object.\_\_rxor\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__rxor__ "Link to this definition")
object.\_\_ror\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ror__ "Link to this definition")
These methods are called to implement the binary arithmetic operations (`+`, `-`, `*`, `@`, `/`, `//`, `%`, [`divmod()`](https://docs.python.org/3/library/functions.html#divmod "divmod"), [`pow()`](https://docs.python.org/3/library/functions.html#pow "pow"), `**`, `<<`, `>>`, `&`, `^`, `|`) with reflected (swapped) operands. These functions are only called if the operands are of different types, when the left operand does not support the corresponding operation [\[3\]](https://docs.python.org/3/reference/datamodel.html#id22), or the right operandâs class is derived from the left operandâs class. [\[4\]](https://docs.python.org/3/reference/datamodel.html#id23) For instance, to evaluate the expression `x - y`, where *y* is an instance of a class that has an `__rsub__()` method, `type(y).__rsub__(y, x)` is called if `type(x).__sub__(x, y)` returns [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented") or `type(y)` is a subclass of `type(x)`. [\[5\]](https://docs.python.org/3/reference/datamodel.html#id24)
Note that `__rpow__()` should be defined to accept an optional third argument if the three-argument version of the built-in [`pow()`](https://docs.python.org/3/library/functions.html#pow "pow") function is to be supported.
Changed in version 3.14: Three-argument [`pow()`](https://docs.python.org/3/library/functions.html#pow "pow") now try calling `__rpow__()` if necessary. Previously it was only called in two-argument `pow()` and the binary power operator.
Note
If the right operandâs type is a subclass of the left operandâs type and that subclass provides a different implementation of the reflected method for the operation, this method will be called before the left operandâs non-reflected method. This behavior allows subclasses to override their ancestorsâ operations.
object.\_\_iadd\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__iadd__ "Link to this definition")
object.\_\_isub\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__isub__ "Link to this definition")
object.\_\_imul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__imul__ "Link to this definition")
object.\_\_imatmul\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__imatmul__ "Link to this definition")
object.\_\_itruediv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__itruediv__ "Link to this definition")
object.\_\_ifloordiv\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ifloordiv__ "Link to this definition")
object.\_\_imod\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__imod__ "Link to this definition")
object.\_\_ipow\_\_(*self*, *other*\[, *modulo*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__ipow__ "Link to this definition")
object.\_\_ilshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ilshift__ "Link to this definition")
object.\_\_irshift\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__irshift__ "Link to this definition")
object.\_\_iand\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__iand__ "Link to this definition")
object.\_\_ixor\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ixor__ "Link to this definition")
object.\_\_ior\_\_(*self*, *other*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ior__ "Link to this definition")
These methods are called to implement the augmented arithmetic assignments (`+=`, `-=`, `*=`, `@=`, `/=`, `//=`, `%=`, `**=`, `<<=`, `>>=`, `&=`, `^=`, `|=`). These methods should attempt to do the operation in-place (modifying *self*) and return the result (which could be, but does not have to be, *self*). If a specific method is not defined, or if that method returns [`NotImplemented`](https://docs.python.org/3/library/constants.html#NotImplemented "NotImplemented"), the augmented assignment falls back to the normal methods. For instance, if *x* is an instance of a class with an `__iadd__()` method, `x += y` is equivalent to `x = x.__iadd__(y)` . If `__iadd__()` does not exist, or if `x.__iadd__(y)` returns `NotImplemented`, `x.__add__(y)` and `y.__radd__(x)` are considered, as with the evaluation of `x + y`. In certain situations, augmented assignment can result in unexpected errors (see [Why does a\_tuple\[i\] += \[âitemâ\] raise an exception when the addition works?](https://docs.python.org/3/faq/programming.html#faq-augmented-assignment-tuple-error)), but this behavior is in fact part of the data model.
object.\_\_neg\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__neg__ "Link to this definition")
object.\_\_pos\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__pos__ "Link to this definition")
object.\_\_abs\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__abs__ "Link to this definition")
object.\_\_invert\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__invert__ "Link to this definition")
Called to implement the unary arithmetic operations (`-`, `+`, [`abs()`](https://docs.python.org/3/library/functions.html#abs "abs") and `~`).
object.\_\_complex\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__complex__ "Link to this definition")
object.\_\_int\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__int__ "Link to this definition")
object.\_\_float\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__float__ "Link to this definition")
Called to implement the built-in functions [`complex()`](https://docs.python.org/3/library/functions.html#complex "complex"), [`int()`](https://docs.python.org/3/library/functions.html#int "int") and [`float()`](https://docs.python.org/3/library/functions.html#float "float"). Should return a value of the appropriate type.
object.\_\_index\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__index__ "Link to this definition")
Called to implement [`operator.index()`](https://docs.python.org/3/library/operator.html#operator.index "operator.index"), and whenever Python needs to losslessly convert the numeric object to an integer object (such as in slicing, or in the built-in [`bin()`](https://docs.python.org/3/library/functions.html#bin "bin"), [`hex()`](https://docs.python.org/3/library/functions.html#hex "hex") and [`oct()`](https://docs.python.org/3/library/functions.html#oct "oct") functions). Presence of this method indicates that the numeric object is an integer type. Must return an integer.
If [`__int__()`](https://docs.python.org/3/reference/datamodel.html#object.__int__ "object.__int__"), [`__float__()`](https://docs.python.org/3/reference/datamodel.html#object.__float__ "object.__float__") and [`__complex__()`](https://docs.python.org/3/reference/datamodel.html#object.__complex__ "object.__complex__") are not defined then corresponding built-in functions [`int()`](https://docs.python.org/3/library/functions.html#int "int"), [`float()`](https://docs.python.org/3/library/functions.html#float "float") and [`complex()`](https://docs.python.org/3/library/functions.html#complex "complex") fall back to `__index__()`.
object.\_\_round\_\_(*self*\[, *ndigits*\])[¶](https://docs.python.org/3/reference/datamodel.html#object.__round__ "Link to this definition")
object.\_\_trunc\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__trunc__ "Link to this definition")
object.\_\_floor\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__floor__ "Link to this definition")
object.\_\_ceil\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__ceil__ "Link to this definition")
Called to implement the built-in function [`round()`](https://docs.python.org/3/library/functions.html#round "round") and [`math`](https://docs.python.org/3/library/math.html#module-math "math: Mathematical functions (sin() etc.).") functions [`trunc()`](https://docs.python.org/3/library/math.html#math.trunc "math.trunc"), [`floor()`](https://docs.python.org/3/library/math.html#math.floor "math.floor") and [`ceil()`](https://docs.python.org/3/library/math.html#math.ceil "math.ceil"). Unless *ndigits* is passed to `__round__()` all these methods should return the value of the object truncated to an [`Integral`](https://docs.python.org/3/library/numbers.html#numbers.Integral "numbers.Integral") (typically an [`int`](https://docs.python.org/3/library/functions.html#int "int")).
Changed in version 3.14: [`int()`](https://docs.python.org/3/library/functions.html#int "int") no longer delegates to the `__trunc__()` method.
### 3\.3.9. With Statement Context Managers[¶](https://docs.python.org/3/reference/datamodel.html#with-statement-context-managers "Link to this heading")
A *context manager* is an object that defines the runtime context to be established when executing a [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement. The context manager handles the entry into, and the exit from, the desired runtime context for the execution of the block of code. Context managers are normally invoked using the `with` statement (described in section [The with statement](https://docs.python.org/3/reference/compound_stmts.html#with)), but can also be used by directly invoking their methods.
Typical uses of context managers include saving and restoring various kinds of global state, locking and unlocking resources, closing opened files, etc.
For more information on context managers, see [Context Manager Types](https://docs.python.org/3/library/stdtypes.html#typecontextmanager). The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide the context manager methods.
object.\_\_enter\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__enter__ "Link to this definition")
Enter the runtime context related to this object. The [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement will bind this methodâs return value to the target(s) specified in the `as` clause of the statement, if any.
object.\_\_exit\_\_(*self*, *exc\_type*, *exc\_value*, *traceback*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__exit__ "Link to this definition")
Exit the runtime context related to this object. The parameters describe the exception that caused the context to be exited. If the context was exited without an exception, all three arguments will be [`None`](https://docs.python.org/3/library/constants.html#None "None").
If an exception is supplied, and the method wishes to suppress the exception (i.e., prevent it from being propagated), it should return a true value. Otherwise, the exception will be processed normally upon exit from this method.
Note that `__exit__()` methods should not reraise the passed-in exception; this is the callerâs responsibility.
See also
[**PEP 343**](https://peps.python.org/pep-0343/) - The âwithâ statement
The specification, background, and examples for the Python [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement.
### 3\.3.10. Customizing positional arguments in class pattern matching[¶](https://docs.python.org/3/reference/datamodel.html#customizing-positional-arguments-in-class-pattern-matching "Link to this heading")
When using a class name in a pattern, positional arguments in the pattern are not allowed by default, i.e. `case MyClass(x, y)` is typically invalid without special support in `MyClass`. To be able to use that kind of pattern, the class needs to define a *\_\_match\_args\_\_* attribute.
object.\_\_match\_args\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__match_args__ "Link to this definition")
This class variable can be assigned a tuple of strings. When this class is used in a class pattern with positional arguments, each positional argument will be converted into a keyword argument, using the corresponding value in *\_\_match\_args\_\_* as the keyword. The absence of this attribute is equivalent to setting it to `()`.
For example, if `MyClass.__match_args__` is `("left", "center", "right")` that means that `case MyClass(x, y)` is equivalent to `case MyClass(left=x, center=y)`. Note that the number of arguments in the pattern must be smaller than or equal to the number of elements in *\_\_match\_args\_\_*; if it is larger, the pattern match attempt will raise a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError").
Added in version 3.10.
See also
[**PEP 634**](https://peps.python.org/pep-0634/) - Structural Pattern Matching
The specification for the Python `match` statement.
### 3\.3.11. Emulating buffer types[¶](https://docs.python.org/3/reference/datamodel.html#emulating-buffer-types "Link to this heading")
The [buffer protocol](https://docs.python.org/3/c-api/buffer.html#bufferobjects) provides a way for Python objects to expose efficient access to a low-level memory array. This protocol is implemented by builtin types such as [`bytes`](https://docs.python.org/3/library/stdtypes.html#bytes "bytes") and [`memoryview`](https://docs.python.org/3/library/stdtypes.html#memoryview "memoryview"), and third-party libraries may define additional buffer types.
While buffer types are usually implemented in C, it is also possible to implement the protocol in Python.
object.\_\_buffer\_\_(*self*, *flags*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__buffer__ "Link to this definition")
Called when a buffer is requested from *self* (for example, by the [`memoryview`](https://docs.python.org/3/library/stdtypes.html#memoryview "memoryview") constructor). The *flags* argument is an integer representing the kind of buffer requested, affecting for example whether the returned buffer is read-only or writable. [`inspect.BufferFlags`](https://docs.python.org/3/library/inspect.html#inspect.BufferFlags "inspect.BufferFlags") provides a convenient way to interpret the flags. The method must return a `memoryview` object.
**Thread safety:** In [free-threaded](https://docs.python.org/3/glossary.html#term-free-threading) Python, implementations must manage any internal export counter using atomic operations. The method must be safe to call concurrently from multiple threads, and the returned bufferâs underlying data must remain valid until the corresponding [`__release_buffer__()`](https://docs.python.org/3/reference/datamodel.html#object.__release_buffer__ "object.__release_buffer__") call completes. See [Thread safety for memoryview objects](https://docs.python.org/3/library/threadsafety.html#thread-safety-memoryview) for details.
object.\_\_release\_buffer\_\_(*self*, *buffer*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__release_buffer__ "Link to this definition")
Called when a buffer is no longer needed. The *buffer* argument is a [`memoryview`](https://docs.python.org/3/library/stdtypes.html#memoryview "memoryview") object that was previously returned by [`__buffer__()`](https://docs.python.org/3/reference/datamodel.html#object.__buffer__ "object.__buffer__"). The method must release any resources associated with the buffer. This method should return `None`.
**Thread safety:** In [free-threaded](https://docs.python.org/3/glossary.html#term-free-threading) Python, any export counter decrement must use atomic operations. Resource cleanup must be thread-safe, as the final release may race with concurrent releases from other threads.
Buffer objects that do not need to perform any cleanup are not required to implement this method.
Added in version 3.12.
See also
[**PEP 688**](https://peps.python.org/pep-0688/) - Making the buffer protocol accessible in Python
Introduces the Python `__buffer__` and `__release_buffer__` methods.
[`collections.abc.Buffer`](https://docs.python.org/3/library/collections.abc.html#collections.abc.Buffer "collections.abc.Buffer")
ABC for buffer types.
### 3\.3.12. Annotations[¶](https://docs.python.org/3/reference/datamodel.html#annotations "Link to this heading")
Functions, classes, and modules may contain [annotations](https://docs.python.org/3/glossary.html#term-annotation), which are a way to associate information (usually [type hints](https://docs.python.org/3/glossary.html#term-type-hint)) with a symbol.
object.\_\_annotations\_\_[¶](https://docs.python.org/3/reference/datamodel.html#object.__annotations__ "Link to this definition")
This attribute contains the annotations for an object. It is [lazily evaluated](https://docs.python.org/3/reference/executionmodel.html#lazy-evaluation), so accessing the attribute may execute arbitrary code and raise exceptions. If evaluation is successful, the attribute is set to a dictionary mapping from variable names to annotations.
Changed in version 3.14: Annotations are now lazily evaluated.
object.\_\_annotate\_\_(*format*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__annotate__ "Link to this definition")
An [annotate function](https://docs.python.org/3/glossary.html#term-annotate-function). Returns a new dictionary object mapping attribute/parameter names to their annotation values.
Takes a format parameter specifying the format in which annotations values should be provided. It must be a member of the [`annotationlib.Format`](https://docs.python.org/3/library/annotationlib.html#annotationlib.Format "annotationlib.Format") enum, or an integer with a value corresponding to a member of the enum.
If an annotate function doesnât support the requested format, it must raise [`NotImplementedError`](https://docs.python.org/3/library/exceptions.html#NotImplementedError "NotImplementedError"). Annotate functions must always support [`VALUE`](https://docs.python.org/3/library/annotationlib.html#annotationlib.Format.VALUE "annotationlib.Format.VALUE") format; they must not raise [`NotImplementedError()`](https://docs.python.org/3/library/exceptions.html#NotImplementedError "NotImplementedError") when called with this format.
When called with [`VALUE`](https://docs.python.org/3/library/annotationlib.html#annotationlib.Format.VALUE "annotationlib.Format.VALUE") format, an annotate function may raise [`NameError`](https://docs.python.org/3/library/exceptions.html#NameError "NameError"); it must not raise `NameError` when called requesting any other format.
If an object does not have any annotations, [`__annotate__`](https://docs.python.org/3/reference/datamodel.html#object.__annotate__ "object.__annotate__") should preferably be set to `None` (it canât be deleted), rather than set to a function that returns an empty dict.
Added in version 3.14.
See also
[**PEP 649**](https://peps.python.org/pep-0649/) â Deferred evaluation of annotation using descriptors
Introduces lazy evaluation of annotations and the `__annotate__` function.
### 3\.3.13. Special method lookup[¶](https://docs.python.org/3/reference/datamodel.html#special-method-lookup "Link to this heading")
For custom classes, implicit invocations of special methods are only guaranteed to work correctly if defined on an objectâs type, not in the objectâs instance dictionary. That behaviour is the reason why the following code raises an exception:
```
>>> class C:
... pass
...
>>> c = C()
>>> c.__len__ = lambda: 5
>>> len(c)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: object of type 'C' has no len()
```
The rationale behind this behaviour lies with a number of special methods such as [`__hash__()`](https://docs.python.org/3/reference/datamodel.html#object.__hash__ "object.__hash__") and [`__repr__()`](https://docs.python.org/3/reference/datamodel.html#object.__repr__ "object.__repr__") that are implemented by all objects, including type objects. If the implicit lookup of these methods used the conventional lookup process, they would fail when invoked on the type object itself:
```
>>> 1 .__hash__() == hash(1)
True
>>> int.__hash__() == hash(int)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: descriptor '__hash__' of 'int' object needs an argument
```
Incorrectly attempting to invoke an unbound method of a class in this way is sometimes referred to as âmetaclass confusionâ, and is avoided by bypassing the instance when looking up special methods:
```
>>> type(1).__hash__(1) == hash(1)
True
>>> type(int).__hash__(int) == hash(int)
True
```
In addition to bypassing any instance attributes in the interest of correctness, implicit special method lookup generally also bypasses the [`__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__") method even of the objectâs metaclass:
```
>>> class Meta(type):
... def __getattribute__(*args):
... print("Metaclass getattribute invoked")
... return type.__getattribute__(*args)
...
>>> class C(object, metaclass=Meta):
... def __len__(self):
... return 10
... def __getattribute__(*args):
... print("Class getattribute invoked")
... return object.__getattribute__(*args)
...
>>> c = C()
>>> c.__len__() # Explicit lookup via instance
Class getattribute invoked
10
>>> type(c).__len__(c) # Explicit lookup via type
Metaclass getattribute invoked
10
>>> len(c) # Implicit lookup
10
```
Bypassing the [`__getattribute__()`](https://docs.python.org/3/reference/datamodel.html#object.__getattribute__ "object.__getattribute__") machinery in this fashion provides significant scope for speed optimisations within the interpreter, at the cost of some flexibility in the handling of special methods (the special method *must* be set on the class object itself in order to be consistently invoked by the interpreter).
## 3\.4. Coroutines[¶](https://docs.python.org/3/reference/datamodel.html#coroutines "Link to this heading")
### 3\.4.1. Awaitable Objects[¶](https://docs.python.org/3/reference/datamodel.html#awaitable-objects "Link to this heading")
An [awaitable](https://docs.python.org/3/glossary.html#term-awaitable) object generally implements an [`__await__()`](https://docs.python.org/3/reference/datamodel.html#object.__await__ "object.__await__") method. [Coroutine objects](https://docs.python.org/3/glossary.html#term-coroutine) returned from [`async def`](https://docs.python.org/3/reference/compound_stmts.html#async-def) functions are awaitable.
object.\_\_await\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__await__ "Link to this definition")
Must return an [iterator](https://docs.python.org/3/glossary.html#term-iterator). Should be used to implement [awaitable](https://docs.python.org/3/glossary.html#term-awaitable) objects. For instance, [`asyncio.Future`](https://docs.python.org/3/library/asyncio-future.html#asyncio.Future "asyncio.Future") implements this method to be compatible with the [`await`](https://docs.python.org/3/reference/expressions.html#await) expression. The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself is not awaitable and does not provide this method.
Note
The language doesnât place any restriction on the type or value of the objects yielded by the iterator returned by `__await__`, as this is specific to the implementation of the asynchronous execution framework (e.g. [`asyncio`](https://docs.python.org/3/library/asyncio.html#module-asyncio "asyncio: Asynchronous I/O.")) that will be managing the [awaitable](https://docs.python.org/3/glossary.html#term-awaitable) object.
Added in version 3.5.
See also
[**PEP 492**](https://peps.python.org/pep-0492/) for additional information about awaitable objects.
### 3\.4.2. Coroutine Objects[¶](https://docs.python.org/3/reference/datamodel.html#coroutine-objects "Link to this heading")
[Coroutine objects](https://docs.python.org/3/glossary.html#term-coroutine) are [awaitable](https://docs.python.org/3/glossary.html#term-awaitable) objects. A coroutineâs execution can be controlled by calling [`__await__()`](https://docs.python.org/3/reference/datamodel.html#object.__await__ "object.__await__") and iterating over the result. When the coroutine has finished executing and returns, the iterator raises [`StopIteration`](https://docs.python.org/3/library/exceptions.html#StopIteration "StopIteration"), and the exceptionâs [`value`](https://docs.python.org/3/library/exceptions.html#StopIteration.value "StopIteration.value") attribute holds the return value. If the coroutine raises an exception, it is propagated by the iterator. Coroutines should not directly raise unhandled `StopIteration` exceptions.
Coroutines also have the methods listed below, which are analogous to those of generators (see [Generator-iterator methods](https://docs.python.org/3/reference/expressions.html#generator-methods)). However, unlike generators, coroutines do not directly support iteration.
Changed in version 3.5.2: It is a [`RuntimeError`](https://docs.python.org/3/library/exceptions.html#RuntimeError "RuntimeError") to await on a coroutine more than once.
coroutine.send(*value*)[¶](https://docs.python.org/3/reference/datamodel.html#coroutine.send "Link to this definition")
Starts or resumes execution of the coroutine. If *value* is `None`, this is equivalent to advancing the iterator returned by [`__await__()`](https://docs.python.org/3/reference/datamodel.html#object.__await__ "object.__await__"). If *value* is not `None`, this method delegates to the [`send()`](https://docs.python.org/3/reference/expressions.html#generator.send "generator.send") method of the iterator that caused the coroutine to suspend. The result (return value, [`StopIteration`](https://docs.python.org/3/library/exceptions.html#StopIteration "StopIteration"), or other exception) is the same as when iterating over the `__await__()` return value, described above.
coroutine.throw(*value*)[¶](https://docs.python.org/3/reference/datamodel.html#coroutine.throw "Link to this definition")
coroutine.throw(*type*\[, *value*\[, *traceback*\]\])
Raises the specified exception in the coroutine. This method delegates to the [`throw()`](https://docs.python.org/3/reference/expressions.html#generator.throw "generator.throw") method of the iterator that caused the coroutine to suspend, if it has such a method. Otherwise, the exception is raised at the suspension point. The result (return value, [`StopIteration`](https://docs.python.org/3/library/exceptions.html#StopIteration "StopIteration"), or other exception) is the same as when iterating over the [`__await__()`](https://docs.python.org/3/reference/datamodel.html#object.__await__ "object.__await__") return value, described above. If the exception is not caught in the coroutine, it propagates back to the caller.
Changed in version 3.12: The second signature (type\[, value\[, traceback\]\]) is deprecated and may be removed in a future version of Python.
coroutine.close()[¶](https://docs.python.org/3/reference/datamodel.html#coroutine.close "Link to this definition")
Causes the coroutine to clean itself up and exit. If the coroutine is suspended, this method first delegates to the [`close()`](https://docs.python.org/3/reference/expressions.html#generator.close "generator.close") method of the iterator that caused the coroutine to suspend, if it has such a method. Then it raises [`GeneratorExit`](https://docs.python.org/3/library/exceptions.html#GeneratorExit "GeneratorExit") at the suspension point, causing the coroutine to immediately clean itself up. Finally, the coroutine is marked as having finished executing, even if it was never started.
Coroutine objects are automatically closed using the above process when they are about to be destroyed.
### 3\.4.3. Asynchronous Iterators[¶](https://docs.python.org/3/reference/datamodel.html#asynchronous-iterators "Link to this heading")
An *asynchronous iterator* can call asynchronous code in its `__anext__` method.
Asynchronous iterators can be used in an [`async for`](https://docs.python.org/3/reference/compound_stmts.html#async-for) statement.
The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide these methods.
object.\_\_aiter\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__aiter__ "Link to this definition")
Must return an *asynchronous iterator* object.
object.\_\_anext\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__anext__ "Link to this definition")
Must return an *awaitable* resulting in a next value of the iterator. Should raise a [`StopAsyncIteration`](https://docs.python.org/3/library/exceptions.html#StopAsyncIteration "StopAsyncIteration") error when the iteration is over.
An example of an asynchronous iterable object:
```
class Reader:
async def readline(self):
...
def __aiter__(self):
return self
async def __anext__(self):
val = await self.readline()
if val == b'':
raise StopAsyncIteration
return val
```
Added in version 3.5.
Changed in version 3.7: Prior to Python 3.7, [`__aiter__()`](https://docs.python.org/3/reference/datamodel.html#object.__aiter__ "object.__aiter__") could return an *awaitable* that would resolve to an [asynchronous iterator](https://docs.python.org/3/glossary.html#term-asynchronous-iterator).
Starting with Python 3.7, [`__aiter__()`](https://docs.python.org/3/reference/datamodel.html#object.__aiter__ "object.__aiter__") must return an asynchronous iterator object. Returning anything else will result in a [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") error.
### 3\.4.4. Asynchronous Context Managers[¶](https://docs.python.org/3/reference/datamodel.html#asynchronous-context-managers "Link to this heading")
An *asynchronous context manager* is a *context manager* that is able to suspend execution in its `__aenter__` and `__aexit__` methods.
Asynchronous context managers can be used in an [`async with`](https://docs.python.org/3/reference/compound_stmts.html#async-with) statement.
The [`object`](https://docs.python.org/3/library/functions.html#object "object") class itself does not provide these methods.
object.\_\_aenter\_\_(*self*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__aenter__ "Link to this definition")
Semantically similar to [`__enter__()`](https://docs.python.org/3/reference/datamodel.html#object.__enter__ "object.__enter__"), the only difference being that it must return an *awaitable*.
object.\_\_aexit\_\_(*self*, *exc\_type*, *exc\_value*, *traceback*)[¶](https://docs.python.org/3/reference/datamodel.html#object.__aexit__ "Link to this definition")
Semantically similar to [`__exit__()`](https://docs.python.org/3/reference/datamodel.html#object.__exit__ "object.__exit__"), the only difference being that it must return an *awaitable*.
An example of an asynchronous context manager class:
```
class AsyncContextManager:
async def __aenter__(self):
await log('entering context')
async def __aexit__(self, exc_type, exc, tb):
await log('exiting context')
```
Added in version 3.5.
Footnotes |
| Shard | 16 (laksa) |
| Root Hash | 10954876678907435016 |
| Unparsed URL | org,python!docs,/3/reference/datamodel.html s443 |