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| Meta Title | 7. Simple statements — Python 3.14.4 documentation | |||||||||
| Meta Description | A simple statement is comprised within a single logical line. Several simple statements may occur on a single line separated by semicolons. The syntax for simple statements is: Expression statement... | |||||||||
| Meta Canonical | null | |||||||||
| Boilerpipe Text | A simple statement is comprised within a single logical line. Several simple
statements may occur on a single line separated by semicolons. The syntax for
simple statements is:
simple_stmt
:
expression_stmt
|
assert_stmt
|
assignment_stmt
|
augmented_assignment_stmt
|
annotated_assignment_stmt
|
pass_stmt
|
del_stmt
|
return_stmt
|
yield_stmt
|
raise_stmt
|
break_stmt
|
continue_stmt
|
import_stmt
|
future_stmt
|
global_stmt
|
nonlocal_stmt
|
type_stmt
7.1.
Expression statements
¶
Expression statements are used (mostly interactively) to compute and write a
value, or (usually) to call a procedure (a function that returns no meaningful
result; in Python, procedures return the value
None
). Other uses of
expression statements are allowed and occasionally useful. The syntax for an
expression statement is:
expression_stmt
:
starred_expression
An expression statement evaluates the expression list (which may be a single
expression).
In interactive mode, if the value is not
None
, it is converted to a string
using the built-in
repr()
function and the resulting string is written to
standard output on a line by itself (except if the result is
None
, so that
procedure calls do not cause any output.)
7.2.
Assignment statements
¶
Assignment statements are used to (re)bind names to values and to modify
attributes or items of mutable objects:
assignment_stmt
: (
target_list
"="
)+ (
starred_expression
|
yield_expression
)
target_list
:
target
(
","
target
)* [
","
]
target
:
identifier
|
"("
[
target_list
]
")"
|
"["
[
target_list
]
"]"
|
attributeref
|
subscription
|
"*"
target
(See section
Primaries
for the syntax definitions for
attributeref
and
subscription
.)
An assignment statement evaluates the expression list (remember that this can be
a single expression or a comma-separated list, the latter yielding a tuple) and
assigns the single resulting object to each of the target lists, from left to
right.
Assignment is defined recursively depending on the form of the target (list).
When a target is part of a mutable object (an attribute reference or
subscription), the mutable object must ultimately perform the assignment and
decide about its validity, and may raise an exception if the assignment is
unacceptable. The rules observed by various types and the exceptions raised are
given with the definition of the object types (see section
The standard type hierarchy
).
Assignment of an object to a target list, optionally enclosed in parentheses or
square brackets, is recursively defined as follows.
If the target list is a single target with no trailing comma,
optionally in parentheses, the object is assigned to that target.
Else:
If the target list contains one target prefixed with an asterisk, called a
“starred” target: The object must be an iterable with at least as many items
as there are targets in the target list, minus one. The first items of the
iterable are assigned, from left to right, to the targets before the starred
target. The final items of the iterable are assigned to the targets after
the starred target. A list of the remaining items in the iterable is then
assigned to the starred target (the list can be empty).
Else: The object must be an iterable with the same number of items as there
are targets in the target list, and the items are assigned, from left to
right, to the corresponding targets.
Assignment of an object to a single target is recursively defined as follows.
If the target is an identifier (name):
If the name does not occur in a
global
or
nonlocal
statement in the current code block: the name is bound to the object in the
current local namespace.
Otherwise: the name is bound to the object in the global namespace or the
outer namespace determined by
nonlocal
, respectively.
The name is rebound if it was already bound. This may cause the reference
count for the object previously bound to the name to reach zero, causing the
object to be deallocated and its destructor (if it has one) to be called.
If the target is an attribute reference: The primary expression in the
reference is evaluated. It should yield an object with assignable attributes;
if this is not the case,
TypeError
is raised. That object is then
asked to assign the assigned object to the given attribute; if it cannot
perform the assignment, it raises an exception (usually but not necessarily
AttributeError
).
Note: If the object is a class instance and the attribute reference occurs on
both sides of the assignment operator, the right-hand side expression,
a.x
can access
either an instance attribute or (if no instance attribute exists) a class
attribute. The left-hand side target
a.x
is always set as an instance attribute,
creating it if necessary. Thus, the two occurrences of
a.x
do not
necessarily refer to the same attribute: if the right-hand side expression refers to a
class attribute, the left-hand side creates a new instance attribute as the target of the
assignment:
class
Cls
:
x
=
3
# class variable
inst
=
Cls
()
inst
.
x
=
inst
.
x
+
1
# writes inst.x as 4 leaving Cls.x as 3
This description does not necessarily apply to descriptor attributes, such as
properties created with
property()
.
If the target is a subscription: The primary expression in the reference is
evaluated.
Next, the subscript expression is evaluated.
Then, the primary’s
__setitem__()
method is called with
two arguments: the subscript and the assigned object.
Typically,
__setitem__()
is defined on mutable sequence objects
(such as lists) and mapping objects (such as dictionaries), and behaves as
follows.
If the primary is a mutable sequence object (such as a list), the subscript
must yield an integer. If it is negative, the sequence’s length is added to
it. The resulting value must be a nonnegative integer less than the
sequence’s length, and the sequence is asked to assign the assigned object to
its item with that index. If the index is out of range,
IndexError
is
raised (assignment to a subscripted sequence cannot add new items to a list).
If the primary is a mapping object (such as a dictionary), the subscript must
have a type compatible with the mapping’s key type, and the mapping is then
asked to create a key/value pair which maps the subscript to the assigned
object. This can either replace an existing key/value pair with the same key
value, or insert a new key/value pair (if no key with the same value existed).
If the target is a slicing: The primary expression should evaluate to
a mutable sequence object (such as a list).
The assigned object should be
iterable
.
The slicing’s lower and upper bounds should be integers; if they are
None
(or not present), the defaults are zero and the sequence’s length.
If either bound is negative, the sequence’s length is added to it. The
resulting bounds are clipped to lie between zero and the sequence’s length,
inclusive. Finally, the sequence object is asked to replace the slice with
the items of the assigned sequence. The length of the slice may be different
from the length of the assigned sequence, thus changing the length of the
target sequence, if the target sequence allows it.
Although the definition of assignment implies that overlaps between the
left-hand side and the right-hand side are ‘simultaneous’ (for example
a,
b
=
b,
a
swaps two variables), overlaps
within
the collection of assigned-to
variables occur left-to-right, sometimes resulting in confusion. For instance,
the following program prints
[0,
2]
:
x
=
[
0
,
1
]
i
=
0
i
,
x
[
i
]
=
1
,
2
# i is updated, then x[i] is updated
print
(
x
)
See also
PEP 3132
- Extended Iterable Unpacking
The specification for the
*target
feature.
7.2.1.
Augmented assignment statements
¶
Augmented assignment is the combination, in a single statement, of a binary
operation and an assignment statement:
augmented_assignment_stmt
:
augtarget
augop
(
expression_list
|
yield_expression
)
augtarget
:
identifier
|
attributeref
|
subscription
augop
:
"+="
|
"-="
|
"*="
|
"@="
|
"/="
|
"//="
|
"%="
|
"**="
|
">>="
|
"<<="
|
"&="
|
"^="
|
"|="
(See section
Primaries
for the syntax definitions of the last three
symbols.)
An augmented assignment evaluates the target (which, unlike normal assignment
statements, cannot be an unpacking) and the expression list, performs the binary
operation specific to the type of assignment on the two operands, and assigns
the result to the original target. The target is only evaluated once.
An augmented assignment statement like
x
+=
1
can be rewritten as
x
=
x
+
1
to achieve a similar, but not exactly equal effect. In the augmented
version,
x
is only evaluated once. Also, when possible, the actual operation
is performed
in-place
, meaning that rather than creating a new object and
assigning that to the target, the old object is modified instead.
Unlike normal assignments, augmented assignments evaluate the left-hand side
before
evaluating the right-hand side. For example,
a[i]
+=
f(x)
first
looks-up
a[i]
, then it evaluates
f(x)
and performs the addition, and
lastly, it writes the result back to
a[i]
.
With the exception of assigning to tuples and multiple targets in a single
statement, the assignment done by augmented assignment statements is handled the
same way as normal assignments. Similarly, with the exception of the possible
in-place
behavior, the binary operation performed by augmented assignment is
the same as the normal binary operations.
For targets which are attribute references, the same
caveat about class
and instance attributes
applies as for regular assignments.
7.2.2.
Annotated assignment statements
¶
Annotation
assignment is the combination, in a single
statement, of a variable or attribute annotation and an optional assignment statement:
annotated_assignment_stmt
:
augtarget
":"
expression
[
"="
(
starred_expression
|
yield_expression
)]
The difference from normal
Assignment statements
is that only a single target is allowed.
The assignment target is considered “simple” if it consists of a single
name that is not enclosed in parentheses.
For simple assignment targets, if in class or module scope,
the annotations are gathered in a lazily evaluated
annotation scope
. The annotations can be
evaluated using the
__annotations__
attribute of a
class or module, or using the facilities in the
annotationlib
module.
If the assignment target is not simple (an attribute, subscript node, or
parenthesized name), the annotation is never evaluated.
If a name is annotated in a function scope, then this name is local for
that scope. Annotations are never evaluated and stored in function scopes.
If the right hand side is present, an annotated
assignment performs the actual assignment as if there was no annotation
present. If the right hand side is not present for an expression
target, then the interpreter evaluates the target except for the last
__setitem__()
or
__setattr__()
call.
See also
PEP 526
- Syntax for Variable Annotations
The proposal that added syntax for annotating the types of variables
(including class variables and instance variables), instead of expressing
them through comments.
PEP 484
- Type hints
The proposal that added the
typing
module to provide a standard
syntax for type annotations that can be used in static analysis tools and
IDEs.
Changed in version 3.8:
Now annotated assignments allow the same expressions in the right hand side as
regular assignments. Previously, some expressions (like un-parenthesized
tuple expressions) caused a syntax error.
Changed in version 3.14:
Annotations are now lazily evaluated in a separate
annotation scope
.
If the assignment target is not simple, annotations are never evaluated.
7.3.
The
assert
statement
¶
Assert statements are a convenient way to insert debugging assertions into a
program:
assert_stmt
:
"assert"
expression
[
","
expression
]
The simple form,
assert
expression
, is equivalent to
if
__debug__
:
if
not
expression
:
raise
AssertionError
The extended form,
assert
expression1,
expression2
, is equivalent to
if
__debug__
:
if
not
expression1
:
raise
AssertionError
(
expression2
)
These equivalences assume that
__debug__
and
AssertionError
refer to
the built-in variables with those names. In the current implementation, the
built-in variable
__debug__
is
True
under normal circumstances,
False
when optimization is requested (command line option
-O
). The current
code generator emits no code for an
assert
statement when optimization is
requested at compile time. Note that it is unnecessary to include the source
code for the expression that failed in the error message; it will be displayed
as part of the stack trace.
Assignments to
__debug__
are illegal. The value for the built-in variable
is determined when the interpreter starts.
7.4.
The
pass
statement
¶
pass_stmt
:
"pass"
pass
is a null operation — when it is executed, nothing happens.
It is useful as a placeholder when a statement is required syntactically, but no
code needs to be executed, for example:
def
f
(
arg
):
pass
# a function that does nothing (yet)
class
C
:
pass
# a class with no methods (yet)
7.5.
The
del
statement
¶
del_stmt
:
"del"
target_list
Deletion is recursively defined very similar to the way assignment is defined.
Rather than spelling it out in full details, here are some hints.
Deletion of a target list recursively deletes each target, from left to right.
Deletion of a name removes the binding of that name from the local or global
namespace, depending on whether the name occurs in a
global
statement
in the same code block. Trying to delete an unbound name raises a
NameError
exception.
Deletion of attribute references and subscriptions is passed to the
primary object involved; deletion of a slicing is in general equivalent to
assignment of an empty slice of the right type (but even this is determined by
the sliced object).
Changed in version 3.2:
Previously it was illegal to delete a name from the local namespace if it
occurs as a free variable in a nested block.
7.6.
The
return
statement
¶
return_stmt
:
"return"
[
expression_list
]
return
may only occur syntactically nested in a function definition,
not within a nested class definition.
If an expression list is present, it is evaluated, else
None
is substituted.
return
leaves the current function call with the expression list (or
None
) as return value.
When
return
passes control out of a
try
statement with a
finally
clause, that
finally
clause is executed before
really leaving the function.
In a generator function, the
return
statement indicates that the
generator is done and will cause
StopIteration
to be raised. The returned
value (if any) is used as an argument to construct
StopIteration
and
becomes the
StopIteration.value
attribute.
In an asynchronous generator function, an empty
return
statement
indicates that the asynchronous generator is done and will cause
StopAsyncIteration
to be raised. A non-empty
return
statement is a syntax error in an asynchronous generator function.
7.7.
The
yield
statement
¶
yield_stmt
:
yield_expression
A
yield
statement is semantically equivalent to a
yield
expression
. The
yield
statement can be used to omit the
parentheses that would otherwise be required in the equivalent yield expression
statement. For example, the yield statements
yield
<
expr
>
yield from
<
expr
>
are equivalent to the yield expression statements
(
yield
<
expr
>
)
(
yield from
<
expr
>
)
Yield expressions and statements are only used when defining a
generator
function, and are only used in the body of the generator function. Using
yield
in a function definition is sufficient to cause that definition to create a
generator function instead of a normal function.
For full details of
yield
semantics, refer to the
Yield expressions
section.
7.8.
The
raise
statement
¶
raise_stmt
:
"raise"
[
expression
[
"from"
expression
]]
If no expressions are present,
raise
re-raises the
exception that is currently being handled, which is also known as the
active exception
.
If there isn’t currently an active exception, a
RuntimeError
exception is raised
indicating that this is an error.
Otherwise,
raise
evaluates the first expression as the exception
object. It must be either a subclass or an instance of
BaseException
.
If it is a class, the exception instance will be obtained when needed by
instantiating the class with no arguments.
The
type
of the exception is the exception instance’s class, the
value
is the instance itself.
A traceback object is normally created automatically when an exception is raised
and attached to it as the
__traceback__
attribute.
You can create an exception and set your own traceback in one step using the
with_traceback()
exception method (which returns the
same exception instance, with its traceback set to its argument), like so:
raise
Exception
(
"foo occurred"
)
.
with_traceback
(
tracebackobj
)
The
from
clause is used for exception chaining: if given, the second
expression
must be another exception class or instance. If the second
expression is an exception instance, it will be attached to the raised
exception as the
__cause__
attribute (which is writable). If the
expression is an exception class, the class will be instantiated and the
resulting exception instance will be attached to the raised exception as the
__cause__
attribute. If the raised exception is not handled, both
exceptions will be printed:
>>>
try
:
...
print
(
1
/
0
)
...
except
Exception
as
exc
:
...
raise
RuntimeError
(
"Something bad happened"
)
from
exc
...
Traceback (most recent call last):
File
"<stdin>"
, line
2
, in
<module>
print
(
1
/
0
)
~~^~~
ZeroDivisionError
:
division by zero
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File
"<stdin>"
, line
4
, in
<module>
raise
RuntimeError
(
"Something bad happened"
)
from
exc
RuntimeError
:
Something bad happened
A similar mechanism works implicitly if a new exception is raised when
an exception is already being handled. An exception may be handled
when an
except
or
finally
clause, or a
with
statement, is used. The previous exception is then
attached as the new exception’s
__context__
attribute:
>>>
try
:
...
print
(
1
/
0
)
...
except
:
...
raise
RuntimeError
(
"Something bad happened"
)
...
Traceback (most recent call last):
File
"<stdin>"
, line
2
, in
<module>
print
(
1
/
0
)
~~^~~
ZeroDivisionError
:
division by zero
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File
"<stdin>"
, line
4
, in
<module>
raise
RuntimeError
(
"Something bad happened"
)
RuntimeError
:
Something bad happened
Exception chaining can be explicitly suppressed by specifying
None
in
the
from
clause:
>>>
try
:
...
print
(
1
/
0
)
...
except
:
...
raise
RuntimeError
(
"Something bad happened"
)
from
None
...
Traceback (most recent call last):
File
"<stdin>"
, line
4
, in
<module>
RuntimeError
:
Something bad happened
Additional information on exceptions can be found in section
Exceptions
,
and information about handling exceptions is in section
The try statement
.
Changed in version 3.3:
None
is now permitted as
Y
in
raise
X
from
Y
.
Added the
__suppress_context__
attribute to suppress
automatic display of the exception context.
Changed in version 3.11:
If the traceback of the active exception is modified in an
except
clause, a subsequent
raise
statement re-raises the exception with the
modified traceback. Previously, the exception was re-raised with the
traceback it had when it was caught.
7.9.
The
break
statement
¶
break_stmt
:
"break"
break
may only occur syntactically nested in a
for
or
while
loop, but not nested in a function or class definition within
that loop.
It terminates the nearest enclosing loop, skipping the optional
else
clause if the loop has one.
If a
for
loop is terminated by
break
, the loop control
target keeps its current value.
When
break
passes control out of a
try
statement with a
finally
clause, that
finally
clause is executed before
really leaving the loop.
7.10.
The
continue
statement
¶
continue_stmt
:
"continue"
continue
may only occur syntactically nested in a
for
or
while
loop, but not nested in a function or class definition within
that loop. It continues with the next cycle of the nearest enclosing loop.
When
continue
passes control out of a
try
statement with a
finally
clause, that
finally
clause is executed before
really starting the next loop cycle.
7.11.
The
import
statement
¶
import_stmt
:
"import"
module
[
"as"
identifier
] (
","
module
[
"as"
identifier
])*
|
"from"
relative_module
"import"
identifier
[
"as"
identifier
]
(
","
identifier
[
"as"
identifier
])*
|
"from"
relative_module
"import"
"("
identifier
[
"as"
identifier
]
(
","
identifier
[
"as"
identifier
])* [
","
]
")"
|
"from"
relative_module
"import"
"*"
module
: (
identifier
"."
)*
identifier
relative_module
:
"."
*
module
|
"."
+
The basic import statement (no
from
clause) is executed in two
steps:
find a module, loading and initializing it if necessary
define a name or names in the local namespace for the scope where
the
import
statement occurs.
When the statement contains multiple clauses (separated by
commas) the two steps are carried out separately for each clause, just
as though the clauses had been separated out into individual import
statements.
The details of the first step, finding and loading modules, are described in
greater detail in the section on the
import system
,
which also describes the various types of packages and modules that can
be imported, as well as all the hooks that can be used to customize
the import system. Note that failures in this step may indicate either
that the module could not be located,
or
that an error occurred while
initializing the module, which includes execution of the module’s code.
If the requested module is retrieved successfully, it will be made
available in the local namespace in one of three ways:
If the module name is followed by
as
, then the name
following
as
is bound directly to the imported module.
If no other name is specified, and the module being imported is a top
level module, the module’s name is bound in the local namespace as a
reference to the imported module
If the module being imported is
not
a top level module, then the name
of the top level package that contains the module is bound in the local
namespace as a reference to the top level package. The imported module
must be accessed using its full qualified name rather than directly
The
from
form uses a slightly more complex process:
find the module specified in the
from
clause, loading and
initializing it if necessary;
for each of the identifiers specified in the
import
clauses:
check if the imported module has an attribute by that name
if not, attempt to import a submodule with that name and then
check the imported module again for that attribute
if the attribute is not found,
ImportError
is raised.
otherwise, a reference to that value is stored in the local namespace,
using the name in the
as
clause if it is present,
otherwise using the attribute name
Examples:
import
foo
# foo imported and bound locally
import
foo.bar.baz
# foo, foo.bar, and foo.bar.baz imported, foo bound locally
import
foo.bar.baz
as
fbb
# foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as fbb
from
foo.bar
import
baz
# foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as baz
from
foo
import
attr
# foo imported and foo.attr bound as attr
If the list of identifiers is replaced by a star (
'*'
), all public
names defined in the module are bound in the local namespace for the scope
where the
import
statement occurs.
The
public names
defined by a module are determined by checking the module’s
namespace for a variable named
__all__
; if defined, it must be a sequence
of strings which are names defined or imported by that module.
Names containing non-ASCII characters must be in the
normalization form
NFKC; see
Non-ASCII characters in names
for details. The names
given in
__all__
are all considered public and are required to exist. If
__all__
is not defined, the set of public names includes all names found
in the module’s namespace which do not begin with an underscore character
(
'_'
).
__all__
should contain the entire public API. It is intended
to avoid accidentally exporting items that are not part of the API (such as
library modules which were imported and used within the module).
The wild card form of import —
from
module
import
*
— is only allowed at
the module level. Attempting to use it in class or function definitions will
raise a
SyntaxError
.
When specifying what module to import you do not have to specify the absolute
name of the module. When a module or package is contained within another
package it is possible to make a relative import within the same top package
without having to mention the package name. By using leading dots in the
specified module or package after
from
you can specify how high to
traverse up the current package hierarchy without specifying exact names. One
leading dot means the current package where the module making the import
exists. Two dots means up one package level. Three dots is up two levels, etc.
So if you execute
from
.
import
mod
from a module in the
pkg
package
then you will end up importing
pkg.mod
. If you execute
from
..subpkg2
import
mod
from within
pkg.subpkg1
you will import
pkg.subpkg2.mod
.
The specification for relative imports is contained in
the
Package Relative Imports
section.
importlib.import_module()
is provided to support applications that
determine dynamically the modules to be loaded.
Raises an
auditing event
import
with arguments
module
,
filename
,
sys.path
,
sys.meta_path
,
sys.path_hooks
.
7.11.1.
Future statements
¶
A
future statement
is a directive to the compiler that a particular
module should be compiled using syntax or semantics that will be available in a
specified future release of Python where the feature becomes standard.
The future statement is intended to ease migration to future versions of Python
that introduce incompatible changes to the language. It allows use of the new
features on a per-module basis before the release in which the feature becomes
standard.
future_stmt
:
"from"
"__future__"
"import"
feature
[
"as"
identifier
]
(
","
feature
[
"as"
identifier
])*
|
"from"
"__future__"
"import"
"("
feature
[
"as"
identifier
]
(
","
feature
[
"as"
identifier
])* [
","
]
")"
feature
:
identifier
A future statement must appear near the top of the module. The only lines that
can appear before a future statement are:
the module docstring (if any),
comments,
blank lines, and
other future statements.
The only feature that requires using the future statement is
annotations
(see
PEP 563
).
All historical features enabled by the future statement are still recognized
by Python 3. The list includes
absolute_import
,
division
,
generators
,
generator_stop
,
unicode_literals
,
print_function
,
nested_scopes
and
with_statement
. They are
all redundant because they are always enabled, and only kept for
backwards compatibility.
A future statement is recognized and treated specially at compile time: Changes
to the semantics of core constructs are often implemented by generating
different code. It may even be the case that a new feature introduces new
incompatible syntax (such as a new reserved word), in which case the compiler
may need to parse the module differently. Such decisions cannot be pushed off
until runtime.
For any given release, the compiler knows which feature names have been defined,
and raises a compile-time error if a future statement contains a feature not
known to it.
The direct runtime semantics are the same as for any import statement: there is
a standard module
__future__
, described later, and it will be imported in
the usual way at the time the future statement is executed.
The interesting runtime semantics depend on the specific feature enabled by the
future statement.
Note that there is nothing special about the statement:
import
__future__
[
as
name
]
That is not a future statement; it’s an ordinary import statement with no
special semantics or syntax restrictions.
Code compiled by calls to the built-in functions
exec()
and
compile()
that occur in a module
M
containing a future statement will, by default,
use the new syntax or semantics associated with the future statement. This can
be controlled by optional arguments to
compile()
— see the documentation
of that function for details.
A future statement typed at an interactive interpreter prompt will take effect
for the rest of the interpreter session. If an interpreter is started with the
-i
option, is passed a script name to execute, and the script includes
a future statement, it will be in effect in the interactive session started
after the script is executed.
See also
PEP 236
- Back to the __future__
The original proposal for the __future__ mechanism.
7.12.
The
global
statement
¶
global_stmt
:
"global"
identifier
(
","
identifier
)*
The
global
statement causes the listed identifiers to be interpreted
as globals. It would be impossible to assign to a global variable without
global
, although free variables may refer to globals without being
declared global.
The
global
statement applies to the entire current scope
(module, function body or class definition).
A
SyntaxError
is raised if a variable is used or
assigned to prior to its global declaration in the scope.
At the module level, all variables are global, so a
global
statement has no effect.
However, variables must still not be used or
assigned to prior to their
global
declaration.
This requirement is relaxed in the interactive prompt (
REPL
).
Programmer’s note:
global
is a directive to the parser. It
applies only to code parsed at the same time as the
global
statement.
In particular, a
global
statement contained in a string or code
object supplied to the built-in
exec()
function does not affect the code
block
containing
the function call, and code contained in such a string is
unaffected by
global
statements in the code containing the function
call. The same applies to the
eval()
and
compile()
functions.
7.13.
The
nonlocal
statement
¶
nonlocal_stmt
:
"nonlocal"
identifier
(
","
identifier
)*
When the definition of a function or class is nested (enclosed) within
the definitions of other functions, its nonlocal scopes are the local
scopes of the enclosing functions. The
nonlocal
statement
causes the listed identifiers to refer to names previously bound in
nonlocal scopes. It allows encapsulated code to rebind such nonlocal
identifiers. If a name is bound in more than one nonlocal scope, the
nearest binding is used. If a name is not bound in any nonlocal scope,
or if there is no nonlocal scope, a
SyntaxError
is raised.
The
nonlocal
statement applies to the entire scope of a function or
class body. A
SyntaxError
is raised if a variable is used or
assigned to prior to its nonlocal declaration in the scope.
See also
PEP 3104
- Access to Names in Outer Scopes
The specification for the
nonlocal
statement.
Programmer’s note:
nonlocal
is a directive to the parser
and applies only to code parsed along with it. See the note for the
global
statement.
7.14.
The
type
statement
¶
type_stmt
:
'type'
identifier
[
type_params
]
"="
expression
The
type
statement declares a type alias, which is an instance
of
typing.TypeAliasType
.
For example, the following statement creates a type alias:
type
Point
=
tuple
[
float
,
float
]
This code is roughly equivalent to:
annotation
-
def
VALUE_OF_Point
():
return
tuple
[
float
,
float
]
Point
=
typing
.
TypeAliasType
(
"Point"
,
VALUE_OF_Point
())
annotation-def
indicates an
annotation scope
, which behaves
mostly like a function, but with several small differences.
The value of the
type alias is evaluated in the annotation scope. It is not evaluated when the
type alias is created, but only when the value is accessed through the type alias’s
__value__
attribute (see
Lazy evaluation
).
This allows the type alias to refer to names that are not yet defined.
Type aliases may be made generic by adding a
type parameter list
after the name. See
Generic type aliases
for more.
type
is a
soft keyword
.
Added in version 3.12.
See also
PEP 695
- Type Parameter Syntax
Introduced the
type
statement and syntax for
generic classes and functions. | |||||||||
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Theme
### [Table of Contents](https://docs.python.org/3/contents.html)
- [7\. Simple statements](https://docs.python.org/3/reference/simple_stmts.html)
- [7\.1. Expression statements](https://docs.python.org/3/reference/simple_stmts.html#expression-statements)
- [7\.2. Assignment statements](https://docs.python.org/3/reference/simple_stmts.html#assignment-statements)
- [7\.2.1. Augmented assignment statements](https://docs.python.org/3/reference/simple_stmts.html#augmented-assignment-statements)
- [7\.2.2. Annotated assignment statements](https://docs.python.org/3/reference/simple_stmts.html#annotated-assignment-statements)
- [7\.3. The `assert` statement](https://docs.python.org/3/reference/simple_stmts.html#the-assert-statement)
- [7\.4. The `pass` statement](https://docs.python.org/3/reference/simple_stmts.html#the-pass-statement)
- [7\.5. The `del` statement](https://docs.python.org/3/reference/simple_stmts.html#the-del-statement)
- [7\.6. The `return` statement](https://docs.python.org/3/reference/simple_stmts.html#the-return-statement)
- [7\.7. The `yield` statement](https://docs.python.org/3/reference/simple_stmts.html#the-yield-statement)
- [7\.8. The `raise` statement](https://docs.python.org/3/reference/simple_stmts.html#the-raise-statement)
- [7\.9. The `break` statement](https://docs.python.org/3/reference/simple_stmts.html#the-break-statement)
- [7\.10. The `continue` statement](https://docs.python.org/3/reference/simple_stmts.html#the-continue-statement)
- [7\.11. The `import` statement](https://docs.python.org/3/reference/simple_stmts.html#the-import-statement)
- [7\.11.1. Future statements](https://docs.python.org/3/reference/simple_stmts.html#future-statements)
- [7\.12. The `global` statement](https://docs.python.org/3/reference/simple_stmts.html#the-global-statement)
- [7\.13. The `nonlocal` statement](https://docs.python.org/3/reference/simple_stmts.html#the-nonlocal-statement)
- [7\.14. The `type` statement](https://docs.python.org/3/reference/simple_stmts.html#the-type-statement)
#### Previous topic
[6\. Expressions](https://docs.python.org/3/reference/expressions.html "previous chapter")
#### Next topic
[8\. Compound statements](https://docs.python.org/3/reference/compound_stmts.html "next chapter")
### This page
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# 7\. Simple statements[¶](https://docs.python.org/3/reference/simple_stmts.html#simple-statements "Link to this heading")
A simple statement is comprised within a single logical line. Several simple statements may occur on a single line separated by semicolons. The syntax for simple statements is:
```
simple_stmt: expression_stmt
| assert_stmt
| assignment_stmt
| augmented_assignment_stmt
| annotated_assignment_stmt
| pass_stmt
| del_stmt
| return_stmt
| yield_stmt
| raise_stmt
| break_stmt
| continue_stmt
| import_stmt
| future_stmt
| global_stmt
| nonlocal_stmt
| type_stmt
```
## 7\.1. Expression statements[¶](https://docs.python.org/3/reference/simple_stmts.html#expression-statements "Link to this heading")
Expression statements are used (mostly interactively) to compute and write a value, or (usually) to call a procedure (a function that returns no meaningful result; in Python, procedures return the value `None`). Other uses of expression statements are allowed and occasionally useful. The syntax for an expression statement is:
```
expression_stmt: starred_expression
```
An expression statement evaluates the expression list (which may be a single expression).
In interactive mode, if the value is not `None`, it is converted to a string using the built-in [`repr()`](https://docs.python.org/3/library/functions.html#repr "repr") function and the resulting string is written to standard output on a line by itself (except if the result is `None`, so that procedure calls do not cause any output.)
## 7\.2. Assignment statements[¶](https://docs.python.org/3/reference/simple_stmts.html#assignment-statements "Link to this heading")
Assignment statements are used to (re)bind names to values and to modify attributes or items of mutable objects:
```
assignment_stmt: (target_list "=")+ (starred_expression | yield_expression)
target_list: target ("," target)* [","]
target: identifier
| "(" [target_list] ")"
| "[" [target_list] "]"
| attributeref
| subscription
| "*" target
```
(See section [Primaries](https://docs.python.org/3/reference/expressions.html#primaries) for the syntax definitions for *attributeref* and *subscription*.)
An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right.
Assignment is defined recursively depending on the form of the target (list). When a target is part of a mutable object (an attribute reference or subscription), the mutable object must ultimately perform the assignment and decide about its validity, and may raise an exception if the assignment is unacceptable. The rules observed by various types and the exceptions raised are given with the definition of the object types (see section [The standard type hierarchy](https://docs.python.org/3/reference/datamodel.html#types)).
Assignment of an object to a target list, optionally enclosed in parentheses or square brackets, is recursively defined as follows.
- If the target list is a single target with no trailing comma, optionally in parentheses, the object is assigned to that target.
- Else:
- If the target list contains one target prefixed with an asterisk, called a “starred” target: The object must be an iterable with at least as many items as there are targets in the target list, minus one. The first items of the iterable are assigned, from left to right, to the targets before the starred target. The final items of the iterable are assigned to the targets after the starred target. A list of the remaining items in the iterable is then assigned to the starred target (the list can be empty).
- Else: The object must be an iterable with the same number of items as there are targets in the target list, and the items are assigned, from left to right, to the corresponding targets.
Assignment of an object to a single target is recursively defined as follows.
- If the target is an identifier (name):
- If the name does not occur in a [`global`](https://docs.python.org/3/reference/simple_stmts.html#global) or [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) statement in the current code block: the name is bound to the object in the current local namespace.
- Otherwise: the name is bound to the object in the global namespace or the outer namespace determined by [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal), respectively.
The name is rebound if it was already bound. This may cause the reference count for the object previously bound to the name to reach zero, causing the object to be deallocated and its destructor (if it has one) to be called.
- If the target is an attribute reference: The primary expression in the reference is evaluated. It should yield an object with assignable attributes; if this is not the case, [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") is raised. That object is then asked to assign the assigned object to the given attribute; if it cannot perform the assignment, it raises an exception (usually but not necessarily [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError")).
Note: If the object is a class instance and the attribute reference occurs on both sides of the assignment operator, the right-hand side expression, `a.x` can access either an instance attribute or (if no instance attribute exists) a class attribute. The left-hand side target `a.x` is always set as an instance attribute, creating it if necessary. Thus, the two occurrences of `a.x` do not necessarily refer to the same attribute: if the right-hand side expression refers to a class attribute, the left-hand side creates a new instance attribute as the target of the assignment:
Copy
```
class Cls:
x = 3 # class variable
inst = Cls()
inst.x = inst.x + 1 # writes inst.x as 4 leaving Cls.x as 3
```
This description does not necessarily apply to descriptor attributes, such as properties created with [`property()`](https://docs.python.org/3/library/functions.html#property "property").
- If the target is a subscription: The primary expression in the reference is evaluated. Next, the subscript expression is evaluated. Then, the primary’s [`__setitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "object.__setitem__") method is called with two arguments: the subscript and the assigned object.
Typically, [`__setitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "object.__setitem__") is defined on mutable sequence objects (such as lists) and mapping objects (such as dictionaries), and behaves as follows.
If the primary is a mutable sequence object (such as a list), the subscript must yield an integer. If it is negative, the sequence’s length is added to it. The resulting value must be a nonnegative integer less than the sequence’s length, and the sequence is asked to assign the assigned object to its item with that index. If the index is out of range, [`IndexError`](https://docs.python.org/3/library/exceptions.html#IndexError "IndexError") is raised (assignment to a subscripted sequence cannot add new items to a list).
If the primary is a mapping object (such as a dictionary), the subscript must have a type compatible with the mapping’s key type, and the mapping is then asked to create a key/value pair which maps the subscript to the assigned object. This can either replace an existing key/value pair with the same key value, or insert a new key/value pair (if no key with the same value existed).
If the target is a slicing: The primary expression should evaluate to a mutable sequence object (such as a list). The assigned object should be [iterable](https://docs.python.org/3/glossary.html#term-iterable). The slicing’s lower and upper bounds should be integers; if they are `None` (or not present), the defaults are zero and the sequence’s length. If either bound is negative, the sequence’s length is added to it. The resulting bounds are clipped to lie between zero and the sequence’s length, inclusive. Finally, the sequence object is asked to replace the slice with the items of the assigned sequence. The length of the slice may be different from the length of the assigned sequence, thus changing the length of the target sequence, if the target sequence allows it.
Although the definition of assignment implies that overlaps between the left-hand side and the right-hand side are ‘simultaneous’ (for example swaps two variables), overlaps *within* the collection of assigned-to variables occur left-to-right, sometimes resulting in confusion. For instance, the following program prints `[0, 2]`:
Copy
```
x = [0, 1]
i = 0
i, x[i] = 1, 2 # i is updated, then x[i] is updated
print(x)
```
See also
[**PEP 3132**](https://peps.python.org/pep-3132/) - Extended Iterable Unpacking
The specification for the `*target` feature.
### 7\.2.1. Augmented assignment statements[¶](https://docs.python.org/3/reference/simple_stmts.html#augmented-assignment-statements "Link to this heading")
Augmented assignment is the combination, in a single statement, of a binary operation and an assignment statement:
```
augmented_assignment_stmt: augtarget augop (expression_list | yield_expression)
augtarget: identifier | attributeref | subscription
augop: "+=" | "-=" | "*=" | "@=" | "/=" | "//=" | "%=" | "**="
| ">>=" | "<<=" | "&=" | "^=" | "|="
```
(See section [Primaries](https://docs.python.org/3/reference/expressions.html#primaries) for the syntax definitions of the last three symbols.)
An augmented assignment evaluates the target (which, unlike normal assignment statements, cannot be an unpacking) and the expression list, performs the binary operation specific to the type of assignment on the two operands, and assigns the result to the original target. The target is only evaluated once.
An augmented assignment statement like `x += 1` can be rewritten as to achieve a similar, but not exactly equal effect. In the augmented version, `x` is only evaluated once. Also, when possible, the actual operation is performed *in-place*, meaning that rather than creating a new object and assigning that to the target, the old object is modified instead.
Unlike normal assignments, augmented assignments evaluate the left-hand side *before* evaluating the right-hand side. For example, `a[i] += f(x)` first looks-up `a[i]`, then it evaluates `f(x)` and performs the addition, and lastly, it writes the result back to `a[i]`.
With the exception of assigning to tuples and multiple targets in a single statement, the assignment done by augmented assignment statements is handled the same way as normal assignments. Similarly, with the exception of the possible *in-place* behavior, the binary operation performed by augmented assignment is the same as the normal binary operations.
For targets which are attribute references, the same [caveat about class and instance attributes](https://docs.python.org/3/reference/simple_stmts.html#attr-target-note) applies as for regular assignments.
### 7\.2.2. Annotated assignment statements[¶](https://docs.python.org/3/reference/simple_stmts.html#annotated-assignment-statements "Link to this heading")
[Annotation](https://docs.python.org/3/glossary.html#term-variable-annotation) assignment is the combination, in a single statement, of a variable or attribute annotation and an optional assignment statement:
```
annotated_assignment_stmt: augtarget ":" expression
["=" (starred_expression | yield_expression)]
```
The difference from normal [Assignment statements](https://docs.python.org/3/reference/simple_stmts.html#assignment) is that only a single target is allowed.
The assignment target is considered “simple” if it consists of a single name that is not enclosed in parentheses. For simple assignment targets, if in class or module scope, the annotations are gathered in a lazily evaluated [annotation scope](https://docs.python.org/3/reference/executionmodel.html#annotation-scopes). The annotations can be evaluated using the [`__annotations__`](https://docs.python.org/3/reference/datamodel.html#object.__annotations__ "object.__annotations__") attribute of a class or module, or using the facilities in the [`annotationlib`](https://docs.python.org/3/library/annotationlib.html#module-annotationlib "annotationlib: Functionality for introspecting annotations") module.
If the assignment target is not simple (an attribute, subscript node, or parenthesized name), the annotation is never evaluated.
If a name is annotated in a function scope, then this name is local for that scope. Annotations are never evaluated and stored in function scopes.
If the right hand side is present, an annotated assignment performs the actual assignment as if there was no annotation present. If the right hand side is not present for an expression target, then the interpreter evaluates the target except for the last [`__setitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "object.__setitem__") or [`__setattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__setattr__ "object.__setattr__") call.
See also
[**PEP 526**](https://peps.python.org/pep-0526/) - Syntax for Variable Annotations
The proposal that added syntax for annotating the types of variables (including class variables and instance variables), instead of expressing them through comments.
[**PEP 484**](https://peps.python.org/pep-0484/) - Type hints
The proposal that added the [`typing`](https://docs.python.org/3/library/typing.html#module-typing "typing: Support for type hints (see :pep:`484`).") module to provide a standard syntax for type annotations that can be used in static analysis tools and IDEs.
Changed in version 3.8: Now annotated assignments allow the same expressions in the right hand side as regular assignments. Previously, some expressions (like un-parenthesized tuple expressions) caused a syntax error.
Changed in version 3.14: Annotations are now lazily evaluated in a separate [annotation scope](https://docs.python.org/3/reference/executionmodel.html#annotation-scopes). If the assignment target is not simple, annotations are never evaluated.
## 7\.3. The `assert` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-assert-statement "Link to this heading")
Assert statements are a convenient way to insert debugging assertions into a program:
```
assert_stmt: "assert" expression ["," expression]
```
The simple form, `assert expression`, is equivalent to
Copy
```
if __debug__:
if not expression: raise AssertionError
```
The extended form, `assert expression1, expression2`, is equivalent to
Copy
```
if __debug__:
if not expression1: raise AssertionError(expression2)
```
These equivalences assume that [`__debug__`](https://docs.python.org/3/library/constants.html#debug__ "__debug__") and [`AssertionError`](https://docs.python.org/3/library/exceptions.html#AssertionError "AssertionError") refer to the built-in variables with those names. In the current implementation, the built-in variable `__debug__` is `True` under normal circumstances, `False` when optimization is requested (command line option [`-O`](https://docs.python.org/3/using/cmdline.html#cmdoption-O)). The current code generator emits no code for an [`assert`](https://docs.python.org/3/reference/simple_stmts.html#assert) statement when optimization is requested at compile time. Note that it is unnecessary to include the source code for the expression that failed in the error message; it will be displayed as part of the stack trace.
Assignments to [`__debug__`](https://docs.python.org/3/library/constants.html#debug__ "__debug__") are illegal. The value for the built-in variable is determined when the interpreter starts.
## 7\.4. The `pass` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-pass-statement "Link to this heading")
```
pass_stmt: "pass"
```
[`pass`](https://docs.python.org/3/reference/simple_stmts.html#pass) is a null operation — when it is executed, nothing happens. It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed, for example:
Copy
```
def f(arg): pass # a function that does nothing (yet)
class C: pass # a class with no methods (yet)
```
## 7\.5. The `del` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-del-statement "Link to this heading")
```
del_stmt: "del" target_list
```
Deletion is recursively defined very similar to the way assignment is defined. Rather than spelling it out in full details, here are some hints.
Deletion of a target list recursively deletes each target, from left to right.
Deletion of a name removes the binding of that name from the local or global namespace, depending on whether the name occurs in a [`global`](https://docs.python.org/3/reference/simple_stmts.html#global) statement in the same code block. Trying to delete an unbound name raises a [`NameError`](https://docs.python.org/3/library/exceptions.html#NameError "NameError") exception.
Deletion of attribute references and subscriptions is passed to the primary object involved; deletion of a slicing is in general equivalent to assignment of an empty slice of the right type (but even this is determined by the sliced object).
Changed in version 3.2: Previously it was illegal to delete a name from the local namespace if it occurs as a free variable in a nested block.
## 7\.6. The `return` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-return-statement "Link to this heading")
```
return_stmt: "return" [expression_list]
```
[`return`](https://docs.python.org/3/reference/simple_stmts.html#return) may only occur syntactically nested in a function definition, not within a nested class definition.
If an expression list is present, it is evaluated, else `None` is substituted.
[`return`](https://docs.python.org/3/reference/simple_stmts.html#return) leaves the current function call with the expression list (or `None`) as return value.
When [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) passes control out of a [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement with a [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause, that `finally` clause is executed before really leaving the function.
In a generator function, the [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement indicates that the generator is done and will cause [`StopIteration`](https://docs.python.org/3/library/exceptions.html#StopIteration "StopIteration") to be raised. The returned value (if any) is used as an argument to construct `StopIteration` and becomes the [`StopIteration.value`](https://docs.python.org/3/library/exceptions.html#StopIteration.value "StopIteration.value") attribute.
In an asynchronous generator function, an empty [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement indicates that the asynchronous generator is done and will cause [`StopAsyncIteration`](https://docs.python.org/3/library/exceptions.html#StopAsyncIteration "StopAsyncIteration") to be raised. A non-empty `return` statement is a syntax error in an asynchronous generator function.
## 7\.7. The `yield` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-yield-statement "Link to this heading")
```
yield_stmt: yield_expression
```
A [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) statement is semantically equivalent to a [yield expression](https://docs.python.org/3/reference/expressions.html#yieldexpr). The `yield` statement can be used to omit the parentheses that would otherwise be required in the equivalent yield expression statement. For example, the yield statements
Copy
```
yield <expr>
yield from <expr>
```
are equivalent to the yield expression statements
Copy
```
(yield <expr>)
(yield from <expr>)
```
Yield expressions and statements are only used when defining a [generator](https://docs.python.org/3/glossary.html#term-generator) function, and are only used in the body of the generator function. Using [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) in a function definition is sufficient to cause that definition to create a generator function instead of a normal function.
For full details of [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) semantics, refer to the [Yield expressions](https://docs.python.org/3/reference/expressions.html#yieldexpr) section.
## 7\.8. The `raise` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-raise-statement "Link to this heading")
```
raise_stmt: "raise" [expression ["from" expression]]
```
If no expressions are present, [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) re-raises the exception that is currently being handled, which is also known as the *active exception*. If there isn’t currently an active exception, a [`RuntimeError`](https://docs.python.org/3/library/exceptions.html#RuntimeError "RuntimeError") exception is raised indicating that this is an error.
Otherwise, [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) evaluates the first expression as the exception object. It must be either a subclass or an instance of [`BaseException`](https://docs.python.org/3/library/exceptions.html#BaseException "BaseException"). If it is a class, the exception instance will be obtained when needed by instantiating the class with no arguments.
The *type* of the exception is the exception instance’s class, the *value* is the instance itself.
A traceback object is normally created automatically when an exception is raised and attached to it as the [`__traceback__`](https://docs.python.org/3/library/exceptions.html#BaseException.__traceback__ "BaseException.__traceback__") attribute. You can create an exception and set your own traceback in one step using the [`with_traceback()`](https://docs.python.org/3/library/exceptions.html#BaseException.with_traceback "BaseException.with_traceback") exception method (which returns the same exception instance, with its traceback set to its argument), like so:
Copy
```
raise Exception("foo occurred").with_traceback(tracebackobj)
```
The `from` clause is used for exception chaining: if given, the second *expression* must be another exception class or instance. If the second expression is an exception instance, it will be attached to the raised exception as the [`__cause__`](https://docs.python.org/3/library/exceptions.html#BaseException.__cause__ "BaseException.__cause__") attribute (which is writable). If the expression is an exception class, the class will be instantiated and the resulting exception instance will be attached to the raised exception as the `__cause__` attribute. If the raised exception is not handled, both exceptions will be printed:
Copy
```
>>> try:
... print(1 / 0)
... except Exception as exc:
... raise RuntimeError("Something bad happened") from exc
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
print(1 / 0)
~~^~~
ZeroDivisionError: division by zero
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError("Something bad happened") from exc
RuntimeError: Something bad happened
```
A similar mechanism works implicitly if a new exception is raised when an exception is already being handled. An exception may be handled when an [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) or [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause, or a [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement, is used. The previous exception is then attached as the new exception’s [`__context__`](https://docs.python.org/3/library/exceptions.html#BaseException.__context__ "BaseException.__context__") attribute:
Copy
```
>>> try:
... print(1 / 0)
... except:
... raise RuntimeError("Something bad happened")
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
print(1 / 0)
~~^~~
ZeroDivisionError: division by zero
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError("Something bad happened")
RuntimeError: Something bad happened
```
Exception chaining can be explicitly suppressed by specifying [`None`](https://docs.python.org/3/library/constants.html#None "None") in the `from` clause:
Copy
```
>>> try:
... print(1 / 0)
... except:
... raise RuntimeError("Something bad happened") from None
...
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
RuntimeError: Something bad happened
```
Additional information on exceptions can be found in section [Exceptions](https://docs.python.org/3/reference/executionmodel.html#exceptions), and information about handling exceptions is in section [The try statement](https://docs.python.org/3/reference/compound_stmts.html#try).
Changed in version 3.3: [`None`](https://docs.python.org/3/library/constants.html#None "None") is now permitted as `Y` in `raise X from Y`.
Added the [`__suppress_context__`](https://docs.python.org/3/library/exceptions.html#BaseException.__suppress_context__ "BaseException.__suppress_context__") attribute to suppress automatic display of the exception context.
Changed in version 3.11: If the traceback of the active exception is modified in an [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) clause, a subsequent `raise` statement re-raises the exception with the modified traceback. Previously, the exception was re-raised with the traceback it had when it was caught.
## 7\.9. The `break` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-break-statement "Link to this heading")
```
break_stmt: "break"
```
[`break`](https://docs.python.org/3/reference/simple_stmts.html#break) may only occur syntactically nested in a [`for`](https://docs.python.org/3/reference/compound_stmts.html#for) or [`while`](https://docs.python.org/3/reference/compound_stmts.html#while) loop, but not nested in a function or class definition within that loop.
It terminates the nearest enclosing loop, skipping the optional `else` clause if the loop has one.
If a [`for`](https://docs.python.org/3/reference/compound_stmts.html#for) loop is terminated by [`break`](https://docs.python.org/3/reference/simple_stmts.html#break), the loop control target keeps its current value.
When [`break`](https://docs.python.org/3/reference/simple_stmts.html#break) passes control out of a [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement with a [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause, that `finally` clause is executed before really leaving the loop.
## 7\.10. The `continue` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-continue-statement "Link to this heading")
```
continue_stmt: "continue"
```
[`continue`](https://docs.python.org/3/reference/simple_stmts.html#continue) may only occur syntactically nested in a [`for`](https://docs.python.org/3/reference/compound_stmts.html#for) or [`while`](https://docs.python.org/3/reference/compound_stmts.html#while) loop, but not nested in a function or class definition within that loop. It continues with the next cycle of the nearest enclosing loop.
When [`continue`](https://docs.python.org/3/reference/simple_stmts.html#continue) passes control out of a [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement with a [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause, that `finally` clause is executed before really starting the next loop cycle.
## 7\.11. The `import` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-import-statement "Link to this heading")
```
import_stmt: "import" module ["as" identifier] ("," module ["as" identifier])*
| "from" relative_module "import" identifier ["as" identifier]
("," identifier ["as" identifier])*
| "from" relative_module "import" "(" identifier ["as" identifier]
("," identifier ["as" identifier])* [","] ")"
| "from" relative_module "import" "*"
module: (identifier ".")* identifier
relative_module: "."* module | "."+
```
The basic import statement (no [`from`](https://docs.python.org/3/reference/simple_stmts.html#from) clause) is executed in two steps:
1. find a module, loading and initializing it if necessary
2. define a name or names in the local namespace for the scope where the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement occurs.
When the statement contains multiple clauses (separated by commas) the two steps are carried out separately for each clause, just as though the clauses had been separated out into individual import statements.
The details of the first step, finding and loading modules, are described in greater detail in the section on the [import system](https://docs.python.org/3/reference/import.html#importsystem), which also describes the various types of packages and modules that can be imported, as well as all the hooks that can be used to customize the import system. Note that failures in this step may indicate either that the module could not be located, *or* that an error occurred while initializing the module, which includes execution of the module’s code.
If the requested module is retrieved successfully, it will be made available in the local namespace in one of three ways:
- If the module name is followed by `as`, then the name following `as` is bound directly to the imported module.
- If no other name is specified, and the module being imported is a top level module, the module’s name is bound in the local namespace as a reference to the imported module
- If the module being imported is *not* a top level module, then the name of the top level package that contains the module is bound in the local namespace as a reference to the top level package. The imported module must be accessed using its full qualified name rather than directly
The [`from`](https://docs.python.org/3/reference/simple_stmts.html#from) form uses a slightly more complex process:
1. find the module specified in the [`from`](https://docs.python.org/3/reference/simple_stmts.html#from) clause, loading and initializing it if necessary;
2. for each of the identifiers specified in the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) clauses:
1. check if the imported module has an attribute by that name
2. if not, attempt to import a submodule with that name and then check the imported module again for that attribute
3. if the attribute is not found, [`ImportError`](https://docs.python.org/3/library/exceptions.html#ImportError "ImportError") is raised.
4. otherwise, a reference to that value is stored in the local namespace, using the name in the `as` clause if it is present, otherwise using the attribute name
Examples:
Copy
```
import foo # foo imported and bound locally
import foo.bar.baz # foo, foo.bar, and foo.bar.baz imported, foo bound locally
import foo.bar.baz as fbb # foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as fbb
from foo.bar import baz # foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as baz
from foo import attr # foo imported and foo.attr bound as attr
```
If the list of identifiers is replaced by a star (`'*'`), all public names defined in the module are bound in the local namespace for the scope where the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement occurs.
The *public names* defined by a module are determined by checking the module’s namespace for a variable named `__all__`; if defined, it must be a sequence of strings which are names defined or imported by that module. Names containing non-ASCII characters must be in the [normalization form](https://www.unicode.org/reports/tr15/#Norm_Forms) NFKC; see [Non-ASCII characters in names](https://docs.python.org/3/reference/lexical_analysis.html#lexical-names-nonascii) for details. The names given in `__all__` are all considered public and are required to exist. If `__all__` is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character (`'_'`). `__all__` should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).
The wild card form of import — `from module import *` — is only allowed at the module level. Attempting to use it in class or function definitions will raise a [`SyntaxError`](https://docs.python.org/3/library/exceptions.html#SyntaxError "SyntaxError").
When specifying what module to import you do not have to specify the absolute name of the module. When a module or package is contained within another package it is possible to make a relative import within the same top package without having to mention the package name. By using leading dots in the specified module or package after [`from`](https://docs.python.org/3/reference/simple_stmts.html#from) you can specify how high to traverse up the current package hierarchy without specifying exact names. One leading dot means the current package where the module making the import exists. Two dots means up one package level. Three dots is up two levels, etc. So if you execute `from . import mod` from a module in the `pkg` package then you will end up importing `pkg.mod`. If you execute from within `pkg.subpkg1` you will import `pkg.subpkg2.mod`. The specification for relative imports is contained in the [Package Relative Imports](https://docs.python.org/3/reference/import.html#relativeimports) section.
[`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module") is provided to support applications that determine dynamically the modules to be loaded.
Raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `import` with arguments `module`, `filename`, `sys.path`, `sys.meta_path`, `sys.path_hooks`.
### 7\.11.1. Future statements[¶](https://docs.python.org/3/reference/simple_stmts.html#future-statements "Link to this heading")
A *future statement* is a directive to the compiler that a particular module should be compiled using syntax or semantics that will be available in a specified future release of Python where the feature becomes standard.
The future statement is intended to ease migration to future versions of Python that introduce incompatible changes to the language. It allows use of the new features on a per-module basis before the release in which the feature becomes standard.
```
future_stmt: "from" "__future__" "import" feature ["as" identifier]
("," feature ["as" identifier])*
| "from" "__future__" "import" "(" feature ["as" identifier]
("," feature ["as" identifier])* [","] ")"
feature: identifier
```
A future statement must appear near the top of the module. The only lines that can appear before a future statement are:
- the module docstring (if any),
- comments,
- blank lines, and
- other future statements.
The only feature that requires using the future statement is `annotations` (see [**PEP 563**](https://peps.python.org/pep-0563/)).
All historical features enabled by the future statement are still recognized by Python 3. The list includes `absolute_import`, `division`, `generators`, `generator_stop`, `unicode_literals`, `print_function`, `nested_scopes` and `with_statement`. They are all redundant because they are always enabled, and only kept for backwards compatibility.
A future statement is recognized and treated specially at compile time: Changes to the semantics of core constructs are often implemented by generating different code. It may even be the case that a new feature introduces new incompatible syntax (such as a new reserved word), in which case the compiler may need to parse the module differently. Such decisions cannot be pushed off until runtime.
For any given release, the compiler knows which feature names have been defined, and raises a compile-time error if a future statement contains a feature not known to it.
The direct runtime semantics are the same as for any import statement: there is a standard module [`__future__`](https://docs.python.org/3/library/__future__.html#module-__future__ "__future__: Future statement definitions"), described later, and it will be imported in the usual way at the time the future statement is executed.
The interesting runtime semantics depend on the specific feature enabled by the future statement.
Note that there is nothing special about the statement:
Copy
```
import __future__ [as name]
```
That is not a future statement; it’s an ordinary import statement with no special semantics or syntax restrictions.
Code compiled by calls to the built-in functions [`exec()`](https://docs.python.org/3/library/functions.html#exec "exec") and [`compile()`](https://docs.python.org/3/library/functions.html#compile "compile") that occur in a module `M` containing a future statement will, by default, use the new syntax or semantics associated with the future statement. This can be controlled by optional arguments to `compile()` — see the documentation of that function for details.
A future statement typed at an interactive interpreter prompt will take effect for the rest of the interpreter session. If an interpreter is started with the [`-i`](https://docs.python.org/3/using/cmdline.html#cmdoption-i) option, is passed a script name to execute, and the script includes a future statement, it will be in effect in the interactive session started after the script is executed.
See also
[**PEP 236**](https://peps.python.org/pep-0236/) - Back to the \_\_future\_\_
The original proposal for the \_\_future\_\_ mechanism.
## 7\.12. The `global` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-global-statement "Link to this heading")
```
global_stmt: "global" identifier ("," identifier)*
```
The [`global`](https://docs.python.org/3/reference/simple_stmts.html#global) statement causes the listed identifiers to be interpreted as globals. It would be impossible to assign to a global variable without `global`, although free variables may refer to globals without being declared global.
The `global` statement applies to the entire current scope (module, function body or class definition). A [`SyntaxError`](https://docs.python.org/3/library/exceptions.html#SyntaxError "SyntaxError") is raised if a variable is used or assigned to prior to its global declaration in the scope.
At the module level, all variables are global, so a `global` statement has no effect. However, variables must still not be used or assigned to prior to their `global` declaration. This requirement is relaxed in the interactive prompt ([REPL](https://docs.python.org/3/glossary.html#term-REPL)).
**Programmer’s note:** [`global`](https://docs.python.org/3/reference/simple_stmts.html#global) is a directive to the parser. It applies only to code parsed at the same time as the `global` statement. In particular, a `global` statement contained in a string or code object supplied to the built-in [`exec()`](https://docs.python.org/3/library/functions.html#exec "exec") function does not affect the code block *containing* the function call, and code contained in such a string is unaffected by `global` statements in the code containing the function call. The same applies to the [`eval()`](https://docs.python.org/3/library/functions.html#eval "eval") and [`compile()`](https://docs.python.org/3/library/functions.html#compile "compile") functions.
## 7\.13. The `nonlocal` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-nonlocal-statement "Link to this heading")
```
nonlocal_stmt: "nonlocal" identifier ("," identifier)*
```
When the definition of a function or class is nested (enclosed) within the definitions of other functions, its nonlocal scopes are the local scopes of the enclosing functions. The [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) statement causes the listed identifiers to refer to names previously bound in nonlocal scopes. It allows encapsulated code to rebind such nonlocal identifiers. If a name is bound in more than one nonlocal scope, the nearest binding is used. If a name is not bound in any nonlocal scope, or if there is no nonlocal scope, a [`SyntaxError`](https://docs.python.org/3/library/exceptions.html#SyntaxError "SyntaxError") is raised.
The [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) statement applies to the entire scope of a function or class body. A [`SyntaxError`](https://docs.python.org/3/library/exceptions.html#SyntaxError "SyntaxError") is raised if a variable is used or assigned to prior to its nonlocal declaration in the scope.
See also
[**PEP 3104**](https://peps.python.org/pep-3104/) - Access to Names in Outer Scopes
The specification for the [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) statement.
**Programmer’s note:** [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) is a directive to the parser and applies only to code parsed along with it. See the note for the [`global`](https://docs.python.org/3/reference/simple_stmts.html#global) statement.
## 7\.14. The `type` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-type-statement "Link to this heading")
```
type_stmt: 'type' identifier [type_params] "=" expression
```
The `type` statement declares a type alias, which is an instance of [`typing.TypeAliasType`](https://docs.python.org/3/library/typing.html#typing.TypeAliasType "typing.TypeAliasType").
For example, the following statement creates a type alias:
Copy
```
type Point = tuple[float, float]
```
This code is roughly equivalent to:
Copy
```
annotation-def VALUE_OF_Point():
return tuple[float, float]
Point = typing.TypeAliasType("Point", VALUE_OF_Point())
```
`annotation-def` indicates an [annotation scope](https://docs.python.org/3/reference/executionmodel.html#annotation-scopes), which behaves mostly like a function, but with several small differences.
The value of the type alias is evaluated in the annotation scope. It is not evaluated when the type alias is created, but only when the value is accessed through the type alias’s `__value__` attribute (see [Lazy evaluation](https://docs.python.org/3/reference/executionmodel.html#lazy-evaluation)). This allows the type alias to refer to names that are not yet defined.
Type aliases may be made generic by adding a [type parameter list](https://docs.python.org/3/reference/compound_stmts.html#type-params) after the name. See [Generic type aliases](https://docs.python.org/3/reference/compound_stmts.html#generic-type-aliases) for more.
`type` is a [soft keyword](https://docs.python.org/3/reference/lexical_analysis.html#soft-keywords).
Added in version 3.12.
See also
[**PEP 695**](https://peps.python.org/pep-0695/) - Type Parameter Syntax
Introduced the `type` statement and syntax for generic classes and functions.
### [Table of Contents](https://docs.python.org/3/contents.html)
- [7\. Simple statements](https://docs.python.org/3/reference/simple_stmts.html)
- [7\.1. Expression statements](https://docs.python.org/3/reference/simple_stmts.html#expression-statements)
- [7\.2. Assignment statements](https://docs.python.org/3/reference/simple_stmts.html#assignment-statements)
- [7\.2.1. Augmented assignment statements](https://docs.python.org/3/reference/simple_stmts.html#augmented-assignment-statements)
- [7\.2.2. Annotated assignment statements](https://docs.python.org/3/reference/simple_stmts.html#annotated-assignment-statements)
- [7\.3. The `assert` statement](https://docs.python.org/3/reference/simple_stmts.html#the-assert-statement)
- [7\.4. The `pass` statement](https://docs.python.org/3/reference/simple_stmts.html#the-pass-statement)
- [7\.5. The `del` statement](https://docs.python.org/3/reference/simple_stmts.html#the-del-statement)
- [7\.6. The `return` statement](https://docs.python.org/3/reference/simple_stmts.html#the-return-statement)
- [7\.7. The `yield` statement](https://docs.python.org/3/reference/simple_stmts.html#the-yield-statement)
- [7\.8. The `raise` statement](https://docs.python.org/3/reference/simple_stmts.html#the-raise-statement)
- [7\.9. The `break` statement](https://docs.python.org/3/reference/simple_stmts.html#the-break-statement)
- [7\.10. The `continue` statement](https://docs.python.org/3/reference/simple_stmts.html#the-continue-statement)
- [7\.11. The `import` statement](https://docs.python.org/3/reference/simple_stmts.html#the-import-statement)
- [7\.11.1. Future statements](https://docs.python.org/3/reference/simple_stmts.html#future-statements)
- [7\.12. The `global` statement](https://docs.python.org/3/reference/simple_stmts.html#the-global-statement)
- [7\.13. The `nonlocal` statement](https://docs.python.org/3/reference/simple_stmts.html#the-nonlocal-statement)
- [7\.14. The `type` statement](https://docs.python.org/3/reference/simple_stmts.html#the-type-statement)
#### Previous topic
[6\. Expressions](https://docs.python.org/3/reference/expressions.html "previous chapter")
#### Next topic
[8\. Compound statements](https://docs.python.org/3/reference/compound_stmts.html "next chapter")
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| Readable Markdown | A simple statement is comprised within a single logical line. Several simple statements may occur on a single line separated by semicolons. The syntax for simple statements is:
```
simple_stmt: expression_stmt
| assert_stmt
| assignment_stmt
| augmented_assignment_stmt
| annotated_assignment_stmt
| pass_stmt
| del_stmt
| return_stmt
| yield_stmt
| raise_stmt
| break_stmt
| continue_stmt
| import_stmt
| future_stmt
| global_stmt
| nonlocal_stmt
| type_stmt
```
## 7\.1. Expression statements[¶](https://docs.python.org/3/reference/simple_stmts.html#expression-statements "Link to this heading")
Expression statements are used (mostly interactively) to compute and write a value, or (usually) to call a procedure (a function that returns no meaningful result; in Python, procedures return the value `None`). Other uses of expression statements are allowed and occasionally useful. The syntax for an expression statement is:
```
expression_stmt: starred_expression
```
An expression statement evaluates the expression list (which may be a single expression).
In interactive mode, if the value is not `None`, it is converted to a string using the built-in [`repr()`](https://docs.python.org/3/library/functions.html#repr "repr") function and the resulting string is written to standard output on a line by itself (except if the result is `None`, so that procedure calls do not cause any output.)
## 7\.2. Assignment statements[¶](https://docs.python.org/3/reference/simple_stmts.html#assignment-statements "Link to this heading")
Assignment statements are used to (re)bind names to values and to modify attributes or items of mutable objects:
```
assignment_stmt: (target_list "=")+ (starred_expression | yield_expression)
target_list: target ("," target)* [","]
target: identifier
| "(" [target_list] ")"
| "[" [target_list] "]"
| attributeref
| subscription
| "*" target
```
(See section [Primaries](https://docs.python.org/3/reference/expressions.html#primaries) for the syntax definitions for *attributeref* and *subscription*.)
An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right.
Assignment is defined recursively depending on the form of the target (list). When a target is part of a mutable object (an attribute reference or subscription), the mutable object must ultimately perform the assignment and decide about its validity, and may raise an exception if the assignment is unacceptable. The rules observed by various types and the exceptions raised are given with the definition of the object types (see section [The standard type hierarchy](https://docs.python.org/3/reference/datamodel.html#types)).
Assignment of an object to a target list, optionally enclosed in parentheses or square brackets, is recursively defined as follows.
- If the target list is a single target with no trailing comma, optionally in parentheses, the object is assigned to that target.
- Else:
- If the target list contains one target prefixed with an asterisk, called a “starred” target: The object must be an iterable with at least as many items as there are targets in the target list, minus one. The first items of the iterable are assigned, from left to right, to the targets before the starred target. The final items of the iterable are assigned to the targets after the starred target. A list of the remaining items in the iterable is then assigned to the starred target (the list can be empty).
- Else: The object must be an iterable with the same number of items as there are targets in the target list, and the items are assigned, from left to right, to the corresponding targets.
Assignment of an object to a single target is recursively defined as follows.
- If the target is an identifier (name):
- If the name does not occur in a [`global`](https://docs.python.org/3/reference/simple_stmts.html#global) or [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) statement in the current code block: the name is bound to the object in the current local namespace.
- Otherwise: the name is bound to the object in the global namespace or the outer namespace determined by [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal), respectively.
The name is rebound if it was already bound. This may cause the reference count for the object previously bound to the name to reach zero, causing the object to be deallocated and its destructor (if it has one) to be called.
- If the target is an attribute reference: The primary expression in the reference is evaluated. It should yield an object with assignable attributes; if this is not the case, [`TypeError`](https://docs.python.org/3/library/exceptions.html#TypeError "TypeError") is raised. That object is then asked to assign the assigned object to the given attribute; if it cannot perform the assignment, it raises an exception (usually but not necessarily [`AttributeError`](https://docs.python.org/3/library/exceptions.html#AttributeError "AttributeError")).
Note: If the object is a class instance and the attribute reference occurs on both sides of the assignment operator, the right-hand side expression, `a.x` can access either an instance attribute or (if no instance attribute exists) a class attribute. The left-hand side target `a.x` is always set as an instance attribute, creating it if necessary. Thus, the two occurrences of `a.x` do not necessarily refer to the same attribute: if the right-hand side expression refers to a class attribute, the left-hand side creates a new instance attribute as the target of the assignment:
```
class Cls:
x = 3 # class variable
inst = Cls()
inst.x = inst.x + 1 # writes inst.x as 4 leaving Cls.x as 3
```
This description does not necessarily apply to descriptor attributes, such as properties created with [`property()`](https://docs.python.org/3/library/functions.html#property "property").
- If the target is a subscription: The primary expression in the reference is evaluated. Next, the subscript expression is evaluated. Then, the primary’s [`__setitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "object.__setitem__") method is called with two arguments: the subscript and the assigned object.
Typically, [`__setitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "object.__setitem__") is defined on mutable sequence objects (such as lists) and mapping objects (such as dictionaries), and behaves as follows.
If the primary is a mutable sequence object (such as a list), the subscript must yield an integer. If it is negative, the sequence’s length is added to it. The resulting value must be a nonnegative integer less than the sequence’s length, and the sequence is asked to assign the assigned object to its item with that index. If the index is out of range, [`IndexError`](https://docs.python.org/3/library/exceptions.html#IndexError "IndexError") is raised (assignment to a subscripted sequence cannot add new items to a list).
If the primary is a mapping object (such as a dictionary), the subscript must have a type compatible with the mapping’s key type, and the mapping is then asked to create a key/value pair which maps the subscript to the assigned object. This can either replace an existing key/value pair with the same key value, or insert a new key/value pair (if no key with the same value existed).
If the target is a slicing: The primary expression should evaluate to a mutable sequence object (such as a list). The assigned object should be [iterable](https://docs.python.org/3/glossary.html#term-iterable). The slicing’s lower and upper bounds should be integers; if they are `None` (or not present), the defaults are zero and the sequence’s length. If either bound is negative, the sequence’s length is added to it. The resulting bounds are clipped to lie between zero and the sequence’s length, inclusive. Finally, the sequence object is asked to replace the slice with the items of the assigned sequence. The length of the slice may be different from the length of the assigned sequence, thus changing the length of the target sequence, if the target sequence allows it.
Although the definition of assignment implies that overlaps between the left-hand side and the right-hand side are ‘simultaneous’ (for example swaps two variables), overlaps *within* the collection of assigned-to variables occur left-to-right, sometimes resulting in confusion. For instance, the following program prints `[0, 2]`:
```
x = [0, 1]
i = 0
i, x[i] = 1, 2 # i is updated, then x[i] is updated
print(x)
```
See also
[**PEP 3132**](https://peps.python.org/pep-3132/) - Extended Iterable Unpacking
The specification for the `*target` feature.
### 7\.2.1. Augmented assignment statements[¶](https://docs.python.org/3/reference/simple_stmts.html#augmented-assignment-statements "Link to this heading")
Augmented assignment is the combination, in a single statement, of a binary operation and an assignment statement:
```
augmented_assignment_stmt: augtarget augop (expression_list | yield_expression)
augtarget: identifier | attributeref | subscription
augop: "+=" | "-=" | "*=" | "@=" | "/=" | "//=" | "%=" | "**="
| ">>=" | "<<=" | "&=" | "^=" | "|="
```
(See section [Primaries](https://docs.python.org/3/reference/expressions.html#primaries) for the syntax definitions of the last three symbols.)
An augmented assignment evaluates the target (which, unlike normal assignment statements, cannot be an unpacking) and the expression list, performs the binary operation specific to the type of assignment on the two operands, and assigns the result to the original target. The target is only evaluated once.
An augmented assignment statement like `x += 1` can be rewritten as to achieve a similar, but not exactly equal effect. In the augmented version, `x` is only evaluated once. Also, when possible, the actual operation is performed *in-place*, meaning that rather than creating a new object and assigning that to the target, the old object is modified instead.
Unlike normal assignments, augmented assignments evaluate the left-hand side *before* evaluating the right-hand side. For example, `a[i] += f(x)` first looks-up `a[i]`, then it evaluates `f(x)` and performs the addition, and lastly, it writes the result back to `a[i]`.
With the exception of assigning to tuples and multiple targets in a single statement, the assignment done by augmented assignment statements is handled the same way as normal assignments. Similarly, with the exception of the possible *in-place* behavior, the binary operation performed by augmented assignment is the same as the normal binary operations.
For targets which are attribute references, the same [caveat about class and instance attributes](https://docs.python.org/3/reference/simple_stmts.html#attr-target-note) applies as for regular assignments.
### 7\.2.2. Annotated assignment statements[¶](https://docs.python.org/3/reference/simple_stmts.html#annotated-assignment-statements "Link to this heading")
[Annotation](https://docs.python.org/3/glossary.html#term-variable-annotation) assignment is the combination, in a single statement, of a variable or attribute annotation and an optional assignment statement:
```
annotated_assignment_stmt: augtarget ":" expression
["=" (starred_expression | yield_expression)]
```
The difference from normal [Assignment statements](https://docs.python.org/3/reference/simple_stmts.html#assignment) is that only a single target is allowed.
The assignment target is considered “simple” if it consists of a single name that is not enclosed in parentheses. For simple assignment targets, if in class or module scope, the annotations are gathered in a lazily evaluated [annotation scope](https://docs.python.org/3/reference/executionmodel.html#annotation-scopes). The annotations can be evaluated using the [`__annotations__`](https://docs.python.org/3/reference/datamodel.html#object.__annotations__ "object.__annotations__") attribute of a class or module, or using the facilities in the [`annotationlib`](https://docs.python.org/3/library/annotationlib.html#module-annotationlib "annotationlib: Functionality for introspecting annotations") module.
If the assignment target is not simple (an attribute, subscript node, or parenthesized name), the annotation is never evaluated.
If a name is annotated in a function scope, then this name is local for that scope. Annotations are never evaluated and stored in function scopes.
If the right hand side is present, an annotated assignment performs the actual assignment as if there was no annotation present. If the right hand side is not present for an expression target, then the interpreter evaluates the target except for the last [`__setitem__()`](https://docs.python.org/3/reference/datamodel.html#object.__setitem__ "object.__setitem__") or [`__setattr__()`](https://docs.python.org/3/reference/datamodel.html#object.__setattr__ "object.__setattr__") call.
See also
[**PEP 526**](https://peps.python.org/pep-0526/) - Syntax for Variable Annotations
The proposal that added syntax for annotating the types of variables (including class variables and instance variables), instead of expressing them through comments.
[**PEP 484**](https://peps.python.org/pep-0484/) - Type hints
The proposal that added the [`typing`](https://docs.python.org/3/library/typing.html#module-typing "typing: Support for type hints (see :pep:`484`).") module to provide a standard syntax for type annotations that can be used in static analysis tools and IDEs.
Changed in version 3.8: Now annotated assignments allow the same expressions in the right hand side as regular assignments. Previously, some expressions (like un-parenthesized tuple expressions) caused a syntax error.
Changed in version 3.14: Annotations are now lazily evaluated in a separate [annotation scope](https://docs.python.org/3/reference/executionmodel.html#annotation-scopes). If the assignment target is not simple, annotations are never evaluated.
## 7\.3. The `assert` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-assert-statement "Link to this heading")
Assert statements are a convenient way to insert debugging assertions into a program:
```
assert_stmt: "assert" expression ["," expression]
```
The simple form, `assert expression`, is equivalent to
```
if __debug__:
if not expression: raise AssertionError
```
The extended form, `assert expression1, expression2`, is equivalent to
```
if __debug__:
if not expression1: raise AssertionError(expression2)
```
These equivalences assume that [`__debug__`](https://docs.python.org/3/library/constants.html#debug__ "__debug__") and [`AssertionError`](https://docs.python.org/3/library/exceptions.html#AssertionError "AssertionError") refer to the built-in variables with those names. In the current implementation, the built-in variable `__debug__` is `True` under normal circumstances, `False` when optimization is requested (command line option [`-O`](https://docs.python.org/3/using/cmdline.html#cmdoption-O)). The current code generator emits no code for an [`assert`](https://docs.python.org/3/reference/simple_stmts.html#assert) statement when optimization is requested at compile time. Note that it is unnecessary to include the source code for the expression that failed in the error message; it will be displayed as part of the stack trace.
Assignments to [`__debug__`](https://docs.python.org/3/library/constants.html#debug__ "__debug__") are illegal. The value for the built-in variable is determined when the interpreter starts.
## 7\.4. The `pass` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-pass-statement "Link to this heading")
```
pass_stmt: "pass"
```
[`pass`](https://docs.python.org/3/reference/simple_stmts.html#pass) is a null operation — when it is executed, nothing happens. It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed, for example:
```
def f(arg): pass # a function that does nothing (yet)
class C: pass # a class with no methods (yet)
```
## 7\.5. The `del` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-del-statement "Link to this heading")
```
del_stmt: "del" target_list
```
Deletion is recursively defined very similar to the way assignment is defined. Rather than spelling it out in full details, here are some hints.
Deletion of a target list recursively deletes each target, from left to right.
Deletion of a name removes the binding of that name from the local or global namespace, depending on whether the name occurs in a [`global`](https://docs.python.org/3/reference/simple_stmts.html#global) statement in the same code block. Trying to delete an unbound name raises a [`NameError`](https://docs.python.org/3/library/exceptions.html#NameError "NameError") exception.
Deletion of attribute references and subscriptions is passed to the primary object involved; deletion of a slicing is in general equivalent to assignment of an empty slice of the right type (but even this is determined by the sliced object).
Changed in version 3.2: Previously it was illegal to delete a name from the local namespace if it occurs as a free variable in a nested block.
## 7\.6. The `return` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-return-statement "Link to this heading")
```
return_stmt: "return" [expression_list]
```
[`return`](https://docs.python.org/3/reference/simple_stmts.html#return) may only occur syntactically nested in a function definition, not within a nested class definition.
If an expression list is present, it is evaluated, else `None` is substituted.
[`return`](https://docs.python.org/3/reference/simple_stmts.html#return) leaves the current function call with the expression list (or `None`) as return value.
When [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) passes control out of a [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement with a [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause, that `finally` clause is executed before really leaving the function.
In a generator function, the [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement indicates that the generator is done and will cause [`StopIteration`](https://docs.python.org/3/library/exceptions.html#StopIteration "StopIteration") to be raised. The returned value (if any) is used as an argument to construct `StopIteration` and becomes the [`StopIteration.value`](https://docs.python.org/3/library/exceptions.html#StopIteration.value "StopIteration.value") attribute.
In an asynchronous generator function, an empty [`return`](https://docs.python.org/3/reference/simple_stmts.html#return) statement indicates that the asynchronous generator is done and will cause [`StopAsyncIteration`](https://docs.python.org/3/library/exceptions.html#StopAsyncIteration "StopAsyncIteration") to be raised. A non-empty `return` statement is a syntax error in an asynchronous generator function.
## 7\.7. The `yield` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-yield-statement "Link to this heading")
```
yield_stmt: yield_expression
```
A [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) statement is semantically equivalent to a [yield expression](https://docs.python.org/3/reference/expressions.html#yieldexpr). The `yield` statement can be used to omit the parentheses that would otherwise be required in the equivalent yield expression statement. For example, the yield statements
```
yield <expr>
yield from <expr>
```
are equivalent to the yield expression statements
```
(yield <expr>)
(yield from <expr>)
```
Yield expressions and statements are only used when defining a [generator](https://docs.python.org/3/glossary.html#term-generator) function, and are only used in the body of the generator function. Using [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) in a function definition is sufficient to cause that definition to create a generator function instead of a normal function.
For full details of [`yield`](https://docs.python.org/3/reference/simple_stmts.html#yield) semantics, refer to the [Yield expressions](https://docs.python.org/3/reference/expressions.html#yieldexpr) section.
## 7\.8. The `raise` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-raise-statement "Link to this heading")
```
raise_stmt: "raise" [expression ["from" expression]]
```
If no expressions are present, [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) re-raises the exception that is currently being handled, which is also known as the *active exception*. If there isn’t currently an active exception, a [`RuntimeError`](https://docs.python.org/3/library/exceptions.html#RuntimeError "RuntimeError") exception is raised indicating that this is an error.
Otherwise, [`raise`](https://docs.python.org/3/reference/simple_stmts.html#raise) evaluates the first expression as the exception object. It must be either a subclass or an instance of [`BaseException`](https://docs.python.org/3/library/exceptions.html#BaseException "BaseException"). If it is a class, the exception instance will be obtained when needed by instantiating the class with no arguments.
The *type* of the exception is the exception instance’s class, the *value* is the instance itself.
A traceback object is normally created automatically when an exception is raised and attached to it as the [`__traceback__`](https://docs.python.org/3/library/exceptions.html#BaseException.__traceback__ "BaseException.__traceback__") attribute. You can create an exception and set your own traceback in one step using the [`with_traceback()`](https://docs.python.org/3/library/exceptions.html#BaseException.with_traceback "BaseException.with_traceback") exception method (which returns the same exception instance, with its traceback set to its argument), like so:
```
raise Exception("foo occurred").with_traceback(tracebackobj)
```
The `from` clause is used for exception chaining: if given, the second *expression* must be another exception class or instance. If the second expression is an exception instance, it will be attached to the raised exception as the [`__cause__`](https://docs.python.org/3/library/exceptions.html#BaseException.__cause__ "BaseException.__cause__") attribute (which is writable). If the expression is an exception class, the class will be instantiated and the resulting exception instance will be attached to the raised exception as the `__cause__` attribute. If the raised exception is not handled, both exceptions will be printed:
```
>>> try:
... print(1 / 0)
... except Exception as exc:
... raise RuntimeError("Something bad happened") from exc
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
print(1 / 0)
~~^~~
ZeroDivisionError: division by zero
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError("Something bad happened") from exc
RuntimeError: Something bad happened
```
A similar mechanism works implicitly if a new exception is raised when an exception is already being handled. An exception may be handled when an [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) or [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause, or a [`with`](https://docs.python.org/3/reference/compound_stmts.html#with) statement, is used. The previous exception is then attached as the new exception’s [`__context__`](https://docs.python.org/3/library/exceptions.html#BaseException.__context__ "BaseException.__context__") attribute:
```
>>> try:
... print(1 / 0)
... except:
... raise RuntimeError("Something bad happened")
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
print(1 / 0)
~~^~~
ZeroDivisionError: division by zero
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
raise RuntimeError("Something bad happened")
RuntimeError: Something bad happened
```
Exception chaining can be explicitly suppressed by specifying [`None`](https://docs.python.org/3/library/constants.html#None "None") in the `from` clause:
```
>>> try:
... print(1 / 0)
... except:
... raise RuntimeError("Something bad happened") from None
...
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
RuntimeError: Something bad happened
```
Additional information on exceptions can be found in section [Exceptions](https://docs.python.org/3/reference/executionmodel.html#exceptions), and information about handling exceptions is in section [The try statement](https://docs.python.org/3/reference/compound_stmts.html#try).
Changed in version 3.3: [`None`](https://docs.python.org/3/library/constants.html#None "None") is now permitted as `Y` in `raise X from Y`.
Added the [`__suppress_context__`](https://docs.python.org/3/library/exceptions.html#BaseException.__suppress_context__ "BaseException.__suppress_context__") attribute to suppress automatic display of the exception context.
Changed in version 3.11: If the traceback of the active exception is modified in an [`except`](https://docs.python.org/3/reference/compound_stmts.html#except) clause, a subsequent `raise` statement re-raises the exception with the modified traceback. Previously, the exception was re-raised with the traceback it had when it was caught.
## 7\.9. The `break` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-break-statement "Link to this heading")
```
break_stmt: "break"
```
[`break`](https://docs.python.org/3/reference/simple_stmts.html#break) may only occur syntactically nested in a [`for`](https://docs.python.org/3/reference/compound_stmts.html#for) or [`while`](https://docs.python.org/3/reference/compound_stmts.html#while) loop, but not nested in a function or class definition within that loop.
It terminates the nearest enclosing loop, skipping the optional `else` clause if the loop has one.
If a [`for`](https://docs.python.org/3/reference/compound_stmts.html#for) loop is terminated by [`break`](https://docs.python.org/3/reference/simple_stmts.html#break), the loop control target keeps its current value.
When [`break`](https://docs.python.org/3/reference/simple_stmts.html#break) passes control out of a [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement with a [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause, that `finally` clause is executed before really leaving the loop.
## 7\.10. The `continue` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-continue-statement "Link to this heading")
```
continue_stmt: "continue"
```
[`continue`](https://docs.python.org/3/reference/simple_stmts.html#continue) may only occur syntactically nested in a [`for`](https://docs.python.org/3/reference/compound_stmts.html#for) or [`while`](https://docs.python.org/3/reference/compound_stmts.html#while) loop, but not nested in a function or class definition within that loop. It continues with the next cycle of the nearest enclosing loop.
When [`continue`](https://docs.python.org/3/reference/simple_stmts.html#continue) passes control out of a [`try`](https://docs.python.org/3/reference/compound_stmts.html#try) statement with a [`finally`](https://docs.python.org/3/reference/compound_stmts.html#finally) clause, that `finally` clause is executed before really starting the next loop cycle.
## 7\.11. The `import` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-import-statement "Link to this heading")
```
import_stmt: "import" module ["as" identifier] ("," module ["as" identifier])*
| "from" relative_module "import" identifier ["as" identifier]
("," identifier ["as" identifier])*
| "from" relative_module "import" "(" identifier ["as" identifier]
("," identifier ["as" identifier])* [","] ")"
| "from" relative_module "import" "*"
module: (identifier ".")* identifier
relative_module: "."* module | "."+
```
The basic import statement (no [`from`](https://docs.python.org/3/reference/simple_stmts.html#from) clause) is executed in two steps:
1. find a module, loading and initializing it if necessary
2. define a name or names in the local namespace for the scope where the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement occurs.
When the statement contains multiple clauses (separated by commas) the two steps are carried out separately for each clause, just as though the clauses had been separated out into individual import statements.
The details of the first step, finding and loading modules, are described in greater detail in the section on the [import system](https://docs.python.org/3/reference/import.html#importsystem), which also describes the various types of packages and modules that can be imported, as well as all the hooks that can be used to customize the import system. Note that failures in this step may indicate either that the module could not be located, *or* that an error occurred while initializing the module, which includes execution of the module’s code.
If the requested module is retrieved successfully, it will be made available in the local namespace in one of three ways:
- If the module name is followed by `as`, then the name following `as` is bound directly to the imported module.
- If no other name is specified, and the module being imported is a top level module, the module’s name is bound in the local namespace as a reference to the imported module
- If the module being imported is *not* a top level module, then the name of the top level package that contains the module is bound in the local namespace as a reference to the top level package. The imported module must be accessed using its full qualified name rather than directly
The [`from`](https://docs.python.org/3/reference/simple_stmts.html#from) form uses a slightly more complex process:
1. find the module specified in the [`from`](https://docs.python.org/3/reference/simple_stmts.html#from) clause, loading and initializing it if necessary;
2. for each of the identifiers specified in the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) clauses:
1. check if the imported module has an attribute by that name
2. if not, attempt to import a submodule with that name and then check the imported module again for that attribute
3. if the attribute is not found, [`ImportError`](https://docs.python.org/3/library/exceptions.html#ImportError "ImportError") is raised.
4. otherwise, a reference to that value is stored in the local namespace, using the name in the `as` clause if it is present, otherwise using the attribute name
Examples:
```
import foo # foo imported and bound locally
import foo.bar.baz # foo, foo.bar, and foo.bar.baz imported, foo bound locally
import foo.bar.baz as fbb # foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as fbb
from foo.bar import baz # foo, foo.bar, and foo.bar.baz imported, foo.bar.baz bound as baz
from foo import attr # foo imported and foo.attr bound as attr
```
If the list of identifiers is replaced by a star (`'*'`), all public names defined in the module are bound in the local namespace for the scope where the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement occurs.
The *public names* defined by a module are determined by checking the module’s namespace for a variable named `__all__`; if defined, it must be a sequence of strings which are names defined or imported by that module. Names containing non-ASCII characters must be in the [normalization form](https://www.unicode.org/reports/tr15/#Norm_Forms) NFKC; see [Non-ASCII characters in names](https://docs.python.org/3/reference/lexical_analysis.html#lexical-names-nonascii) for details. The names given in `__all__` are all considered public and are required to exist. If `__all__` is not defined, the set of public names includes all names found in the module’s namespace which do not begin with an underscore character (`'_'`). `__all__` should contain the entire public API. It is intended to avoid accidentally exporting items that are not part of the API (such as library modules which were imported and used within the module).
The wild card form of import — `from module import *` — is only allowed at the module level. Attempting to use it in class or function definitions will raise a [`SyntaxError`](https://docs.python.org/3/library/exceptions.html#SyntaxError "SyntaxError").
When specifying what module to import you do not have to specify the absolute name of the module. When a module or package is contained within another package it is possible to make a relative import within the same top package without having to mention the package name. By using leading dots in the specified module or package after [`from`](https://docs.python.org/3/reference/simple_stmts.html#from) you can specify how high to traverse up the current package hierarchy without specifying exact names. One leading dot means the current package where the module making the import exists. Two dots means up one package level. Three dots is up two levels, etc. So if you execute `from . import mod` from a module in the `pkg` package then you will end up importing `pkg.mod`. If you execute from within `pkg.subpkg1` you will import `pkg.subpkg2.mod`. The specification for relative imports is contained in the [Package Relative Imports](https://docs.python.org/3/reference/import.html#relativeimports) section.
[`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module") is provided to support applications that determine dynamically the modules to be loaded.
Raises an [auditing event](https://docs.python.org/3/library/sys.html#auditing) `import` with arguments `module`, `filename`, `sys.path`, `sys.meta_path`, `sys.path_hooks`.
### 7\.11.1. Future statements[¶](https://docs.python.org/3/reference/simple_stmts.html#future-statements "Link to this heading")
A *future statement* is a directive to the compiler that a particular module should be compiled using syntax or semantics that will be available in a specified future release of Python where the feature becomes standard.
The future statement is intended to ease migration to future versions of Python that introduce incompatible changes to the language. It allows use of the new features on a per-module basis before the release in which the feature becomes standard.
```
future_stmt: "from" "__future__" "import" feature ["as" identifier]
("," feature ["as" identifier])*
| "from" "__future__" "import" "(" feature ["as" identifier]
("," feature ["as" identifier])* [","] ")"
feature: identifier
```
A future statement must appear near the top of the module. The only lines that can appear before a future statement are:
- the module docstring (if any),
- comments,
- blank lines, and
- other future statements.
The only feature that requires using the future statement is `annotations` (see [**PEP 563**](https://peps.python.org/pep-0563/)).
All historical features enabled by the future statement are still recognized by Python 3. The list includes `absolute_import`, `division`, `generators`, `generator_stop`, `unicode_literals`, `print_function`, `nested_scopes` and `with_statement`. They are all redundant because they are always enabled, and only kept for backwards compatibility.
A future statement is recognized and treated specially at compile time: Changes to the semantics of core constructs are often implemented by generating different code. It may even be the case that a new feature introduces new incompatible syntax (such as a new reserved word), in which case the compiler may need to parse the module differently. Such decisions cannot be pushed off until runtime.
For any given release, the compiler knows which feature names have been defined, and raises a compile-time error if a future statement contains a feature not known to it.
The direct runtime semantics are the same as for any import statement: there is a standard module [`__future__`](https://docs.python.org/3/library/__future__.html#module-__future__ "__future__: Future statement definitions"), described later, and it will be imported in the usual way at the time the future statement is executed.
The interesting runtime semantics depend on the specific feature enabled by the future statement.
Note that there is nothing special about the statement:
```
import __future__ [as name]
```
That is not a future statement; it’s an ordinary import statement with no special semantics or syntax restrictions.
Code compiled by calls to the built-in functions [`exec()`](https://docs.python.org/3/library/functions.html#exec "exec") and [`compile()`](https://docs.python.org/3/library/functions.html#compile "compile") that occur in a module `M` containing a future statement will, by default, use the new syntax or semantics associated with the future statement. This can be controlled by optional arguments to `compile()` — see the documentation of that function for details.
A future statement typed at an interactive interpreter prompt will take effect for the rest of the interpreter session. If an interpreter is started with the [`-i`](https://docs.python.org/3/using/cmdline.html#cmdoption-i) option, is passed a script name to execute, and the script includes a future statement, it will be in effect in the interactive session started after the script is executed.
See also
[**PEP 236**](https://peps.python.org/pep-0236/) - Back to the \_\_future\_\_
The original proposal for the \_\_future\_\_ mechanism.
## 7\.12. The `global` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-global-statement "Link to this heading")
```
global_stmt: "global" identifier ("," identifier)*
```
The [`global`](https://docs.python.org/3/reference/simple_stmts.html#global) statement causes the listed identifiers to be interpreted as globals. It would be impossible to assign to a global variable without `global`, although free variables may refer to globals without being declared global.
The `global` statement applies to the entire current scope (module, function body or class definition). A [`SyntaxError`](https://docs.python.org/3/library/exceptions.html#SyntaxError "SyntaxError") is raised if a variable is used or assigned to prior to its global declaration in the scope.
At the module level, all variables are global, so a `global` statement has no effect. However, variables must still not be used or assigned to prior to their `global` declaration. This requirement is relaxed in the interactive prompt ([REPL](https://docs.python.org/3/glossary.html#term-REPL)).
**Programmer’s note:** [`global`](https://docs.python.org/3/reference/simple_stmts.html#global) is a directive to the parser. It applies only to code parsed at the same time as the `global` statement. In particular, a `global` statement contained in a string or code object supplied to the built-in [`exec()`](https://docs.python.org/3/library/functions.html#exec "exec") function does not affect the code block *containing* the function call, and code contained in such a string is unaffected by `global` statements in the code containing the function call. The same applies to the [`eval()`](https://docs.python.org/3/library/functions.html#eval "eval") and [`compile()`](https://docs.python.org/3/library/functions.html#compile "compile") functions.
## 7\.13. The `nonlocal` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-nonlocal-statement "Link to this heading")
```
nonlocal_stmt: "nonlocal" identifier ("," identifier)*
```
When the definition of a function or class is nested (enclosed) within the definitions of other functions, its nonlocal scopes are the local scopes of the enclosing functions. The [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) statement causes the listed identifiers to refer to names previously bound in nonlocal scopes. It allows encapsulated code to rebind such nonlocal identifiers. If a name is bound in more than one nonlocal scope, the nearest binding is used. If a name is not bound in any nonlocal scope, or if there is no nonlocal scope, a [`SyntaxError`](https://docs.python.org/3/library/exceptions.html#SyntaxError "SyntaxError") is raised.
The [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) statement applies to the entire scope of a function or class body. A [`SyntaxError`](https://docs.python.org/3/library/exceptions.html#SyntaxError "SyntaxError") is raised if a variable is used or assigned to prior to its nonlocal declaration in the scope.
See also
[**PEP 3104**](https://peps.python.org/pep-3104/) - Access to Names in Outer Scopes
The specification for the [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) statement.
**Programmer’s note:** [`nonlocal`](https://docs.python.org/3/reference/simple_stmts.html#nonlocal) is a directive to the parser and applies only to code parsed along with it. See the note for the [`global`](https://docs.python.org/3/reference/simple_stmts.html#global) statement.
## 7\.14. The `type` statement[¶](https://docs.python.org/3/reference/simple_stmts.html#the-type-statement "Link to this heading")
```
type_stmt: 'type' identifier [type_params] "=" expression
```
The `type` statement declares a type alias, which is an instance of [`typing.TypeAliasType`](https://docs.python.org/3/library/typing.html#typing.TypeAliasType "typing.TypeAliasType").
For example, the following statement creates a type alias:
```
type Point = tuple[float, float]
```
This code is roughly equivalent to:
```
annotation-def VALUE_OF_Point():
return tuple[float, float]
Point = typing.TypeAliasType("Point", VALUE_OF_Point())
```
`annotation-def` indicates an [annotation scope](https://docs.python.org/3/reference/executionmodel.html#annotation-scopes), which behaves mostly like a function, but with several small differences.
The value of the type alias is evaluated in the annotation scope. It is not evaluated when the type alias is created, but only when the value is accessed through the type alias’s `__value__` attribute (see [Lazy evaluation](https://docs.python.org/3/reference/executionmodel.html#lazy-evaluation)). This allows the type alias to refer to names that are not yet defined.
Type aliases may be made generic by adding a [type parameter list](https://docs.python.org/3/reference/compound_stmts.html#type-params) after the name. See [Generic type aliases](https://docs.python.org/3/reference/compound_stmts.html#generic-type-aliases) for more.
`type` is a [soft keyword](https://docs.python.org/3/reference/lexical_analysis.html#soft-keywords).
Added in version 3.12.
See also
[**PEP 695**](https://peps.python.org/pep-0695/) - Type Parameter Syntax
Introduced the `type` statement and syntax for generic classes and functions. | |||||||||
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