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| Meta Title | 5. The import system — Python 3.14.4 documentation |
| Meta Description | Python code in one module gains access to the code in another module by the process of importing it. The import statement is the most common way of invoking the import machinery, but it is not the ... |
| Meta Canonical | null |
| Boilerpipe Text | Python code in one
module
gains access to the code in another module
by the process of
importing
it. The
import
statement is
the most common way of invoking the import machinery, but it is not the only
way. Functions such as
importlib.import_module()
and built-in
__import__()
can also be used to invoke the import machinery.
The
import
statement combines two operations; it searches for the
named module, then it binds the results of that search to a name in the local
scope. The search operation of the
import
statement is defined as
a call to the
__import__()
function, with the appropriate arguments.
The return value of
__import__()
is used to perform the name
binding operation of the
import
statement. See the
import
statement for the exact details of that name binding
operation.
A direct call to
__import__()
performs only the module search and, if
found, the module creation operation. While certain side-effects may occur,
such as the importing of parent packages, and the updating of various caches
(including
sys.modules
), only the
import
statement performs
a name binding operation.
When an
import
statement is executed, the standard builtin
__import__()
function is called. Other mechanisms for invoking the
import system (such as
importlib.import_module()
) may choose to bypass
__import__()
and use their own solutions to implement import semantics.
When a module is first imported, Python searches for the module and if found,
it creates a module object
[
1
]
, initializing it. If the named module
cannot be found, a
ModuleNotFoundError
is raised. Python implements various
strategies to search for the named module when the import machinery is
invoked. These strategies can be modified and extended by using various hooks
described in the sections below.
Changed in version 3.3:
The import system has been updated to fully implement the second phase
of
PEP 302
. There is no longer any implicit import machinery - the full
import system is exposed through
sys.meta_path
. In addition,
native namespace package support has been implemented (see
PEP 420
).
5.1.
importlib
¶
The
importlib
module provides a rich API for interacting with the
import system. For example
importlib.import_module()
provides a
recommended, simpler API than built-in
__import__()
for invoking the
import machinery. Refer to the
importlib
library documentation for
additional detail.
5.2.
Packages
¶
Python has only one type of module object, and all modules are of this type,
regardless of whether the module is implemented in Python, C, or something
else. To help organize modules and provide a naming hierarchy, Python has a
concept of
packages
.
You can think of packages as the directories on a file system and modules as
files within directories, but don’t take this analogy too literally since
packages and modules need not originate from the file system. For the
purposes of this documentation, we’ll use this convenient analogy of
directories and files. Like file system directories, packages are organized
hierarchically, and packages may themselves contain subpackages, as well as
regular modules.
It’s important to keep in mind that all packages are modules, but not all
modules are packages. Or put another way, packages are just a special kind of
module. Specifically, any module that contains a
__path__
attribute is
considered a package.
All modules have a name. Subpackage names are separated from their parent
package name by a dot, akin to Python’s standard attribute access syntax. Thus
you might have a package called
email
, which in turn has a subpackage
called
email.mime
and a module within that subpackage called
email.mime.text
.
5.2.1.
Regular packages
¶
Python defines two types of packages,
regular packages
and
namespace packages
. Regular
packages are traditional packages as they existed in Python 3.2 and earlier.
A regular package is typically implemented as a directory containing an
__init__.py
file. When a regular package is imported, this
__init__.py
file is implicitly executed, and the objects it defines are
bound to names in the package’s namespace. The
__init__.py
file can
contain the same Python code that any other module can contain, and Python
will add some additional attributes to the module when it is imported.
For example, the following file system layout defines a top level
parent
package with three subpackages:
parent
/
__init__
.
py
one
/
__init__
.
py
two
/
__init__
.
py
three
/
__init__
.
py
Importing
parent.one
will implicitly execute
parent/__init__.py
and
parent/one/__init__.py
. Subsequent imports of
parent.two
or
parent.three
will execute
parent/two/__init__.py
and
parent/three/__init__.py
respectively.
5.2.2.
Namespace packages
¶
A namespace package is a composite of various
portions
,
where each portion contributes a subpackage to the parent package. Portions
may reside in different locations on the file system. Portions may also be
found in zip files, on the network, or anywhere else that Python searches
during import. Namespace packages may or may not correspond directly to
objects on the file system; they may be virtual modules that have no concrete
representation.
Namespace packages do not use an ordinary list for their
__path__
attribute. They instead use a custom iterable type which will automatically
perform a new search for package portions on the next import attempt within
that package if the path of their parent package (or
sys.path
for a
top level package) changes.
With namespace packages, there is no
parent/__init__.py
file. In fact,
there may be multiple
parent
directories found during import search, where
each one is provided by a different portion. Thus
parent/one
may not be
physically located next to
parent/two
. In this case, Python will create a
namespace package for the top-level
parent
package whenever it or one of
its subpackages is imported.
See also
PEP 420
for the namespace package specification.
5.3.
Searching
¶
To begin the search, Python needs the
fully qualified
name of the module (or package, but for the purposes of this discussion, the
difference is immaterial) being imported. This name may come from various
arguments to the
import
statement, or from the parameters to the
importlib.import_module()
or
__import__()
functions.
This name will be used in various phases of the import search, and it may be
the dotted path to a submodule, e.g.
foo.bar.baz
. In this case, Python
first tries to import
foo
, then
foo.bar
, and finally
foo.bar.baz
.
If any of the intermediate imports fail, a
ModuleNotFoundError
is raised.
5.3.1.
The module cache
¶
The first place checked during import search is
sys.modules
. This
mapping serves as a cache of all modules that have been previously imported,
including the intermediate paths. So if
foo.bar.baz
was previously
imported,
sys.modules
will contain entries for
foo
,
foo.bar
,
and
foo.bar.baz
. Each key will have as its value the corresponding module
object.
During import, the module name is looked up in
sys.modules
and if
present, the associated value is the module satisfying the import, and the
process completes. However, if the value is
None
, then a
ModuleNotFoundError
is raised. If the module name is missing, Python will
continue searching for the module.
sys.modules
is writable. Deleting a key may not destroy the
associated module (as other modules may hold references to it),
but it will invalidate the cache entry for the named module, causing
Python to search anew for the named module upon its next
import. The key can also be assigned to
None
, forcing the next import
of the module to result in a
ModuleNotFoundError
.
Beware though, as if you keep a reference to the module object,
invalidate its cache entry in
sys.modules
, and then re-import the
named module, the two module objects will
not
be the same. By contrast,
importlib.reload()
will reuse the
same
module object, and simply
reinitialise the module contents by rerunning the module’s code.
5.3.2.
Finders and loaders
¶
If the named module is not found in
sys.modules
, then Python’s import
protocol is invoked to find and load the module. This protocol consists of
two conceptual objects,
finders
and
loaders
.
A finder’s job is to determine whether it can find the named module using
whatever strategy it knows about. Objects that implement both of these
interfaces are referred to as
importers
- they return
themselves when they find that they can load the requested module.
Python includes a number of default finders and importers. The first one
knows how to locate built-in modules, and the second knows how to locate
frozen modules. A third default finder searches an
import path
for modules. The
import path
is a list of locations that may
name file system paths or zip files. It can also be extended to search
for any locatable resource, such as those identified by URLs.
The import machinery is extensible, so new finders can be added to extend the
range and scope of module searching.
Finders do not actually load modules. If they can find the named module, they
return a
module spec
, an encapsulation of the module’s import-related
information, which the import machinery then uses when loading the module.
The following sections describe the protocol for finders and loaders in more
detail, including how you can create and register new ones to extend the
import machinery.
Changed in version 3.4:
In previous versions of Python, finders returned
loaders
directly, whereas now they return module specs which
contain
loaders.
Loaders are still used during import but have fewer responsibilities.
5.3.3.
Import hooks
¶
The import machinery is designed to be extensible; the primary mechanism for
this are the
import hooks
. There are two types of import hooks:
meta
hooks
and
import path hooks
.
Meta hooks are called at the start of import processing, before any other
import processing has occurred, other than
sys.modules
cache look up.
This allows meta hooks to override
sys.path
processing, frozen
modules, or even built-in modules. Meta hooks are registered by adding new
finder objects to
sys.meta_path
, as described below.
Import path hooks are called as part of
sys.path
(or
package.__path__
) processing, at the point where their associated path
item is encountered. Import path hooks are registered by adding new callables
to
sys.path_hooks
as described below.
5.3.4.
The meta path
¶
When the named module is not found in
sys.modules
, Python next
searches
sys.meta_path
, which contains a list of meta path finder
objects. These finders are queried in order to see if they know how to handle
the named module. Meta path finders must implement a method called
find_spec()
which takes three arguments:
a name, an import path, and (optionally) a target module. The meta path
finder can use any strategy it wants to determine whether it can handle
the named module or not.
If the meta path finder knows how to handle the named module, it returns a
spec object. If it cannot handle the named module, it returns
None
. If
sys.meta_path
processing reaches the end of its list without returning
a spec, then a
ModuleNotFoundError
is raised. Any other exceptions
raised are simply propagated up, aborting the import process.
The
find_spec()
method of meta path
finders is called with two or three arguments. The first is the fully
qualified name of the module being imported, for example
foo.bar.baz
.
The second argument is the path entries to use for the module search. For
top-level modules, the second argument is
None
, but for submodules or
subpackages, the second argument is the value of the parent package’s
__path__
attribute. If the appropriate
__path__
attribute cannot
be accessed, a
ModuleNotFoundError
is raised. The third argument
is an existing module object that will be the target of loading later.
The import system passes in a target module only during reload.
The meta path may be traversed multiple times for a single import request.
For example, assuming none of the modules involved has already been cached,
importing
foo.bar.baz
will first perform a top level import, calling
mpf.find_spec("foo",
None,
None)
on each meta path finder (
mpf
). After
foo
has been imported,
foo.bar
will be imported by traversing the
meta path a second time, calling
mpf.find_spec("foo.bar",
foo.__path__,
None)
. Once
foo.bar
has been
imported, the final traversal will call
mpf.find_spec("foo.bar.baz",
foo.bar.__path__,
None)
.
Some meta path finders only support top level imports. These importers will
always return
None
when anything other than
None
is passed as the
second argument.
Python’s default
sys.meta_path
has three meta path finders, one that
knows how to import built-in modules, one that knows how to import frozen
modules, and one that knows how to import modules from an
import path
(i.e. the
path based finder
).
Changed in version 3.4:
The
find_spec()
method of meta path
finders replaced
find_module()
, which
is now deprecated. While it will continue to work without change, the
import machinery will try it only if the finder does not implement
find_spec()
.
Changed in version 3.10:
Use of
find_module()
by the import system
now raises
ImportWarning
.
Changed in version 3.12:
find_module()
has been removed.
Use
find_spec()
instead.
5.4.
Loading
¶
If and when a module spec is found, the import machinery will use it (and
the loader it contains) when loading the module. Here is an approximation
of what happens during the loading portion of import:
module
=
None
if
spec
.
loader
is
not
None
and
hasattr
(
spec
.
loader
,
'create_module'
):
# It is assumed 'exec_module' will also be defined on the loader.
module
=
spec
.
loader
.
create_module
(
spec
)
if
module
is
None
:
module
=
ModuleType
(
spec
.
name
)
# The import-related module attributes get set here:
_init_module_attrs
(
spec
,
module
)
if
spec
.
loader
is
None
:
# unsupported
raise
ImportError
if
spec
.
origin
is
None
and
spec
.
submodule_search_locations
is
not
None
:
# namespace package
sys
.
modules
[
spec
.
name
]
=
module
elif
not
hasattr
(
spec
.
loader
,
'exec_module'
):
module
=
spec
.
loader
.
load_module
(
spec
.
name
)
else
:
sys
.
modules
[
spec
.
name
]
=
module
try
:
spec
.
loader
.
exec_module
(
module
)
except
BaseException
:
try
:
del
sys
.
modules
[
spec
.
name
]
except
KeyError
:
pass
raise
return
sys
.
modules
[
spec
.
name
]
Note the following details:
If there is an existing module object with the given name in
sys.modules
, import will have already returned it.
The module will exist in
sys.modules
before the loader
executes the module code. This is crucial because the module code may
(directly or indirectly) import itself; adding it to
sys.modules
beforehand prevents unbounded recursion in the worst case and multiple
loading in the best.
If loading fails, the failing module – and only the failing module –
gets removed from
sys.modules
. Any module already in the
sys.modules
cache, and any module that was successfully loaded
as a side-effect, must remain in the cache. This contrasts with
reloading where even the failing module is left in
sys.modules
.
After the module is created but before execution, the import machinery
sets the import-related module attributes (“_init_module_attrs” in
the pseudo-code example above), as summarized in a
later section
.
Module execution is the key moment of loading in which the module’s
namespace gets populated. Execution is entirely delegated to the
loader, which gets to decide what gets populated and how.
The module created during loading and passed to exec_module() may
not be the one returned at the end of import
[
2
]
.
Changed in version 3.4:
The import system has taken over the boilerplate responsibilities of
loaders. These were previously performed by the
importlib.abc.Loader.load_module()
method.
5.4.1.
Loaders
¶
Module loaders provide the critical function of loading: module execution.
The import machinery calls the
importlib.abc.Loader.exec_module()
method with a single argument, the module object to execute. Any value
returned from
exec_module()
is ignored.
Loaders must satisfy the following requirements:
If the module is a Python module (as opposed to a built-in module or a
dynamically loaded extension), the loader should execute the module’s code
in the module’s global name space (
module.__dict__
).
If the loader cannot execute the module, it should raise an
ImportError
, although any other exception raised during
exec_module()
will be propagated.
In many cases, the finder and loader can be the same object; in such cases the
find_spec()
method would just return a
spec with the loader set to
self
.
Module loaders may opt in to creating the module object during loading
by implementing a
create_module()
method.
It takes one argument, the module spec, and returns the new module object
to use during loading.
create_module()
does not need to set any attributes
on the module object. If the method returns
None
, the
import machinery will create the new module itself.
Changed in version 3.4:
The
load_module()
method was replaced by
exec_module()
and the import
machinery assumed all the boilerplate responsibilities of loading.
For compatibility with existing loaders, the import machinery will use
the
load_module()
method of loaders if it exists and the loader does
not also implement
exec_module()
. However,
load_module()
has been
deprecated and loaders should implement
exec_module()
instead.
The
load_module()
method must implement all the boilerplate loading
functionality described above in addition to executing the module. All
the same constraints apply, with some additional clarification:
If there is an existing module object with the given name in
sys.modules
, the loader must use that existing module.
(Otherwise,
importlib.reload()
will not work correctly.) If the
named module does not exist in
sys.modules
, the loader
must create a new module object and add it to
sys.modules
.
The module
must
exist in
sys.modules
before the loader
executes the module code, to prevent unbounded recursion or multiple
loading.
If loading fails, the loader must remove any modules it has inserted
into
sys.modules
, but it must remove
only
the failing
module(s), and only if the loader itself has loaded the module(s)
explicitly.
Changed in version 3.5:
A
DeprecationWarning
is raised when
exec_module()
is defined but
create_module()
is not.
Changed in version 3.6:
An
ImportError
is raised when
exec_module()
is defined but
create_module()
is not.
Changed in version 3.10:
Use of
load_module()
will raise
ImportWarning
.
5.4.2.
Submodules
¶
When a submodule is loaded using any mechanism (e.g.
importlib
APIs, the
import
or
import-from
statements, or built-in
__import__()
) a
binding is placed in the parent module’s namespace to the submodule object.
For example, if package
spam
has a submodule
foo
, after importing
spam.foo
,
spam
will have an attribute
foo
which is bound to the
submodule. Let’s say you have the following directory structure:
spam
/
__init__
.
py
foo
.
py
and
spam/__init__.py
has the following line in it:
from
.foo
import
Foo
then executing the following puts name bindings for
foo
and
Foo
in the
spam
module:
>>>
import
spam
>>>
spam
.
foo
<module 'spam.foo' from '/tmp/imports/spam/foo.py'>
>>>
spam
.
Foo
<class 'spam.foo.Foo'>
Given Python’s familiar name binding rules this might seem surprising, but
it’s actually a fundamental feature of the import system. The invariant
holding is that if you have
sys.modules['spam']
and
sys.modules['spam.foo']
(as you would after the above import), the latter
must appear as the
foo
attribute of the former.
5.4.3.
Module specs
¶
The import machinery uses a variety of information about each module
during import, especially before loading. Most of the information is
common to all modules. The purpose of a module’s spec is to encapsulate
this import-related information on a per-module basis.
Using a spec during import allows state to be transferred between import
system components, e.g. between the finder that creates the module spec
and the loader that executes it. Most importantly, it allows the
import machinery to perform the boilerplate operations of loading,
whereas without a module spec the loader had that responsibility.
The module’s spec is exposed as
module.__spec__
. Setting
__spec__
appropriately applies equally to
modules initialized during interpreter startup
.
The one exception is
__main__
, where
__spec__
is
set to None in some cases
.
See
ModuleSpec
for details on the contents of
the module spec.
Added in version 3.4.
5.4.4.
__path__ attributes on modules
¶
The
__path__
attribute should be a (possibly empty)
sequence
of strings enumerating the locations where the package’s
submodules will be found. By definition, if a module has a
__path__
attribute, it is a
package
.
A package’s
__path__
attribute is used during imports of its
subpackages.
Within the import machinery, it functions much the same as
sys.path
,
i.e. providing a list of locations to search for modules during import.
However,
__path__
is typically much more constrained than
sys.path
.
The same rules used for
sys.path
also apply to a package’s
__path__
.
sys.path_hooks
(described below) are
consulted when traversing a package’s
__path__
.
A package’s
__init__.py
file may set or alter the package’s
__path__
attribute, and this was typically the way namespace packages were implemented
prior to
PEP 420
. With the adoption of
PEP 420
, namespace packages no
longer need to supply
__init__.py
files containing only
__path__
manipulation code; the import machinery automatically sets
__path__
correctly for the namespace package.
5.4.5.
Module reprs
¶
By default, all modules have a usable repr, however depending on the
attributes set above, and in the module’s spec, you can more explicitly
control the repr of module objects.
If the module has a spec (
__spec__
), the import machinery will try
to generate a repr from it. If that fails or there is no spec, the import
system will craft a default repr using whatever information is available
on the module. It will try to use the
module.__name__
,
module.__file__
, and
module.__loader__
as input into the repr,
with defaults for whatever information is missing.
Here are the exact rules used:
If the module has a
__spec__
attribute, the information in the spec
is used to generate the repr. The “name”, “loader”, “origin”, and
“has_location” attributes are consulted.
If the module has a
__file__
attribute, this is used as part of the
module’s repr.
If the module has no
__file__
but does have a
__loader__
that is not
None
, then the loader’s repr is used as part of the module’s repr.
Otherwise, just use the module’s
__name__
in the repr.
Changed in version 3.12:
Use of
module_repr()
, having been deprecated since Python 3.4, was
removed in Python 3.12 and is no longer called during the resolution of a
module’s repr.
5.4.6.
Cached bytecode invalidation
¶
Before Python loads cached bytecode from a
.pyc
file, it checks whether the
cache is up-to-date with the source
.py
file. By default, Python does this
by storing the source’s last-modified timestamp and size in the cache file when
writing it. At runtime, the import system then validates the cache file by
checking the stored metadata in the cache file against the source’s
metadata.
Python also supports “hash-based” cache files, which store a hash of the source
file’s contents rather than its metadata. There are two variants of hash-based
.pyc
files: checked and unchecked. For checked hash-based
.pyc
files,
Python validates the cache file by hashing the source file and comparing the
resulting hash with the hash in the cache file. If a checked hash-based cache
file is found to be invalid, Python regenerates it and writes a new checked
hash-based cache file. For unchecked hash-based
.pyc
files, Python simply
assumes the cache file is valid if it exists. Hash-based
.pyc
files
validation behavior may be overridden with the
--check-hash-based-pycs
flag.
Changed in version 3.7:
Added hash-based
.pyc
files. Previously, Python only supported
timestamp-based invalidation of bytecode caches.
5.5.
The Path Based Finder
¶
As mentioned previously, Python comes with several default meta path finders.
One of these, called the
path based finder
(
PathFinder
), searches an
import path
,
which contains a list of
path entries
. Each path
entry names a location to search for modules.
The path based finder itself doesn’t know how to import anything. Instead, it
traverses the individual path entries, associating each of them with a
path entry finder that knows how to handle that particular kind of path.
The default set of path entry finders implement all the semantics for finding
modules on the file system, handling special file types such as Python source
code (
.py
files), Python byte code (
.pyc
files) and
shared libraries (e.g.
.so
files). When supported by the
zipimport
module in the standard library, the default path entry finders also handle
loading all of these file types (other than shared libraries) from zipfiles.
Path entries need not be limited to file system locations. They can refer to
URLs, database queries, or any other location that can be specified as a
string.
The path based finder provides additional hooks and protocols so that you
can extend and customize the types of searchable path entries. For example,
if you wanted to support path entries as network URLs, you could write a hook
that implements HTTP semantics to find modules on the web. This hook (a
callable) would return a
path entry finder
supporting the protocol
described below, which was then used to get a loader for the module from the
web.
A word of warning: this section and the previous both use the term
finder
,
distinguishing between them by using the terms
meta path finder
and
path entry finder
. These two types of finders are very similar,
support similar protocols, and function in similar ways during the import
process, but it’s important to keep in mind that they are subtly different.
In particular, meta path finders operate at the beginning of the import
process, as keyed off the
sys.meta_path
traversal.
By contrast, path entry finders are in a sense an implementation detail
of the path based finder, and in fact, if the path based finder were to be
removed from
sys.meta_path
, none of the path entry finder semantics
would be invoked.
5.5.1.
Path entry finders
¶
The
path based finder
is responsible for finding and loading
Python modules and packages whose location is specified with a string
path entry
. Most path entries name locations in the file system,
but they need not be limited to this.
As a meta path finder, the
path based finder
implements the
find_spec()
protocol previously
described, however it exposes additional hooks that can be used to
customize how modules are found and loaded from the
import path
.
Three variables are used by the
path based finder
,
sys.path
,
sys.path_hooks
and
sys.path_importer_cache
. The
__path__
attributes on package objects are also used. These provide additional ways
that the import machinery can be customized.
sys.path
contains a list of strings providing search locations for
modules and packages. It is initialized from the
PYTHONPATH
environment variable and various other installation- and
implementation-specific defaults. Entries in
sys.path
can name
directories on the file system, zip files, and potentially other “locations”
(see the
site
module) that should be searched for modules, such as
URLs, or database queries. Only strings should be present on
sys.path
; all other data types are ignored.
The
path based finder
is a
meta path finder
, so the import
machinery begins the
import path
search by calling the path
based finder’s
find_spec()
method as
described previously. When the
path
argument to
find_spec()
is given, it will be a
list of string paths to traverse - typically a package’s
__path__
attribute for an import within that package. If the
path
argument is
None
, this indicates a top level import and
sys.path
is used.
The path based finder iterates over every entry in the search path, and
for each of these, looks for an appropriate
path entry finder
(
PathEntryFinder
) for the
path entry. Because this can be an expensive operation (e.g. there may be
stat()
call overheads for this search), the path based finder maintains
a cache mapping path entries to path entry finders. This cache is maintained
in
sys.path_importer_cache
(despite the name, this cache actually
stores finder objects rather than being limited to
importer
objects).
In this way, the expensive search for a particular
path entry
location’s
path entry finder
need only be done once. User code is
free to remove cache entries from
sys.path_importer_cache
forcing
the path based finder to perform the path entry search again.
If the path entry is not present in the cache, the path based finder iterates
over every callable in
sys.path_hooks
. Each of the
path entry
hooks
in this list is called with a single argument, the
path entry to be searched. This callable may either return a
path
entry finder
that can handle the path entry, or it may raise
ImportError
. An
ImportError
is used by the path based finder to
signal that the hook cannot find a
path entry finder
for that
path entry
. The
exception is ignored and
import path
iteration continues. The hook
should expect either a string or bytes object; the encoding of bytes objects
is up to the hook (e.g. it may be a file system encoding, UTF-8, or something
else), and if the hook cannot decode the argument, it should raise
ImportError
.
If
sys.path_hooks
iteration ends with no
path entry finder
being returned, then the path based finder’s
find_spec()
method will store
None
in
sys.path_importer_cache
(to indicate that there is no finder for
this path entry) and return
None
, indicating that this
meta path finder
could not find the module.
If a
path entry finder
is
returned by one of the
path entry
hook
callables on
sys.path_hooks
, then the following protocol is used
to ask the finder for a module spec, which is then used when loading the
module.
The current working directory – denoted by an empty string – is handled
slightly differently from other entries on
sys.path
. First, if the
current working directory cannot be determined or is found not to exist, no
value is stored in
sys.path_importer_cache
. Second, the value for the
current working directory is looked up fresh for each module lookup. Third,
the path used for
sys.path_importer_cache
and returned by
importlib.machinery.PathFinder.find_spec()
will be the actual current
working directory and not the empty string.
5.5.2.
Path entry finder protocol
¶
In order to support imports of modules and initialized packages and also to
contribute portions to namespace packages, path entry finders must implement
the
find_spec()
method.
find_spec()
takes two arguments: the
fully qualified name of the module being imported, and the (optional) target
module.
find_spec()
returns a fully populated spec for the module.
This spec will always have “loader” set (with one exception).
To indicate to the import machinery that the spec represents a namespace
portion
, the path entry finder sets
submodule_search_locations
to
a list containing the portion.
Changed in version 3.4:
find_spec()
replaced
find_loader()
and
find_module()
, both of which
are now deprecated, but will be used if
find_spec()
is not defined.
Older path entry finders may implement one of these two deprecated methods
instead of
find_spec()
. The methods are still respected for the
sake of backward compatibility. However, if
find_spec()
is
implemented on the path entry finder, the legacy methods are ignored.
find_loader()
takes one argument, the
fully qualified name of the module being imported.
find_loader()
returns a 2-tuple where the first item is the loader and the second item
is a namespace
portion
.
For backwards compatibility with other implementations of the import
protocol, many path entry finders also support the same,
traditional
find_module()
method that meta path finders support.
However path entry finder
find_module()
methods are never called
with a
path
argument (they are expected to record the appropriate
path information from the initial call to the path hook).
The
find_module()
method on path entry finders is deprecated,
as it does not allow the path entry finder to contribute portions to
namespace packages. If both
find_loader()
and
find_module()
exist on a path entry finder, the import system will always call
find_loader()
in preference to
find_module()
.
Changed in version 3.10:
Calls to
find_module()
and
find_loader()
by the import
system will raise
ImportWarning
.
Changed in version 3.12:
find_module()
and
find_loader()
have been removed.
5.6.
Replacing the standard import system
¶
The most reliable mechanism for replacing the entire import system is to
delete the default contents of
sys.meta_path
, replacing them
entirely with a custom meta path hook.
If it is acceptable to only alter the behaviour of import statements
without affecting other APIs that access the import system, then replacing
the builtin
__import__()
function may be sufficient.
To selectively prevent the import of some modules from a hook early on the
meta path (rather than disabling the standard import system entirely),
it is sufficient to raise
ModuleNotFoundError
directly from
find_spec()
instead of returning
None
. The latter indicates that the meta path search should continue,
while raising an exception terminates it immediately.
5.7.
Package Relative Imports
¶
Relative imports use leading dots. A single leading dot indicates a relative
import, starting with the current package. Two or more leading dots indicate a
relative import to the parent(s) of the current package, one level per dot
after the first. For example, given the following package layout:
package
/
__init__
.
py
subpackage1
/
__init__
.
py
moduleX
.
py
moduleY
.
py
subpackage2
/
__init__
.
py
moduleZ
.
py
moduleA
.
py
In either
subpackage1/moduleX.py
or
subpackage1/__init__.py
,
the following are valid relative imports:
from
.moduleY
import
spam
from
.moduleY
import
spam
as
ham
from
.
import
moduleY
from
..subpackage1
import
moduleY
from
..subpackage2.moduleZ
import
eggs
from
..moduleA
import
foo
Absolute imports may use either the
import
<>
or
from
<>
import
<>
syntax, but relative imports may only use the second form; the reason
for this is that:
import
XXX.YYY.ZZZ
should expose
XXX.YYY.ZZZ
as a usable expression, but .moduleY is
not a valid expression.
5.8.
Special considerations for __main__
¶
The
__main__
module is a special case relative to Python’s import
system. As noted
elsewhere
, the
__main__
module
is directly initialized at interpreter startup, much like
sys
and
builtins
. However, unlike those two, it doesn’t strictly
qualify as a built-in module. This is because the manner in which
__main__
is initialized depends on the flags and other options with
which the interpreter is invoked.
5.8.1.
__main__.__spec__
¶
Depending on how
__main__
is initialized,
__main__.__spec__
gets set appropriately or to
None
.
When Python is started with the
-m
option,
__spec__
is set
to the module spec of the corresponding module or package.
__spec__
is
also populated when the
__main__
module is loaded as part of executing a
directory, zipfile or other
sys.path
entry.
In
the remaining cases
__main__.__spec__
is set to
None
, as the code used to populate the
__main__
does not correspond directly with an importable module:
interactive prompt
-c
option
running from stdin
running directly from a source or bytecode file
Note that
__main__.__spec__
is always
None
in the last case,
even if
the file could technically be imported directly as a module
instead. Use the
-m
switch if valid module metadata is desired
in
__main__
.
Note also that even when
__main__
corresponds with an importable module
and
__main__.__spec__
is set accordingly, they’re still considered
distinct
modules. This is due to the fact that blocks guarded by
if
__name__
==
"__main__":
checks only execute when the module is used
to populate the
__main__
namespace, and not during normal import.
5.9.
References
¶
The import machinery has evolved considerably since Python’s early days. The
original
specification for packages
is still available to read,
although some details have changed since the writing of that document.
The original specification for
sys.meta_path
was
PEP 302
, with
subsequent extension in
PEP 420
.
PEP 420
introduced
namespace packages
for
Python 3.3.
PEP 420
also introduced the
find_loader()
protocol as an
alternative to
find_module()
.
PEP 366
describes the addition of the
__package__
attribute for
explicit relative imports in main modules.
PEP 328
introduced absolute and explicit relative imports and initially
proposed
__name__
for semantics
PEP 366
would eventually specify for
__package__
.
PEP 338
defines executing modules as scripts.
PEP 451
adds the encapsulation of per-module import state in spec
objects. It also off-loads most of the boilerplate responsibilities of
loaders back onto the import machinery. These changes allow the
deprecation of several APIs in the import system and also addition of new
methods to finders and loaders.
Footnotes |
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### [Table of Contents](https://docs.python.org/3/contents.html)
- [5\. The import system](https://docs.python.org/3/reference/import.html)
- [5\.1. `importlib`](https://docs.python.org/3/reference/import.html#importlib)
- [5\.2. Packages](https://docs.python.org/3/reference/import.html#packages)
- [5\.2.1. Regular packages](https://docs.python.org/3/reference/import.html#regular-packages)
- [5\.2.2. Namespace packages](https://docs.python.org/3/reference/import.html#namespace-packages)
- [5\.3. Searching](https://docs.python.org/3/reference/import.html#searching)
- [5\.3.1. The module cache](https://docs.python.org/3/reference/import.html#the-module-cache)
- [5\.3.2. Finders and loaders](https://docs.python.org/3/reference/import.html#finders-and-loaders)
- [5\.3.3. Import hooks](https://docs.python.org/3/reference/import.html#import-hooks)
- [5\.3.4. The meta path](https://docs.python.org/3/reference/import.html#the-meta-path)
- [5\.4. Loading](https://docs.python.org/3/reference/import.html#loading)
- [5\.4.1. Loaders](https://docs.python.org/3/reference/import.html#loaders)
- [5\.4.2. Submodules](https://docs.python.org/3/reference/import.html#submodules)
- [5\.4.3. Module specs](https://docs.python.org/3/reference/import.html#module-specs)
- [5\.4.4. \_\_path\_\_ attributes on modules](https://docs.python.org/3/reference/import.html#path-attributes-on-modules)
- [5\.4.5. Module reprs](https://docs.python.org/3/reference/import.html#module-reprs)
- [5\.4.6. Cached bytecode invalidation](https://docs.python.org/3/reference/import.html#cached-bytecode-invalidation)
- [5\.5. The Path Based Finder](https://docs.python.org/3/reference/import.html#the-path-based-finder)
- [5\.5.1. Path entry finders](https://docs.python.org/3/reference/import.html#path-entry-finders)
- [5\.5.2. Path entry finder protocol](https://docs.python.org/3/reference/import.html#path-entry-finder-protocol)
- [5\.6. Replacing the standard import system](https://docs.python.org/3/reference/import.html#replacing-the-standard-import-system)
- [5\.7. Package Relative Imports](https://docs.python.org/3/reference/import.html#package-relative-imports)
- [5\.8. Special considerations for \_\_main\_\_](https://docs.python.org/3/reference/import.html#special-considerations-for-main)
- [5\.8.1. \_\_main\_\_.\_\_spec\_\_](https://docs.python.org/3/reference/import.html#main-spec)
- [5\.9. References](https://docs.python.org/3/reference/import.html#references)
#### Previous topic
[4\. Execution model](https://docs.python.org/3/reference/executionmodel.html "previous chapter")
#### Next topic
[6\. Expressions](https://docs.python.org/3/reference/expressions.html "next chapter")
### This page
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# 5\. The import system[¶](https://docs.python.org/3/reference/import.html#the-import-system "Link to this heading")
Python code in one [module](https://docs.python.org/3/glossary.html#term-module) gains access to the code in another module by the process of [importing](https://docs.python.org/3/glossary.html#term-importing) it. The [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement is the most common way of invoking the import machinery, but it is not the only way. Functions such as [`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module") and built-in [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") can also be used to invoke the import machinery.
The [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement combines two operations; it searches for the named module, then it binds the results of that search to a name in the local scope. The search operation of the `import` statement is defined as a call to the [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") function, with the appropriate arguments. The return value of `__import__()` is used to perform the name binding operation of the `import` statement. See the `import` statement for the exact details of that name binding operation.
A direct call to [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") performs only the module search and, if found, the module creation operation. While certain side-effects may occur, such as the importing of parent packages, and the updating of various caches (including [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules")), only the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement performs a name binding operation.
When an [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement is executed, the standard builtin [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") function is called. Other mechanisms for invoking the import system (such as [`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module")) may choose to bypass `__import__()` and use their own solutions to implement import semantics.
When a module is first imported, Python searches for the module and if found, it creates a module object [\[1\]](https://docs.python.org/3/reference/import.html#fnmo), initializing it. If the named module cannot be found, a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") is raised. Python implements various strategies to search for the named module when the import machinery is invoked. These strategies can be modified and extended by using various hooks described in the sections below.
Changed in version 3.3: The import system has been updated to fully implement the second phase of [**PEP 302**](https://peps.python.org/pep-0302/). There is no longer any implicit import machinery - the full import system is exposed through [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path"). In addition, native namespace package support has been implemented (see [**PEP 420**](https://peps.python.org/pep-0420/)).
## 5\.1. [`importlib`](https://docs.python.org/3/library/importlib.html#module-importlib "importlib: The implementation of the import machinery.")[¶](https://docs.python.org/3/reference/import.html#importlib "Link to this heading")
The [`importlib`](https://docs.python.org/3/library/importlib.html#module-importlib "importlib: The implementation of the import machinery.") module provides a rich API for interacting with the import system. For example [`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module") provides a recommended, simpler API than built-in [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") for invoking the import machinery. Refer to the `importlib` library documentation for additional detail.
## 5\.2. Packages[¶](https://docs.python.org/3/reference/import.html#packages "Link to this heading")
Python has only one type of module object, and all modules are of this type, regardless of whether the module is implemented in Python, C, or something else. To help organize modules and provide a naming hierarchy, Python has a concept of [packages](https://docs.python.org/3/glossary.html#term-package).
You can think of packages as the directories on a file system and modules as files within directories, but don’t take this analogy too literally since packages and modules need not originate from the file system. For the purposes of this documentation, we’ll use this convenient analogy of directories and files. Like file system directories, packages are organized hierarchically, and packages may themselves contain subpackages, as well as regular modules.
It’s important to keep in mind that all packages are modules, but not all modules are packages. Or put another way, packages are just a special kind of module. Specifically, any module that contains a `__path__` attribute is considered a package.
All modules have a name. Subpackage names are separated from their parent package name by a dot, akin to Python’s standard attribute access syntax. Thus you might have a package called [`email`](https://docs.python.org/3/library/email.html#module-email "email: Package supporting the parsing, manipulating, and generating email messages."), which in turn has a subpackage called [`email.mime`](https://docs.python.org/3/library/email.mime.html#module-email.mime "email.mime: Build MIME messages.") and a module within that subpackage called [`email.mime.text`](https://docs.python.org/3/library/email.mime.html#module-email.mime.text "email.mime.text").
### 5\.2.1. Regular packages[¶](https://docs.python.org/3/reference/import.html#regular-packages "Link to this heading")
Python defines two types of packages, [regular packages](https://docs.python.org/3/glossary.html#term-regular-package) and [namespace packages](https://docs.python.org/3/glossary.html#term-namespace-package). Regular packages are traditional packages as they existed in Python 3.2 and earlier. A regular package is typically implemented as a directory containing an `__init__.py` file. When a regular package is imported, this `__init__.py` file is implicitly executed, and the objects it defines are bound to names in the package’s namespace. The `__init__.py` file can contain the same Python code that any other module can contain, and Python will add some additional attributes to the module when it is imported.
For example, the following file system layout defines a top level `parent` package with three subpackages:
Copy
```
parent/
__init__.py
one/
__init__.py
two/
__init__.py
three/
__init__.py
```
Importing `parent.one` will implicitly execute `parent/__init__.py` and `parent/one/__init__.py`. Subsequent imports of `parent.two` or `parent.three` will execute `parent/two/__init__.py` and `parent/three/__init__.py` respectively.
### 5\.2.2. Namespace packages[¶](https://docs.python.org/3/reference/import.html#namespace-packages "Link to this heading")
A namespace package is a composite of various [portions](https://docs.python.org/3/glossary.html#term-portion), where each portion contributes a subpackage to the parent package. Portions may reside in different locations on the file system. Portions may also be found in zip files, on the network, or anywhere else that Python searches during import. Namespace packages may or may not correspond directly to objects on the file system; they may be virtual modules that have no concrete representation.
Namespace packages do not use an ordinary list for their `__path__` attribute. They instead use a custom iterable type which will automatically perform a new search for package portions on the next import attempt within that package if the path of their parent package (or [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") for a top level package) changes.
With namespace packages, there is no `parent/__init__.py` file. In fact, there may be multiple `parent` directories found during import search, where each one is provided by a different portion. Thus `parent/one` may not be physically located next to `parent/two`. In this case, Python will create a namespace package for the top-level `parent` package whenever it or one of its subpackages is imported.
See also [**PEP 420**](https://peps.python.org/pep-0420/) for the namespace package specification.
## 5\.3. Searching[¶](https://docs.python.org/3/reference/import.html#searching "Link to this heading")
To begin the search, Python needs the [fully qualified](https://docs.python.org/3/glossary.html#term-qualified-name) name of the module (or package, but for the purposes of this discussion, the difference is immaterial) being imported. This name may come from various arguments to the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement, or from the parameters to the [`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module") or [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") functions.
This name will be used in various phases of the import search, and it may be the dotted path to a submodule, e.g. `foo.bar.baz`. In this case, Python first tries to import `foo`, then `foo.bar`, and finally `foo.bar.baz`. If any of the intermediate imports fail, a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") is raised.
### 5\.3.1. The module cache[¶](https://docs.python.org/3/reference/import.html#the-module-cache "Link to this heading")
The first place checked during import search is [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"). This mapping serves as a cache of all modules that have been previously imported, including the intermediate paths. So if `foo.bar.baz` was previously imported, `sys.modules` will contain entries for `foo`, `foo.bar`, and `foo.bar.baz`. Each key will have as its value the corresponding module object.
During import, the module name is looked up in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules") and if present, the associated value is the module satisfying the import, and the process completes. However, if the value is `None`, then a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") is raised. If the module name is missing, Python will continue searching for the module.
[`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules") is writable. Deleting a key may not destroy the associated module (as other modules may hold references to it), but it will invalidate the cache entry for the named module, causing Python to search anew for the named module upon its next import. The key can also be assigned to `None`, forcing the next import of the module to result in a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError").
Beware though, as if you keep a reference to the module object, invalidate its cache entry in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), and then re-import the named module, the two module objects will *not* be the same. By contrast, [`importlib.reload()`](https://docs.python.org/3/library/importlib.html#importlib.reload "importlib.reload") will reuse the *same* module object, and simply reinitialise the module contents by rerunning the module’s code.
### 5\.3.2. Finders and loaders[¶](https://docs.python.org/3/reference/import.html#finders-and-loaders "Link to this heading")
If the named module is not found in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), then Python’s import protocol is invoked to find and load the module. This protocol consists of two conceptual objects, [finders](https://docs.python.org/3/glossary.html#term-finder) and [loaders](https://docs.python.org/3/glossary.html#term-loader). A finder’s job is to determine whether it can find the named module using whatever strategy it knows about. Objects that implement both of these interfaces are referred to as [importers](https://docs.python.org/3/glossary.html#term-importer) - they return themselves when they find that they can load the requested module.
Python includes a number of default finders and importers. The first one knows how to locate built-in modules, and the second knows how to locate frozen modules. A third default finder searches an [import path](https://docs.python.org/3/glossary.html#term-import-path) for modules. The import path is a list of locations that may name file system paths or zip files. It can also be extended to search for any locatable resource, such as those identified by URLs.
The import machinery is extensible, so new finders can be added to extend the range and scope of module searching.
Finders do not actually load modules. If they can find the named module, they return a *module spec*, an encapsulation of the module’s import-related information, which the import machinery then uses when loading the module.
The following sections describe the protocol for finders and loaders in more detail, including how you can create and register new ones to extend the import machinery.
Changed in version 3.4: In previous versions of Python, finders returned [loaders](https://docs.python.org/3/glossary.html#term-loader) directly, whereas now they return module specs which *contain* loaders. Loaders are still used during import but have fewer responsibilities.
### 5\.3.3. Import hooks[¶](https://docs.python.org/3/reference/import.html#import-hooks "Link to this heading")
The import machinery is designed to be extensible; the primary mechanism for this are the *import hooks*. There are two types of import hooks: *meta hooks* and *import path hooks*.
Meta hooks are called at the start of import processing, before any other import processing has occurred, other than [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules") cache look up. This allows meta hooks to override [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") processing, frozen modules, or even built-in modules. Meta hooks are registered by adding new finder objects to [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path"), as described below.
Import path hooks are called as part of [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") (or `package.__path__`) processing, at the point where their associated path item is encountered. Import path hooks are registered by adding new callables to [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks") as described below.
### 5\.3.4. The meta path[¶](https://docs.python.org/3/reference/import.html#the-meta-path "Link to this heading")
When the named module is not found in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), Python next searches [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path"), which contains a list of meta path finder objects. These finders are queried in order to see if they know how to handle the named module. Meta path finders must implement a method called [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") which takes three arguments: a name, an import path, and (optionally) a target module. The meta path finder can use any strategy it wants to determine whether it can handle the named module or not.
If the meta path finder knows how to handle the named module, it returns a spec object. If it cannot handle the named module, it returns `None`. If [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path") processing reaches the end of its list without returning a spec, then a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") is raised. Any other exceptions raised are simply propagated up, aborting the import process.
The [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") method of meta path finders is called with two or three arguments. The first is the fully qualified name of the module being imported, for example `foo.bar.baz`. The second argument is the path entries to use for the module search. For top-level modules, the second argument is `None`, but for submodules or subpackages, the second argument is the value of the parent package’s `__path__` attribute. If the appropriate `__path__` attribute cannot be accessed, a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") is raised. The third argument is an existing module object that will be the target of loading later. The import system passes in a target module only during reload.
The meta path may be traversed multiple times for a single import request. For example, assuming none of the modules involved has already been cached, importing `foo.bar.baz` will first perform a top level import, calling `mpf.find_spec("foo", None, None)` on each meta path finder (`mpf`). After `foo` has been imported, `foo.bar` will be imported by traversing the meta path a second time, calling `mpf.find_spec("foo.bar", foo.__path__, None)`. Once `foo.bar` has been imported, the final traversal will call `mpf.find_spec("foo.bar.baz", foo.bar.__path__, None)`.
Some meta path finders only support top level imports. These importers will always return `None` when anything other than `None` is passed as the second argument.
Python’s default [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path") has three meta path finders, one that knows how to import built-in modules, one that knows how to import frozen modules, and one that knows how to import modules from an [import path](https://docs.python.org/3/glossary.html#term-import-path) (i.e. the [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder)).
Changed in version 3.4: The [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") method of meta path finders replaced `find_module()`, which is now deprecated. While it will continue to work without change, the import machinery will try it only if the finder does not implement `find_spec()`.
Changed in version 3.10: Use of `find_module()` by the import system now raises [`ImportWarning`](https://docs.python.org/3/library/exceptions.html#ImportWarning "ImportWarning").
Changed in version 3.12: `find_module()` has been removed. Use [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") instead.
## 5\.4. Loading[¶](https://docs.python.org/3/reference/import.html#loading "Link to this heading")
If and when a module spec is found, the import machinery will use it (and the loader it contains) when loading the module. Here is an approximation of what happens during the loading portion of import:
Copy
```
module = None
if spec.loader is not None and hasattr(spec.loader, 'create_module'):
# It is assumed 'exec_module' will also be defined on the loader.
module = spec.loader.create_module(spec)
if module is None:
module = ModuleType(spec.name)
# The import-related module attributes get set here:
_init_module_attrs(spec, module)
if spec.loader is None:
# unsupported
raise ImportError
if spec.origin is None and spec.submodule_search_locations is not None:
# namespace package
sys.modules[spec.name] = module
elif not hasattr(spec.loader, 'exec_module'):
module = spec.loader.load_module(spec.name)
else:
sys.modules[spec.name] = module
try:
spec.loader.exec_module(module)
except BaseException:
try:
del sys.modules[spec.name]
except KeyError:
pass
raise
return sys.modules[spec.name]
```
Note the following details:
- If there is an existing module object with the given name in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), import will have already returned it.
- The module will exist in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules") before the loader executes the module code. This is crucial because the module code may (directly or indirectly) import itself; adding it to `sys.modules` beforehand prevents unbounded recursion in the worst case and multiple loading in the best.
- If loading fails, the failing module – and only the failing module – gets removed from [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"). Any module already in the `sys.modules` cache, and any module that was successfully loaded as a side-effect, must remain in the cache. This contrasts with reloading where even the failing module is left in `sys.modules`.
- After the module is created but before execution, the import machinery sets the import-related module attributes (“\_init\_module\_attrs” in the pseudo-code example above), as summarized in a [later section](https://docs.python.org/3/reference/datamodel.html#import-mod-attrs).
- Module execution is the key moment of loading in which the module’s namespace gets populated. Execution is entirely delegated to the loader, which gets to decide what gets populated and how.
- The module created during loading and passed to exec\_module() may not be the one returned at the end of import [\[2\]](https://docs.python.org/3/reference/import.html#fnlo).
Changed in version 3.4: The import system has taken over the boilerplate responsibilities of loaders. These were previously performed by the [`importlib.abc.Loader.load_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.load_module "importlib.abc.Loader.load_module") method.
### 5\.4.1. Loaders[¶](https://docs.python.org/3/reference/import.html#loaders "Link to this heading")
Module loaders provide the critical function of loading: module execution. The import machinery calls the [`importlib.abc.Loader.exec_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.exec_module "importlib.abc.Loader.exec_module") method with a single argument, the module object to execute. Any value returned from `exec_module()` is ignored.
Loaders must satisfy the following requirements:
- If the module is a Python module (as opposed to a built-in module or a dynamically loaded extension), the loader should execute the module’s code in the module’s global name space (`module.__dict__`).
- If the loader cannot execute the module, it should raise an [`ImportError`](https://docs.python.org/3/library/exceptions.html#ImportError "ImportError"), although any other exception raised during [`exec_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.exec_module "importlib.abc.Loader.exec_module") will be propagated.
In many cases, the finder and loader can be the same object; in such cases the [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") method would just return a spec with the loader set to `self`.
Module loaders may opt in to creating the module object during loading by implementing a [`create_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.create_module "importlib.abc.Loader.create_module") method. It takes one argument, the module spec, and returns the new module object to use during loading. `create_module()` does not need to set any attributes on the module object. If the method returns `None`, the import machinery will create the new module itself.
Added in version 3.4: The [`create_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.create_module "importlib.abc.Loader.create_module") method of loaders.
Changed in version 3.4: The [`load_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.load_module "importlib.abc.Loader.load_module") method was replaced by [`exec_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.exec_module "importlib.abc.Loader.exec_module") and the import machinery assumed all the boilerplate responsibilities of loading.
For compatibility with existing loaders, the import machinery will use the `load_module()` method of loaders if it exists and the loader does not also implement `exec_module()`. However, `load_module()` has been deprecated and loaders should implement `exec_module()` instead.
The `load_module()` method must implement all the boilerplate loading functionality described above in addition to executing the module. All the same constraints apply, with some additional clarification:
- If there is an existing module object with the given name in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), the loader must use that existing module. (Otherwise, [`importlib.reload()`](https://docs.python.org/3/library/importlib.html#importlib.reload "importlib.reload") will not work correctly.) If the named module does not exist in `sys.modules`, the loader must create a new module object and add it to `sys.modules`.
- The module *must* exist in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules") before the loader executes the module code, to prevent unbounded recursion or multiple loading.
- If loading fails, the loader must remove any modules it has inserted into [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), but it must remove **only** the failing module(s), and only if the loader itself has loaded the module(s) explicitly.
Changed in version 3.5: A [`DeprecationWarning`](https://docs.python.org/3/library/exceptions.html#DeprecationWarning "DeprecationWarning") is raised when `exec_module()` is defined but `create_module()` is not.
Changed in version 3.6: An [`ImportError`](https://docs.python.org/3/library/exceptions.html#ImportError "ImportError") is raised when `exec_module()` is defined but `create_module()` is not.
Changed in version 3.10: Use of `load_module()` will raise [`ImportWarning`](https://docs.python.org/3/library/exceptions.html#ImportWarning "ImportWarning").
### 5\.4.2. Submodules[¶](https://docs.python.org/3/reference/import.html#submodules "Link to this heading")
When a submodule is loaded using any mechanism (e.g. `importlib` APIs, the `import` or `import-from` statements, or built-in `__import__()`) a binding is placed in the parent module’s namespace to the submodule object. For example, if package `spam` has a submodule `foo`, after importing `spam.foo`, `spam` will have an attribute `foo` which is bound to the submodule. Let’s say you have the following directory structure:
Copy
```
spam/
__init__.py
foo.py
```
and `spam/__init__.py` has the following line in it:
Copy
```
from .foo import Foo
```
then executing the following puts name bindings for `foo` and `Foo` in the `spam` module:
Copy
```
>>> import spam
>>> spam.foo
<module 'spam.foo' from '/tmp/imports/spam/foo.py'>
>>> spam.Foo
<class 'spam.foo.Foo'>
```
Given Python’s familiar name binding rules this might seem surprising, but it’s actually a fundamental feature of the import system. The invariant holding is that if you have `sys.modules['spam']` and `sys.modules['spam.foo']` (as you would after the above import), the latter must appear as the `foo` attribute of the former.
### 5\.4.3. Module specs[¶](https://docs.python.org/3/reference/import.html#module-specs "Link to this heading")
The import machinery uses a variety of information about each module during import, especially before loading. Most of the information is common to all modules. The purpose of a module’s spec is to encapsulate this import-related information on a per-module basis.
Using a spec during import allows state to be transferred between import system components, e.g. between the finder that creates the module spec and the loader that executes it. Most importantly, it allows the import machinery to perform the boilerplate operations of loading, whereas without a module spec the loader had that responsibility.
The module’s spec is exposed as [`module.__spec__`](https://docs.python.org/3/reference/datamodel.html#module.__spec__ "module.__spec__"). Setting `__spec__` appropriately applies equally to [modules initialized during interpreter startup](https://docs.python.org/3/reference/toplevel_components.html#programs). The one exception is `__main__`, where `__spec__` is [set to None in some cases](https://docs.python.org/3/reference/import.html#main-spec).
See [`ModuleSpec`](https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec "importlib.machinery.ModuleSpec") for details on the contents of the module spec.
Added in version 3.4.
### 5\.4.4. \_\_path\_\_ attributes on modules[¶](https://docs.python.org/3/reference/import.html#path-attributes-on-modules "Link to this heading")
The [`__path__`](https://docs.python.org/3/reference/datamodel.html#module.__path__ "module.__path__") attribute should be a (possibly empty) [sequence](https://docs.python.org/3/glossary.html#term-sequence) of strings enumerating the locations where the package’s submodules will be found. By definition, if a module has a `__path__` attribute, it is a [package](https://docs.python.org/3/glossary.html#term-package).
A package’s [`__path__`](https://docs.python.org/3/reference/datamodel.html#module.__path__ "module.__path__") attribute is used during imports of its subpackages. Within the import machinery, it functions much the same as [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path"), i.e. providing a list of locations to search for modules during import. However, `__path__` is typically much more constrained than `sys.path`.
The same rules used for [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") also apply to a package’s `__path__`. [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks") (described below) are consulted when traversing a package’s `__path__`.
A package’s `__init__.py` file may set or alter the package’s [`__path__`](https://docs.python.org/3/reference/datamodel.html#module.__path__ "module.__path__") attribute, and this was typically the way namespace packages were implemented prior to [**PEP 420**](https://peps.python.org/pep-0420/). With the adoption of [**PEP 420**](https://peps.python.org/pep-0420/), namespace packages no longer need to supply `__init__.py` files containing only `__path__` manipulation code; the import machinery automatically sets `__path__` correctly for the namespace package.
### 5\.4.5. Module reprs[¶](https://docs.python.org/3/reference/import.html#module-reprs "Link to this heading")
By default, all modules have a usable repr, however depending on the attributes set above, and in the module’s spec, you can more explicitly control the repr of module objects.
If the module has a spec (`__spec__`), the import machinery will try to generate a repr from it. If that fails or there is no spec, the import system will craft a default repr using whatever information is available on the module. It will try to use the `module.__name__`, `module.__file__`, and `module.__loader__` as input into the repr, with defaults for whatever information is missing.
Here are the exact rules used:
- If the module has a `__spec__` attribute, the information in the spec is used to generate the repr. The “name”, “loader”, “origin”, and “has\_location” attributes are consulted.
- If the module has a `__file__` attribute, this is used as part of the module’s repr.
- If the module has no `__file__` but does have a `__loader__` that is not `None`, then the loader’s repr is used as part of the module’s repr.
- Otherwise, just use the module’s `__name__` in the repr.
Changed in version 3.12: Use of `module_repr()`, having been deprecated since Python 3.4, was removed in Python 3.12 and is no longer called during the resolution of a module’s repr.
### 5\.4.6. Cached bytecode invalidation[¶](https://docs.python.org/3/reference/import.html#cached-bytecode-invalidation "Link to this heading")
Before Python loads cached bytecode from a `.pyc` file, it checks whether the cache is up-to-date with the source `.py` file. By default, Python does this by storing the source’s last-modified timestamp and size in the cache file when writing it. At runtime, the import system then validates the cache file by checking the stored metadata in the cache file against the source’s metadata.
Python also supports “hash-based” cache files, which store a hash of the source file’s contents rather than its metadata. There are two variants of hash-based `.pyc` files: checked and unchecked. For checked hash-based `.pyc` files, Python validates the cache file by hashing the source file and comparing the resulting hash with the hash in the cache file. If a checked hash-based cache file is found to be invalid, Python regenerates it and writes a new checked hash-based cache file. For unchecked hash-based `.pyc` files, Python simply assumes the cache file is valid if it exists. Hash-based `.pyc` files validation behavior may be overridden with the [`--check-hash-based-pycs`](https://docs.python.org/3/using/cmdline.html#cmdoption-check-hash-based-pycs) flag.
Changed in version 3.7: Added hash-based `.pyc` files. Previously, Python only supported timestamp-based invalidation of bytecode caches.
## 5\.5. The Path Based Finder[¶](https://docs.python.org/3/reference/import.html#the-path-based-finder "Link to this heading")
As mentioned previously, Python comes with several default meta path finders. One of these, called the [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder) ([`PathFinder`](https://docs.python.org/3/library/importlib.html#importlib.machinery.PathFinder "importlib.machinery.PathFinder")), searches an [import path](https://docs.python.org/3/glossary.html#term-import-path), which contains a list of [path entries](https://docs.python.org/3/glossary.html#term-path-entry). Each path entry names a location to search for modules.
The path based finder itself doesn’t know how to import anything. Instead, it traverses the individual path entries, associating each of them with a path entry finder that knows how to handle that particular kind of path.
The default set of path entry finders implement all the semantics for finding modules on the file system, handling special file types such as Python source code (`.py` files), Python byte code (`.pyc` files) and shared libraries (e.g. `.so` files). When supported by the [`zipimport`](https://docs.python.org/3/library/zipimport.html#module-zipimport "zipimport: Support for importing Python modules from ZIP archives.") module in the standard library, the default path entry finders also handle loading all of these file types (other than shared libraries) from zipfiles.
Path entries need not be limited to file system locations. They can refer to URLs, database queries, or any other location that can be specified as a string.
The path based finder provides additional hooks and protocols so that you can extend and customize the types of searchable path entries. For example, if you wanted to support path entries as network URLs, you could write a hook that implements HTTP semantics to find modules on the web. This hook (a callable) would return a [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder) supporting the protocol described below, which was then used to get a loader for the module from the web.
A word of warning: this section and the previous both use the term *finder*, distinguishing between them by using the terms [meta path finder](https://docs.python.org/3/glossary.html#term-meta-path-finder) and [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder). These two types of finders are very similar, support similar protocols, and function in similar ways during the import process, but it’s important to keep in mind that they are subtly different. In particular, meta path finders operate at the beginning of the import process, as keyed off the [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path") traversal.
By contrast, path entry finders are in a sense an implementation detail of the path based finder, and in fact, if the path based finder were to be removed from [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path"), none of the path entry finder semantics would be invoked.
### 5\.5.1. Path entry finders[¶](https://docs.python.org/3/reference/import.html#path-entry-finders "Link to this heading")
The [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder) is responsible for finding and loading Python modules and packages whose location is specified with a string [path entry](https://docs.python.org/3/glossary.html#term-path-entry). Most path entries name locations in the file system, but they need not be limited to this.
As a meta path finder, the [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder) implements the [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") protocol previously described, however it exposes additional hooks that can be used to customize how modules are found and loaded from the [import path](https://docs.python.org/3/glossary.html#term-import-path).
Three variables are used by the [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder), [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path"), [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks") and [`sys.path_importer_cache`](https://docs.python.org/3/library/sys.html#sys.path_importer_cache "sys.path_importer_cache"). The `__path__` attributes on package objects are also used. These provide additional ways that the import machinery can be customized.
[`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") contains a list of strings providing search locations for modules and packages. It is initialized from the [`PYTHONPATH`](https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPATH) environment variable and various other installation- and implementation-specific defaults. Entries in `sys.path` can name directories on the file system, zip files, and potentially other “locations” (see the [`site`](https://docs.python.org/3/library/site.html#module-site "site: Module responsible for site-specific configuration.") module) that should be searched for modules, such as URLs, or database queries. Only strings should be present on `sys.path`; all other data types are ignored.
The [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder) is a [meta path finder](https://docs.python.org/3/glossary.html#term-meta-path-finder), so the import machinery begins the [import path](https://docs.python.org/3/glossary.html#term-import-path) search by calling the path based finder’s [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.machinery.PathFinder.find_spec "importlib.machinery.PathFinder.find_spec") method as described previously. When the `path` argument to `find_spec()` is given, it will be a list of string paths to traverse - typically a package’s `__path__` attribute for an import within that package. If the `path` argument is `None`, this indicates a top level import and [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") is used.
The path based finder iterates over every entry in the search path, and for each of these, looks for an appropriate [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder) ([`PathEntryFinder`](https://docs.python.org/3/library/importlib.html#importlib.abc.PathEntryFinder "importlib.abc.PathEntryFinder")) for the path entry. Because this can be an expensive operation (e.g. there may be `stat()` call overheads for this search), the path based finder maintains a cache mapping path entries to path entry finders. This cache is maintained in [`sys.path_importer_cache`](https://docs.python.org/3/library/sys.html#sys.path_importer_cache "sys.path_importer_cache") (despite the name, this cache actually stores finder objects rather than being limited to [importer](https://docs.python.org/3/glossary.html#term-importer) objects). In this way, the expensive search for a particular [path entry](https://docs.python.org/3/glossary.html#term-path-entry) location’s path entry finder need only be done once. User code is free to remove cache entries from `sys.path_importer_cache` forcing the path based finder to perform the path entry search again.
If the path entry is not present in the cache, the path based finder iterates over every callable in [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks"). Each of the [path entry hooks](https://docs.python.org/3/glossary.html#term-path-entry-hook) in this list is called with a single argument, the path entry to be searched. This callable may either return a [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder) that can handle the path entry, or it may raise [`ImportError`](https://docs.python.org/3/library/exceptions.html#ImportError "ImportError"). An `ImportError` is used by the path based finder to signal that the hook cannot find a path entry finder for that [path entry](https://docs.python.org/3/glossary.html#term-path-entry). The exception is ignored and [import path](https://docs.python.org/3/glossary.html#term-import-path) iteration continues. The hook should expect either a string or bytes object; the encoding of bytes objects is up to the hook (e.g. it may be a file system encoding, UTF-8, or something else), and if the hook cannot decode the argument, it should raise `ImportError`.
If [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks") iteration ends with no [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder) being returned, then the path based finder’s [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.machinery.PathFinder.find_spec "importlib.machinery.PathFinder.find_spec") method will store `None` in [`sys.path_importer_cache`](https://docs.python.org/3/library/sys.html#sys.path_importer_cache "sys.path_importer_cache") (to indicate that there is no finder for this path entry) and return `None`, indicating that this [meta path finder](https://docs.python.org/3/glossary.html#term-meta-path-finder) could not find the module.
If a [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder) *is* returned by one of the [path entry hook](https://docs.python.org/3/glossary.html#term-path-entry-hook) callables on [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks"), then the following protocol is used to ask the finder for a module spec, which is then used when loading the module.
The current working directory – denoted by an empty string – is handled slightly differently from other entries on [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path"). First, if the current working directory cannot be determined or is found not to exist, no value is stored in [`sys.path_importer_cache`](https://docs.python.org/3/library/sys.html#sys.path_importer_cache "sys.path_importer_cache"). Second, the value for the current working directory is looked up fresh for each module lookup. Third, the path used for `sys.path_importer_cache` and returned by [`importlib.machinery.PathFinder.find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.machinery.PathFinder.find_spec "importlib.machinery.PathFinder.find_spec") will be the actual current working directory and not the empty string.
### 5\.5.2. Path entry finder protocol[¶](https://docs.python.org/3/reference/import.html#path-entry-finder-protocol "Link to this heading")
In order to support imports of modules and initialized packages and also to contribute portions to namespace packages, path entry finders must implement the [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.PathEntryFinder.find_spec "importlib.abc.PathEntryFinder.find_spec") method.
[`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.PathEntryFinder.find_spec "importlib.abc.PathEntryFinder.find_spec") takes two arguments: the fully qualified name of the module being imported, and the (optional) target module. `find_spec()` returns a fully populated spec for the module. This spec will always have “loader” set (with one exception).
To indicate to the import machinery that the spec represents a namespace [portion](https://docs.python.org/3/glossary.html#term-portion), the path entry finder sets `submodule_search_locations` to a list containing the portion.
Changed in version 3.4: [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.PathEntryFinder.find_spec "importlib.abc.PathEntryFinder.find_spec") replaced `find_loader()` and `find_module()`, both of which are now deprecated, but will be used if `find_spec()` is not defined.
Older path entry finders may implement one of these two deprecated methods instead of `find_spec()`. The methods are still respected for the sake of backward compatibility. However, if `find_spec()` is implemented on the path entry finder, the legacy methods are ignored.
`find_loader()` takes one argument, the fully qualified name of the module being imported. `find_loader()` returns a 2-tuple where the first item is the loader and the second item is a namespace [portion](https://docs.python.org/3/glossary.html#term-portion).
For backwards compatibility with other implementations of the import protocol, many path entry finders also support the same, traditional `find_module()` method that meta path finders support. However path entry finder `find_module()` methods are never called with a `path` argument (they are expected to record the appropriate path information from the initial call to the path hook).
The `find_module()` method on path entry finders is deprecated, as it does not allow the path entry finder to contribute portions to namespace packages. If both `find_loader()` and `find_module()` exist on a path entry finder, the import system will always call `find_loader()` in preference to `find_module()`.
Changed in version 3.10: Calls to `find_module()` and `find_loader()` by the import system will raise [`ImportWarning`](https://docs.python.org/3/library/exceptions.html#ImportWarning "ImportWarning").
Changed in version 3.12: `find_module()` and `find_loader()` have been removed.
## 5\.6. Replacing the standard import system[¶](https://docs.python.org/3/reference/import.html#replacing-the-standard-import-system "Link to this heading")
The most reliable mechanism for replacing the entire import system is to delete the default contents of [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path"), replacing them entirely with a custom meta path hook.
If it is acceptable to only alter the behaviour of import statements without affecting other APIs that access the import system, then replacing the builtin [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") function may be sufficient.
To selectively prevent the import of some modules from a hook early on the meta path (rather than disabling the standard import system entirely), it is sufficient to raise [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") directly from [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") instead of returning `None`. The latter indicates that the meta path search should continue, while raising an exception terminates it immediately.
## 5\.7. Package Relative Imports[¶](https://docs.python.org/3/reference/import.html#package-relative-imports "Link to this heading")
Relative imports use leading dots. A single leading dot indicates a relative import, starting with the current package. Two or more leading dots indicate a relative import to the parent(s) of the current package, one level per dot after the first. For example, given the following package layout:
Copy
```
package/
__init__.py
subpackage1/
__init__.py
moduleX.py
moduleY.py
subpackage2/
__init__.py
moduleZ.py
moduleA.py
```
In either `subpackage1/moduleX.py` or `subpackage1/__init__.py`, the following are valid relative imports:
Copy
```
from .moduleY import spam
from .moduleY import spam as ham
from . import moduleY
from ..subpackage1 import moduleY
from ..subpackage2.moduleZ import eggs
from ..moduleA import foo
```
Absolute imports may use either the `import <>` or `from <> import <>` syntax, but relative imports may only use the second form; the reason for this is that:
Copy
```
import XXX.YYY.ZZZ
```
should expose `XXX.YYY.ZZZ` as a usable expression, but .moduleY is not a valid expression.
## 5\.8. Special considerations for \_\_main\_\_[¶](https://docs.python.org/3/reference/import.html#special-considerations-for-main "Link to this heading")
The [`__main__`](https://docs.python.org/3/library/__main__.html#module-__main__ "__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``.") module is a special case relative to Python’s import system. As noted [elsewhere](https://docs.python.org/3/reference/toplevel_components.html#programs), the `__main__` module is directly initialized at interpreter startup, much like [`sys`](https://docs.python.org/3/library/sys.html#module-sys "sys: Access system-specific parameters and functions.") and [`builtins`](https://docs.python.org/3/library/builtins.html#module-builtins "builtins: The module that provides the built-in namespace."). However, unlike those two, it doesn’t strictly qualify as a built-in module. This is because the manner in which `__main__` is initialized depends on the flags and other options with which the interpreter is invoked.
### 5\.8.1. \_\_main\_\_.\_\_spec\_\_[¶](https://docs.python.org/3/reference/import.html#main-spec "Link to this heading")
Depending on how [`__main__`](https://docs.python.org/3/library/__main__.html#module-__main__ "__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``.") is initialized, `__main__.__spec__` gets set appropriately or to `None`.
When Python is started with the [`-m`](https://docs.python.org/3/using/cmdline.html#cmdoption-m) option, `__spec__` is set to the module spec of the corresponding module or package. `__spec__` is also populated when the `__main__` module is loaded as part of executing a directory, zipfile or other [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") entry.
In [the remaining cases](https://docs.python.org/3/using/cmdline.html#using-on-interface-options) `__main__.__spec__` is set to `None`, as the code used to populate the [`__main__`](https://docs.python.org/3/library/__main__.html#module-__main__ "__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``.") does not correspond directly with an importable module:
- interactive prompt
- [`-c`](https://docs.python.org/3/using/cmdline.html#cmdoption-c) option
- running from stdin
- running directly from a source or bytecode file
Note that `__main__.__spec__` is always `None` in the last case, *even if* the file could technically be imported directly as a module instead. Use the [`-m`](https://docs.python.org/3/using/cmdline.html#cmdoption-m) switch if valid module metadata is desired in [`__main__`](https://docs.python.org/3/library/__main__.html#module-__main__ "__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``.").
Note also that even when `__main__` corresponds with an importable module and `__main__.__spec__` is set accordingly, they’re still considered *distinct* modules. This is due to the fact that blocks guarded by `if __name__ == "__main__":` checks only execute when the module is used to populate the `__main__` namespace, and not during normal import.
## 5\.9. References[¶](https://docs.python.org/3/reference/import.html#references "Link to this heading")
The import machinery has evolved considerably since Python’s early days. The original [specification for packages](https://www.python.org/doc/essays/packages/) is still available to read, although some details have changed since the writing of that document.
The original specification for [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path") was [**PEP 302**](https://peps.python.org/pep-0302/), with subsequent extension in [**PEP 420**](https://peps.python.org/pep-0420/).
[**PEP 420**](https://peps.python.org/pep-0420/) introduced [namespace packages](https://docs.python.org/3/glossary.html#term-namespace-package) for Python 3.3. [**PEP 420**](https://peps.python.org/pep-0420/) also introduced the `find_loader()` protocol as an alternative to `find_module()`.
[**PEP 366**](https://peps.python.org/pep-0366/) describes the addition of the `__package__` attribute for explicit relative imports in main modules.
[**PEP 328**](https://peps.python.org/pep-0328/) introduced absolute and explicit relative imports and initially proposed `__name__` for semantics [**PEP 366**](https://peps.python.org/pep-0366/) would eventually specify for `__package__`.
[**PEP 338**](https://peps.python.org/pep-0338/) defines executing modules as scripts.
[**PEP 451**](https://peps.python.org/pep-0451/) adds the encapsulation of per-module import state in spec objects. It also off-loads most of the boilerplate responsibilities of loaders back onto the import machinery. These changes allow the deprecation of several APIs in the import system and also addition of new methods to finders and loaders.
Footnotes
\[[1](https://docs.python.org/3/reference/import.html#id1)\]
See [`types.ModuleType`](https://docs.python.org/3/library/types.html#types.ModuleType "types.ModuleType").
\[[2](https://docs.python.org/3/reference/import.html#id3)\]
The importlib implementation avoids using the return value directly. Instead, it gets the module object by looking the module name up in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"). The indirect effect of this is that an imported module may replace itself in `sys.modules`. This is implementation-specific behavior that is not guaranteed to work in other Python implementations.
### [Table of Contents](https://docs.python.org/3/contents.html)
- [5\. The import system](https://docs.python.org/3/reference/import.html)
- [5\.1. `importlib`](https://docs.python.org/3/reference/import.html#importlib)
- [5\.2. Packages](https://docs.python.org/3/reference/import.html#packages)
- [5\.2.1. Regular packages](https://docs.python.org/3/reference/import.html#regular-packages)
- [5\.2.2. Namespace packages](https://docs.python.org/3/reference/import.html#namespace-packages)
- [5\.3. Searching](https://docs.python.org/3/reference/import.html#searching)
- [5\.3.1. The module cache](https://docs.python.org/3/reference/import.html#the-module-cache)
- [5\.3.2. Finders and loaders](https://docs.python.org/3/reference/import.html#finders-and-loaders)
- [5\.3.3. Import hooks](https://docs.python.org/3/reference/import.html#import-hooks)
- [5\.3.4. The meta path](https://docs.python.org/3/reference/import.html#the-meta-path)
- [5\.4. Loading](https://docs.python.org/3/reference/import.html#loading)
- [5\.4.1. Loaders](https://docs.python.org/3/reference/import.html#loaders)
- [5\.4.2. Submodules](https://docs.python.org/3/reference/import.html#submodules)
- [5\.4.3. Module specs](https://docs.python.org/3/reference/import.html#module-specs)
- [5\.4.4. \_\_path\_\_ attributes on modules](https://docs.python.org/3/reference/import.html#path-attributes-on-modules)
- [5\.4.5. Module reprs](https://docs.python.org/3/reference/import.html#module-reprs)
- [5\.4.6. Cached bytecode invalidation](https://docs.python.org/3/reference/import.html#cached-bytecode-invalidation)
- [5\.5. The Path Based Finder](https://docs.python.org/3/reference/import.html#the-path-based-finder)
- [5\.5.1. Path entry finders](https://docs.python.org/3/reference/import.html#path-entry-finders)
- [5\.5.2. Path entry finder protocol](https://docs.python.org/3/reference/import.html#path-entry-finder-protocol)
- [5\.6. Replacing the standard import system](https://docs.python.org/3/reference/import.html#replacing-the-standard-import-system)
- [5\.7. Package Relative Imports](https://docs.python.org/3/reference/import.html#package-relative-imports)
- [5\.8. Special considerations for \_\_main\_\_](https://docs.python.org/3/reference/import.html#special-considerations-for-main)
- [5\.8.1. \_\_main\_\_.\_\_spec\_\_](https://docs.python.org/3/reference/import.html#main-spec)
- [5\.9. References](https://docs.python.org/3/reference/import.html#references)
#### Previous topic
[4\. Execution model](https://docs.python.org/3/reference/executionmodel.html "previous chapter")
#### Next topic
[6\. Expressions](https://docs.python.org/3/reference/expressions.html "next chapter")
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| Readable Markdown | Python code in one [module](https://docs.python.org/3/glossary.html#term-module) gains access to the code in another module by the process of [importing](https://docs.python.org/3/glossary.html#term-importing) it. The [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement is the most common way of invoking the import machinery, but it is not the only way. Functions such as [`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module") and built-in [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") can also be used to invoke the import machinery.
The [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement combines two operations; it searches for the named module, then it binds the results of that search to a name in the local scope. The search operation of the `import` statement is defined as a call to the [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") function, with the appropriate arguments. The return value of `__import__()` is used to perform the name binding operation of the `import` statement. See the `import` statement for the exact details of that name binding operation.
A direct call to [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") performs only the module search and, if found, the module creation operation. While certain side-effects may occur, such as the importing of parent packages, and the updating of various caches (including [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules")), only the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement performs a name binding operation.
When an [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement is executed, the standard builtin [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") function is called. Other mechanisms for invoking the import system (such as [`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module")) may choose to bypass `__import__()` and use their own solutions to implement import semantics.
When a module is first imported, Python searches for the module and if found, it creates a module object [\[1\]](https://docs.python.org/3/reference/import.html#fnmo), initializing it. If the named module cannot be found, a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") is raised. Python implements various strategies to search for the named module when the import machinery is invoked. These strategies can be modified and extended by using various hooks described in the sections below.
Changed in version 3.3: The import system has been updated to fully implement the second phase of [**PEP 302**](https://peps.python.org/pep-0302/). There is no longer any implicit import machinery - the full import system is exposed through [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path"). In addition, native namespace package support has been implemented (see [**PEP 420**](https://peps.python.org/pep-0420/)).
## 5\.1. [`importlib`](https://docs.python.org/3/library/importlib.html#module-importlib "importlib: The implementation of the import machinery.")[¶](https://docs.python.org/3/reference/import.html#importlib "Link to this heading")
The [`importlib`](https://docs.python.org/3/library/importlib.html#module-importlib "importlib: The implementation of the import machinery.") module provides a rich API for interacting with the import system. For example [`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module") provides a recommended, simpler API than built-in [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") for invoking the import machinery. Refer to the `importlib` library documentation for additional detail.
## 5\.2. Packages[¶](https://docs.python.org/3/reference/import.html#packages "Link to this heading")
Python has only one type of module object, and all modules are of this type, regardless of whether the module is implemented in Python, C, or something else. To help organize modules and provide a naming hierarchy, Python has a concept of [packages](https://docs.python.org/3/glossary.html#term-package).
You can think of packages as the directories on a file system and modules as files within directories, but don’t take this analogy too literally since packages and modules need not originate from the file system. For the purposes of this documentation, we’ll use this convenient analogy of directories and files. Like file system directories, packages are organized hierarchically, and packages may themselves contain subpackages, as well as regular modules.
It’s important to keep in mind that all packages are modules, but not all modules are packages. Or put another way, packages are just a special kind of module. Specifically, any module that contains a `__path__` attribute is considered a package.
All modules have a name. Subpackage names are separated from their parent package name by a dot, akin to Python’s standard attribute access syntax. Thus you might have a package called [`email`](https://docs.python.org/3/library/email.html#module-email "email: Package supporting the parsing, manipulating, and generating email messages."), which in turn has a subpackage called [`email.mime`](https://docs.python.org/3/library/email.mime.html#module-email.mime "email.mime: Build MIME messages.") and a module within that subpackage called [`email.mime.text`](https://docs.python.org/3/library/email.mime.html#module-email.mime.text "email.mime.text").
### 5\.2.1. Regular packages[¶](https://docs.python.org/3/reference/import.html#regular-packages "Link to this heading")
Python defines two types of packages, [regular packages](https://docs.python.org/3/glossary.html#term-regular-package) and [namespace packages](https://docs.python.org/3/glossary.html#term-namespace-package). Regular packages are traditional packages as they existed in Python 3.2 and earlier. A regular package is typically implemented as a directory containing an `__init__.py` file. When a regular package is imported, this `__init__.py` file is implicitly executed, and the objects it defines are bound to names in the package’s namespace. The `__init__.py` file can contain the same Python code that any other module can contain, and Python will add some additional attributes to the module when it is imported.
For example, the following file system layout defines a top level `parent` package with three subpackages:
```
parent/
__init__.py
one/
__init__.py
two/
__init__.py
three/
__init__.py
```
Importing `parent.one` will implicitly execute `parent/__init__.py` and `parent/one/__init__.py`. Subsequent imports of `parent.two` or `parent.three` will execute `parent/two/__init__.py` and `parent/three/__init__.py` respectively.
### 5\.2.2. Namespace packages[¶](https://docs.python.org/3/reference/import.html#namespace-packages "Link to this heading")
A namespace package is a composite of various [portions](https://docs.python.org/3/glossary.html#term-portion), where each portion contributes a subpackage to the parent package. Portions may reside in different locations on the file system. Portions may also be found in zip files, on the network, or anywhere else that Python searches during import. Namespace packages may or may not correspond directly to objects on the file system; they may be virtual modules that have no concrete representation.
Namespace packages do not use an ordinary list for their `__path__` attribute. They instead use a custom iterable type which will automatically perform a new search for package portions on the next import attempt within that package if the path of their parent package (or [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") for a top level package) changes.
With namespace packages, there is no `parent/__init__.py` file. In fact, there may be multiple `parent` directories found during import search, where each one is provided by a different portion. Thus `parent/one` may not be physically located next to `parent/two`. In this case, Python will create a namespace package for the top-level `parent` package whenever it or one of its subpackages is imported.
See also [**PEP 420**](https://peps.python.org/pep-0420/) for the namespace package specification.
## 5\.3. Searching[¶](https://docs.python.org/3/reference/import.html#searching "Link to this heading")
To begin the search, Python needs the [fully qualified](https://docs.python.org/3/glossary.html#term-qualified-name) name of the module (or package, but for the purposes of this discussion, the difference is immaterial) being imported. This name may come from various arguments to the [`import`](https://docs.python.org/3/reference/simple_stmts.html#import) statement, or from the parameters to the [`importlib.import_module()`](https://docs.python.org/3/library/importlib.html#importlib.import_module "importlib.import_module") or [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") functions.
This name will be used in various phases of the import search, and it may be the dotted path to a submodule, e.g. `foo.bar.baz`. In this case, Python first tries to import `foo`, then `foo.bar`, and finally `foo.bar.baz`. If any of the intermediate imports fail, a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") is raised.
### 5\.3.1. The module cache[¶](https://docs.python.org/3/reference/import.html#the-module-cache "Link to this heading")
The first place checked during import search is [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"). This mapping serves as a cache of all modules that have been previously imported, including the intermediate paths. So if `foo.bar.baz` was previously imported, `sys.modules` will contain entries for `foo`, `foo.bar`, and `foo.bar.baz`. Each key will have as its value the corresponding module object.
During import, the module name is looked up in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules") and if present, the associated value is the module satisfying the import, and the process completes. However, if the value is `None`, then a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") is raised. If the module name is missing, Python will continue searching for the module.
[`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules") is writable. Deleting a key may not destroy the associated module (as other modules may hold references to it), but it will invalidate the cache entry for the named module, causing Python to search anew for the named module upon its next import. The key can also be assigned to `None`, forcing the next import of the module to result in a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError").
Beware though, as if you keep a reference to the module object, invalidate its cache entry in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), and then re-import the named module, the two module objects will *not* be the same. By contrast, [`importlib.reload()`](https://docs.python.org/3/library/importlib.html#importlib.reload "importlib.reload") will reuse the *same* module object, and simply reinitialise the module contents by rerunning the module’s code.
### 5\.3.2. Finders and loaders[¶](https://docs.python.org/3/reference/import.html#finders-and-loaders "Link to this heading")
If the named module is not found in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), then Python’s import protocol is invoked to find and load the module. This protocol consists of two conceptual objects, [finders](https://docs.python.org/3/glossary.html#term-finder) and [loaders](https://docs.python.org/3/glossary.html#term-loader). A finder’s job is to determine whether it can find the named module using whatever strategy it knows about. Objects that implement both of these interfaces are referred to as [importers](https://docs.python.org/3/glossary.html#term-importer) - they return themselves when they find that they can load the requested module.
Python includes a number of default finders and importers. The first one knows how to locate built-in modules, and the second knows how to locate frozen modules. A third default finder searches an [import path](https://docs.python.org/3/glossary.html#term-import-path) for modules. The import path is a list of locations that may name file system paths or zip files. It can also be extended to search for any locatable resource, such as those identified by URLs.
The import machinery is extensible, so new finders can be added to extend the range and scope of module searching.
Finders do not actually load modules. If they can find the named module, they return a *module spec*, an encapsulation of the module’s import-related information, which the import machinery then uses when loading the module.
The following sections describe the protocol for finders and loaders in more detail, including how you can create and register new ones to extend the import machinery.
Changed in version 3.4: In previous versions of Python, finders returned [loaders](https://docs.python.org/3/glossary.html#term-loader) directly, whereas now they return module specs which *contain* loaders. Loaders are still used during import but have fewer responsibilities.
### 5\.3.3. Import hooks[¶](https://docs.python.org/3/reference/import.html#import-hooks "Link to this heading")
The import machinery is designed to be extensible; the primary mechanism for this are the *import hooks*. There are two types of import hooks: *meta hooks* and *import path hooks*.
Meta hooks are called at the start of import processing, before any other import processing has occurred, other than [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules") cache look up. This allows meta hooks to override [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") processing, frozen modules, or even built-in modules. Meta hooks are registered by adding new finder objects to [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path"), as described below.
Import path hooks are called as part of [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") (or `package.__path__`) processing, at the point where their associated path item is encountered. Import path hooks are registered by adding new callables to [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks") as described below.
### 5\.3.4. The meta path[¶](https://docs.python.org/3/reference/import.html#the-meta-path "Link to this heading")
When the named module is not found in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), Python next searches [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path"), which contains a list of meta path finder objects. These finders are queried in order to see if they know how to handle the named module. Meta path finders must implement a method called [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") which takes three arguments: a name, an import path, and (optionally) a target module. The meta path finder can use any strategy it wants to determine whether it can handle the named module or not.
If the meta path finder knows how to handle the named module, it returns a spec object. If it cannot handle the named module, it returns `None`. If [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path") processing reaches the end of its list without returning a spec, then a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") is raised. Any other exceptions raised are simply propagated up, aborting the import process.
The [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") method of meta path finders is called with two or three arguments. The first is the fully qualified name of the module being imported, for example `foo.bar.baz`. The second argument is the path entries to use for the module search. For top-level modules, the second argument is `None`, but for submodules or subpackages, the second argument is the value of the parent package’s `__path__` attribute. If the appropriate `__path__` attribute cannot be accessed, a [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") is raised. The third argument is an existing module object that will be the target of loading later. The import system passes in a target module only during reload.
The meta path may be traversed multiple times for a single import request. For example, assuming none of the modules involved has already been cached, importing `foo.bar.baz` will first perform a top level import, calling `mpf.find_spec("foo", None, None)` on each meta path finder (`mpf`). After `foo` has been imported, `foo.bar` will be imported by traversing the meta path a second time, calling `mpf.find_spec("foo.bar", foo.__path__, None)`. Once `foo.bar` has been imported, the final traversal will call `mpf.find_spec("foo.bar.baz", foo.bar.__path__, None)`.
Some meta path finders only support top level imports. These importers will always return `None` when anything other than `None` is passed as the second argument.
Python’s default [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path") has three meta path finders, one that knows how to import built-in modules, one that knows how to import frozen modules, and one that knows how to import modules from an [import path](https://docs.python.org/3/glossary.html#term-import-path) (i.e. the [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder)).
Changed in version 3.4: The [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") method of meta path finders replaced `find_module()`, which is now deprecated. While it will continue to work without change, the import machinery will try it only if the finder does not implement `find_spec()`.
Changed in version 3.10: Use of `find_module()` by the import system now raises [`ImportWarning`](https://docs.python.org/3/library/exceptions.html#ImportWarning "ImportWarning").
Changed in version 3.12: `find_module()` has been removed. Use [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") instead.
## 5\.4. Loading[¶](https://docs.python.org/3/reference/import.html#loading "Link to this heading")
If and when a module spec is found, the import machinery will use it (and the loader it contains) when loading the module. Here is an approximation of what happens during the loading portion of import:
```
module = None
if spec.loader is not None and hasattr(spec.loader, 'create_module'):
# It is assumed 'exec_module' will also be defined on the loader.
module = spec.loader.create_module(spec)
if module is None:
module = ModuleType(spec.name)
# The import-related module attributes get set here:
_init_module_attrs(spec, module)
if spec.loader is None:
# unsupported
raise ImportError
if spec.origin is None and spec.submodule_search_locations is not None:
# namespace package
sys.modules[spec.name] = module
elif not hasattr(spec.loader, 'exec_module'):
module = spec.loader.load_module(spec.name)
else:
sys.modules[spec.name] = module
try:
spec.loader.exec_module(module)
except BaseException:
try:
del sys.modules[spec.name]
except KeyError:
pass
raise
return sys.modules[spec.name]
```
Note the following details:
- If there is an existing module object with the given name in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), import will have already returned it.
- The module will exist in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules") before the loader executes the module code. This is crucial because the module code may (directly or indirectly) import itself; adding it to `sys.modules` beforehand prevents unbounded recursion in the worst case and multiple loading in the best.
- If loading fails, the failing module – and only the failing module – gets removed from [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"). Any module already in the `sys.modules` cache, and any module that was successfully loaded as a side-effect, must remain in the cache. This contrasts with reloading where even the failing module is left in `sys.modules`.
- After the module is created but before execution, the import machinery sets the import-related module attributes (“\_init\_module\_attrs” in the pseudo-code example above), as summarized in a [later section](https://docs.python.org/3/reference/datamodel.html#import-mod-attrs).
- Module execution is the key moment of loading in which the module’s namespace gets populated. Execution is entirely delegated to the loader, which gets to decide what gets populated and how.
- The module created during loading and passed to exec\_module() may not be the one returned at the end of import [\[2\]](https://docs.python.org/3/reference/import.html#fnlo).
Changed in version 3.4: The import system has taken over the boilerplate responsibilities of loaders. These were previously performed by the [`importlib.abc.Loader.load_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.load_module "importlib.abc.Loader.load_module") method.
### 5\.4.1. Loaders[¶](https://docs.python.org/3/reference/import.html#loaders "Link to this heading")
Module loaders provide the critical function of loading: module execution. The import machinery calls the [`importlib.abc.Loader.exec_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.exec_module "importlib.abc.Loader.exec_module") method with a single argument, the module object to execute. Any value returned from `exec_module()` is ignored.
Loaders must satisfy the following requirements:
- If the module is a Python module (as opposed to a built-in module or a dynamically loaded extension), the loader should execute the module’s code in the module’s global name space (`module.__dict__`).
- If the loader cannot execute the module, it should raise an [`ImportError`](https://docs.python.org/3/library/exceptions.html#ImportError "ImportError"), although any other exception raised during [`exec_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.exec_module "importlib.abc.Loader.exec_module") will be propagated.
In many cases, the finder and loader can be the same object; in such cases the [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") method would just return a spec with the loader set to `self`.
Module loaders may opt in to creating the module object during loading by implementing a [`create_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.create_module "importlib.abc.Loader.create_module") method. It takes one argument, the module spec, and returns the new module object to use during loading. `create_module()` does not need to set any attributes on the module object. If the method returns `None`, the import machinery will create the new module itself.
Changed in version 3.4: The [`load_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.load_module "importlib.abc.Loader.load_module") method was replaced by [`exec_module()`](https://docs.python.org/3/library/importlib.html#importlib.abc.Loader.exec_module "importlib.abc.Loader.exec_module") and the import machinery assumed all the boilerplate responsibilities of loading.
For compatibility with existing loaders, the import machinery will use the `load_module()` method of loaders if it exists and the loader does not also implement `exec_module()`. However, `load_module()` has been deprecated and loaders should implement `exec_module()` instead.
The `load_module()` method must implement all the boilerplate loading functionality described above in addition to executing the module. All the same constraints apply, with some additional clarification:
- If there is an existing module object with the given name in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), the loader must use that existing module. (Otherwise, [`importlib.reload()`](https://docs.python.org/3/library/importlib.html#importlib.reload "importlib.reload") will not work correctly.) If the named module does not exist in `sys.modules`, the loader must create a new module object and add it to `sys.modules`.
- The module *must* exist in [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules") before the loader executes the module code, to prevent unbounded recursion or multiple loading.
- If loading fails, the loader must remove any modules it has inserted into [`sys.modules`](https://docs.python.org/3/library/sys.html#sys.modules "sys.modules"), but it must remove **only** the failing module(s), and only if the loader itself has loaded the module(s) explicitly.
Changed in version 3.5: A [`DeprecationWarning`](https://docs.python.org/3/library/exceptions.html#DeprecationWarning "DeprecationWarning") is raised when `exec_module()` is defined but `create_module()` is not.
Changed in version 3.6: An [`ImportError`](https://docs.python.org/3/library/exceptions.html#ImportError "ImportError") is raised when `exec_module()` is defined but `create_module()` is not.
Changed in version 3.10: Use of `load_module()` will raise [`ImportWarning`](https://docs.python.org/3/library/exceptions.html#ImportWarning "ImportWarning").
### 5\.4.2. Submodules[¶](https://docs.python.org/3/reference/import.html#submodules "Link to this heading")
When a submodule is loaded using any mechanism (e.g. `importlib` APIs, the `import` or `import-from` statements, or built-in `__import__()`) a binding is placed in the parent module’s namespace to the submodule object. For example, if package `spam` has a submodule `foo`, after importing `spam.foo`, `spam` will have an attribute `foo` which is bound to the submodule. Let’s say you have the following directory structure:
```
spam/
__init__.py
foo.py
```
and `spam/__init__.py` has the following line in it:
```
from .foo import Foo
```
then executing the following puts name bindings for `foo` and `Foo` in the `spam` module:
```
>>> import spam
>>> spam.foo
<module 'spam.foo' from '/tmp/imports/spam/foo.py'>
>>> spam.Foo
<class 'spam.foo.Foo'>
```
Given Python’s familiar name binding rules this might seem surprising, but it’s actually a fundamental feature of the import system. The invariant holding is that if you have `sys.modules['spam']` and `sys.modules['spam.foo']` (as you would after the above import), the latter must appear as the `foo` attribute of the former.
### 5\.4.3. Module specs[¶](https://docs.python.org/3/reference/import.html#module-specs "Link to this heading")
The import machinery uses a variety of information about each module during import, especially before loading. Most of the information is common to all modules. The purpose of a module’s spec is to encapsulate this import-related information on a per-module basis.
Using a spec during import allows state to be transferred between import system components, e.g. between the finder that creates the module spec and the loader that executes it. Most importantly, it allows the import machinery to perform the boilerplate operations of loading, whereas without a module spec the loader had that responsibility.
The module’s spec is exposed as [`module.__spec__`](https://docs.python.org/3/reference/datamodel.html#module.__spec__ "module.__spec__"). Setting `__spec__` appropriately applies equally to [modules initialized during interpreter startup](https://docs.python.org/3/reference/toplevel_components.html#programs). The one exception is `__main__`, where `__spec__` is [set to None in some cases](https://docs.python.org/3/reference/import.html#main-spec).
See [`ModuleSpec`](https://docs.python.org/3/library/importlib.html#importlib.machinery.ModuleSpec "importlib.machinery.ModuleSpec") for details on the contents of the module spec.
Added in version 3.4.
### 5\.4.4. \_\_path\_\_ attributes on modules[¶](https://docs.python.org/3/reference/import.html#path-attributes-on-modules "Link to this heading")
The [`__path__`](https://docs.python.org/3/reference/datamodel.html#module.__path__ "module.__path__") attribute should be a (possibly empty) [sequence](https://docs.python.org/3/glossary.html#term-sequence) of strings enumerating the locations where the package’s submodules will be found. By definition, if a module has a `__path__` attribute, it is a [package](https://docs.python.org/3/glossary.html#term-package).
A package’s [`__path__`](https://docs.python.org/3/reference/datamodel.html#module.__path__ "module.__path__") attribute is used during imports of its subpackages. Within the import machinery, it functions much the same as [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path"), i.e. providing a list of locations to search for modules during import. However, `__path__` is typically much more constrained than `sys.path`.
The same rules used for [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") also apply to a package’s `__path__`. [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks") (described below) are consulted when traversing a package’s `__path__`.
A package’s `__init__.py` file may set or alter the package’s [`__path__`](https://docs.python.org/3/reference/datamodel.html#module.__path__ "module.__path__") attribute, and this was typically the way namespace packages were implemented prior to [**PEP 420**](https://peps.python.org/pep-0420/). With the adoption of [**PEP 420**](https://peps.python.org/pep-0420/), namespace packages no longer need to supply `__init__.py` files containing only `__path__` manipulation code; the import machinery automatically sets `__path__` correctly for the namespace package.
### 5\.4.5. Module reprs[¶](https://docs.python.org/3/reference/import.html#module-reprs "Link to this heading")
By default, all modules have a usable repr, however depending on the attributes set above, and in the module’s spec, you can more explicitly control the repr of module objects.
If the module has a spec (`__spec__`), the import machinery will try to generate a repr from it. If that fails or there is no spec, the import system will craft a default repr using whatever information is available on the module. It will try to use the `module.__name__`, `module.__file__`, and `module.__loader__` as input into the repr, with defaults for whatever information is missing.
Here are the exact rules used:
- If the module has a `__spec__` attribute, the information in the spec is used to generate the repr. The “name”, “loader”, “origin”, and “has\_location” attributes are consulted.
- If the module has a `__file__` attribute, this is used as part of the module’s repr.
- If the module has no `__file__` but does have a `__loader__` that is not `None`, then the loader’s repr is used as part of the module’s repr.
- Otherwise, just use the module’s `__name__` in the repr.
Changed in version 3.12: Use of `module_repr()`, having been deprecated since Python 3.4, was removed in Python 3.12 and is no longer called during the resolution of a module’s repr.
### 5\.4.6. Cached bytecode invalidation[¶](https://docs.python.org/3/reference/import.html#cached-bytecode-invalidation "Link to this heading")
Before Python loads cached bytecode from a `.pyc` file, it checks whether the cache is up-to-date with the source `.py` file. By default, Python does this by storing the source’s last-modified timestamp and size in the cache file when writing it. At runtime, the import system then validates the cache file by checking the stored metadata in the cache file against the source’s metadata.
Python also supports “hash-based” cache files, which store a hash of the source file’s contents rather than its metadata. There are two variants of hash-based `.pyc` files: checked and unchecked. For checked hash-based `.pyc` files, Python validates the cache file by hashing the source file and comparing the resulting hash with the hash in the cache file. If a checked hash-based cache file is found to be invalid, Python regenerates it and writes a new checked hash-based cache file. For unchecked hash-based `.pyc` files, Python simply assumes the cache file is valid if it exists. Hash-based `.pyc` files validation behavior may be overridden with the [`--check-hash-based-pycs`](https://docs.python.org/3/using/cmdline.html#cmdoption-check-hash-based-pycs) flag.
Changed in version 3.7: Added hash-based `.pyc` files. Previously, Python only supported timestamp-based invalidation of bytecode caches.
## 5\.5. The Path Based Finder[¶](https://docs.python.org/3/reference/import.html#the-path-based-finder "Link to this heading")
As mentioned previously, Python comes with several default meta path finders. One of these, called the [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder) ([`PathFinder`](https://docs.python.org/3/library/importlib.html#importlib.machinery.PathFinder "importlib.machinery.PathFinder")), searches an [import path](https://docs.python.org/3/glossary.html#term-import-path), which contains a list of [path entries](https://docs.python.org/3/glossary.html#term-path-entry). Each path entry names a location to search for modules.
The path based finder itself doesn’t know how to import anything. Instead, it traverses the individual path entries, associating each of them with a path entry finder that knows how to handle that particular kind of path.
The default set of path entry finders implement all the semantics for finding modules on the file system, handling special file types such as Python source code (`.py` files), Python byte code (`.pyc` files) and shared libraries (e.g. `.so` files). When supported by the [`zipimport`](https://docs.python.org/3/library/zipimport.html#module-zipimport "zipimport: Support for importing Python modules from ZIP archives.") module in the standard library, the default path entry finders also handle loading all of these file types (other than shared libraries) from zipfiles.
Path entries need not be limited to file system locations. They can refer to URLs, database queries, or any other location that can be specified as a string.
The path based finder provides additional hooks and protocols so that you can extend and customize the types of searchable path entries. For example, if you wanted to support path entries as network URLs, you could write a hook that implements HTTP semantics to find modules on the web. This hook (a callable) would return a [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder) supporting the protocol described below, which was then used to get a loader for the module from the web.
A word of warning: this section and the previous both use the term *finder*, distinguishing between them by using the terms [meta path finder](https://docs.python.org/3/glossary.html#term-meta-path-finder) and [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder). These two types of finders are very similar, support similar protocols, and function in similar ways during the import process, but it’s important to keep in mind that they are subtly different. In particular, meta path finders operate at the beginning of the import process, as keyed off the [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path") traversal.
By contrast, path entry finders are in a sense an implementation detail of the path based finder, and in fact, if the path based finder were to be removed from [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path"), none of the path entry finder semantics would be invoked.
### 5\.5.1. Path entry finders[¶](https://docs.python.org/3/reference/import.html#path-entry-finders "Link to this heading")
The [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder) is responsible for finding and loading Python modules and packages whose location is specified with a string [path entry](https://docs.python.org/3/glossary.html#term-path-entry). Most path entries name locations in the file system, but they need not be limited to this.
As a meta path finder, the [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder) implements the [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") protocol previously described, however it exposes additional hooks that can be used to customize how modules are found and loaded from the [import path](https://docs.python.org/3/glossary.html#term-import-path).
Three variables are used by the [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder), [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path"), [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks") and [`sys.path_importer_cache`](https://docs.python.org/3/library/sys.html#sys.path_importer_cache "sys.path_importer_cache"). The `__path__` attributes on package objects are also used. These provide additional ways that the import machinery can be customized.
[`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") contains a list of strings providing search locations for modules and packages. It is initialized from the [`PYTHONPATH`](https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPATH) environment variable and various other installation- and implementation-specific defaults. Entries in `sys.path` can name directories on the file system, zip files, and potentially other “locations” (see the [`site`](https://docs.python.org/3/library/site.html#module-site "site: Module responsible for site-specific configuration.") module) that should be searched for modules, such as URLs, or database queries. Only strings should be present on `sys.path`; all other data types are ignored.
The [path based finder](https://docs.python.org/3/glossary.html#term-path-based-finder) is a [meta path finder](https://docs.python.org/3/glossary.html#term-meta-path-finder), so the import machinery begins the [import path](https://docs.python.org/3/glossary.html#term-import-path) search by calling the path based finder’s [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.machinery.PathFinder.find_spec "importlib.machinery.PathFinder.find_spec") method as described previously. When the `path` argument to `find_spec()` is given, it will be a list of string paths to traverse - typically a package’s `__path__` attribute for an import within that package. If the `path` argument is `None`, this indicates a top level import and [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") is used.
The path based finder iterates over every entry in the search path, and for each of these, looks for an appropriate [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder) ([`PathEntryFinder`](https://docs.python.org/3/library/importlib.html#importlib.abc.PathEntryFinder "importlib.abc.PathEntryFinder")) for the path entry. Because this can be an expensive operation (e.g. there may be `stat()` call overheads for this search), the path based finder maintains a cache mapping path entries to path entry finders. This cache is maintained in [`sys.path_importer_cache`](https://docs.python.org/3/library/sys.html#sys.path_importer_cache "sys.path_importer_cache") (despite the name, this cache actually stores finder objects rather than being limited to [importer](https://docs.python.org/3/glossary.html#term-importer) objects). In this way, the expensive search for a particular [path entry](https://docs.python.org/3/glossary.html#term-path-entry) location’s path entry finder need only be done once. User code is free to remove cache entries from `sys.path_importer_cache` forcing the path based finder to perform the path entry search again.
If the path entry is not present in the cache, the path based finder iterates over every callable in [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks"). Each of the [path entry hooks](https://docs.python.org/3/glossary.html#term-path-entry-hook) in this list is called with a single argument, the path entry to be searched. This callable may either return a [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder) that can handle the path entry, or it may raise [`ImportError`](https://docs.python.org/3/library/exceptions.html#ImportError "ImportError"). An `ImportError` is used by the path based finder to signal that the hook cannot find a path entry finder for that [path entry](https://docs.python.org/3/glossary.html#term-path-entry). The exception is ignored and [import path](https://docs.python.org/3/glossary.html#term-import-path) iteration continues. The hook should expect either a string or bytes object; the encoding of bytes objects is up to the hook (e.g. it may be a file system encoding, UTF-8, or something else), and if the hook cannot decode the argument, it should raise `ImportError`.
If [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks") iteration ends with no [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder) being returned, then the path based finder’s [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.machinery.PathFinder.find_spec "importlib.machinery.PathFinder.find_spec") method will store `None` in [`sys.path_importer_cache`](https://docs.python.org/3/library/sys.html#sys.path_importer_cache "sys.path_importer_cache") (to indicate that there is no finder for this path entry) and return `None`, indicating that this [meta path finder](https://docs.python.org/3/glossary.html#term-meta-path-finder) could not find the module.
If a [path entry finder](https://docs.python.org/3/glossary.html#term-path-entry-finder) *is* returned by one of the [path entry hook](https://docs.python.org/3/glossary.html#term-path-entry-hook) callables on [`sys.path_hooks`](https://docs.python.org/3/library/sys.html#sys.path_hooks "sys.path_hooks"), then the following protocol is used to ask the finder for a module spec, which is then used when loading the module.
The current working directory – denoted by an empty string – is handled slightly differently from other entries on [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path"). First, if the current working directory cannot be determined or is found not to exist, no value is stored in [`sys.path_importer_cache`](https://docs.python.org/3/library/sys.html#sys.path_importer_cache "sys.path_importer_cache"). Second, the value for the current working directory is looked up fresh for each module lookup. Third, the path used for `sys.path_importer_cache` and returned by [`importlib.machinery.PathFinder.find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.machinery.PathFinder.find_spec "importlib.machinery.PathFinder.find_spec") will be the actual current working directory and not the empty string.
### 5\.5.2. Path entry finder protocol[¶](https://docs.python.org/3/reference/import.html#path-entry-finder-protocol "Link to this heading")
In order to support imports of modules and initialized packages and also to contribute portions to namespace packages, path entry finders must implement the [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.PathEntryFinder.find_spec "importlib.abc.PathEntryFinder.find_spec") method.
[`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.PathEntryFinder.find_spec "importlib.abc.PathEntryFinder.find_spec") takes two arguments: the fully qualified name of the module being imported, and the (optional) target module. `find_spec()` returns a fully populated spec for the module. This spec will always have “loader” set (with one exception).
To indicate to the import machinery that the spec represents a namespace [portion](https://docs.python.org/3/glossary.html#term-portion), the path entry finder sets `submodule_search_locations` to a list containing the portion.
Changed in version 3.4: [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.PathEntryFinder.find_spec "importlib.abc.PathEntryFinder.find_spec") replaced `find_loader()` and `find_module()`, both of which are now deprecated, but will be used if `find_spec()` is not defined.
Older path entry finders may implement one of these two deprecated methods instead of `find_spec()`. The methods are still respected for the sake of backward compatibility. However, if `find_spec()` is implemented on the path entry finder, the legacy methods are ignored.
`find_loader()` takes one argument, the fully qualified name of the module being imported. `find_loader()` returns a 2-tuple where the first item is the loader and the second item is a namespace [portion](https://docs.python.org/3/glossary.html#term-portion).
For backwards compatibility with other implementations of the import protocol, many path entry finders also support the same, traditional `find_module()` method that meta path finders support. However path entry finder `find_module()` methods are never called with a `path` argument (they are expected to record the appropriate path information from the initial call to the path hook).
The `find_module()` method on path entry finders is deprecated, as it does not allow the path entry finder to contribute portions to namespace packages. If both `find_loader()` and `find_module()` exist on a path entry finder, the import system will always call `find_loader()` in preference to `find_module()`.
Changed in version 3.10: Calls to `find_module()` and `find_loader()` by the import system will raise [`ImportWarning`](https://docs.python.org/3/library/exceptions.html#ImportWarning "ImportWarning").
Changed in version 3.12: `find_module()` and `find_loader()` have been removed.
## 5\.6. Replacing the standard import system[¶](https://docs.python.org/3/reference/import.html#replacing-the-standard-import-system "Link to this heading")
The most reliable mechanism for replacing the entire import system is to delete the default contents of [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path"), replacing them entirely with a custom meta path hook.
If it is acceptable to only alter the behaviour of import statements without affecting other APIs that access the import system, then replacing the builtin [`__import__()`](https://docs.python.org/3/library/functions.html#import__ "__import__") function may be sufficient.
To selectively prevent the import of some modules from a hook early on the meta path (rather than disabling the standard import system entirely), it is sufficient to raise [`ModuleNotFoundError`](https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError "ModuleNotFoundError") directly from [`find_spec()`](https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec "importlib.abc.MetaPathFinder.find_spec") instead of returning `None`. The latter indicates that the meta path search should continue, while raising an exception terminates it immediately.
## 5\.7. Package Relative Imports[¶](https://docs.python.org/3/reference/import.html#package-relative-imports "Link to this heading")
Relative imports use leading dots. A single leading dot indicates a relative import, starting with the current package. Two or more leading dots indicate a relative import to the parent(s) of the current package, one level per dot after the first. For example, given the following package layout:
```
package/
__init__.py
subpackage1/
__init__.py
moduleX.py
moduleY.py
subpackage2/
__init__.py
moduleZ.py
moduleA.py
```
In either `subpackage1/moduleX.py` or `subpackage1/__init__.py`, the following are valid relative imports:
```
from .moduleY import spam
from .moduleY import spam as ham
from . import moduleY
from ..subpackage1 import moduleY
from ..subpackage2.moduleZ import eggs
from ..moduleA import foo
```
Absolute imports may use either the `import <>` or `from <> import <>` syntax, but relative imports may only use the second form; the reason for this is that:
```
import XXX.YYY.ZZZ
```
should expose `XXX.YYY.ZZZ` as a usable expression, but .moduleY is not a valid expression.
## 5\.8. Special considerations for \_\_main\_\_[¶](https://docs.python.org/3/reference/import.html#special-considerations-for-main "Link to this heading")
The [`__main__`](https://docs.python.org/3/library/__main__.html#module-__main__ "__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``.") module is a special case relative to Python’s import system. As noted [elsewhere](https://docs.python.org/3/reference/toplevel_components.html#programs), the `__main__` module is directly initialized at interpreter startup, much like [`sys`](https://docs.python.org/3/library/sys.html#module-sys "sys: Access system-specific parameters and functions.") and [`builtins`](https://docs.python.org/3/library/builtins.html#module-builtins "builtins: The module that provides the built-in namespace."). However, unlike those two, it doesn’t strictly qualify as a built-in module. This is because the manner in which `__main__` is initialized depends on the flags and other options with which the interpreter is invoked.
### 5\.8.1. \_\_main\_\_.\_\_spec\_\_[¶](https://docs.python.org/3/reference/import.html#main-spec "Link to this heading")
Depending on how [`__main__`](https://docs.python.org/3/library/__main__.html#module-__main__ "__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``.") is initialized, `__main__.__spec__` gets set appropriately or to `None`.
When Python is started with the [`-m`](https://docs.python.org/3/using/cmdline.html#cmdoption-m) option, `__spec__` is set to the module spec of the corresponding module or package. `__spec__` is also populated when the `__main__` module is loaded as part of executing a directory, zipfile or other [`sys.path`](https://docs.python.org/3/library/sys.html#sys.path "sys.path") entry.
In [the remaining cases](https://docs.python.org/3/using/cmdline.html#using-on-interface-options) `__main__.__spec__` is set to `None`, as the code used to populate the [`__main__`](https://docs.python.org/3/library/__main__.html#module-__main__ "__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``.") does not correspond directly with an importable module:
- interactive prompt
- [`-c`](https://docs.python.org/3/using/cmdline.html#cmdoption-c) option
- running from stdin
- running directly from a source or bytecode file
Note that `__main__.__spec__` is always `None` in the last case, *even if* the file could technically be imported directly as a module instead. Use the [`-m`](https://docs.python.org/3/using/cmdline.html#cmdoption-m) switch if valid module metadata is desired in [`__main__`](https://docs.python.org/3/library/__main__.html#module-__main__ "__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``.").
Note also that even when `__main__` corresponds with an importable module and `__main__.__spec__` is set accordingly, they’re still considered *distinct* modules. This is due to the fact that blocks guarded by `if __name__ == "__main__":` checks only execute when the module is used to populate the `__main__` namespace, and not during normal import.
## 5\.9. References[¶](https://docs.python.org/3/reference/import.html#references "Link to this heading")
The import machinery has evolved considerably since Python’s early days. The original [specification for packages](https://www.python.org/doc/essays/packages/) is still available to read, although some details have changed since the writing of that document.
The original specification for [`sys.meta_path`](https://docs.python.org/3/library/sys.html#sys.meta_path "sys.meta_path") was [**PEP 302**](https://peps.python.org/pep-0302/), with subsequent extension in [**PEP 420**](https://peps.python.org/pep-0420/).
[**PEP 420**](https://peps.python.org/pep-0420/) introduced [namespace packages](https://docs.python.org/3/glossary.html#term-namespace-package) for Python 3.3. [**PEP 420**](https://peps.python.org/pep-0420/) also introduced the `find_loader()` protocol as an alternative to `find_module()`.
[**PEP 366**](https://peps.python.org/pep-0366/) describes the addition of the `__package__` attribute for explicit relative imports in main modules.
[**PEP 328**](https://peps.python.org/pep-0328/) introduced absolute and explicit relative imports and initially proposed `__name__` for semantics [**PEP 366**](https://peps.python.org/pep-0366/) would eventually specify for `__package__`.
[**PEP 338**](https://peps.python.org/pep-0338/) defines executing modules as scripts.
[**PEP 451**](https://peps.python.org/pep-0451/) adds the encapsulation of per-module import state in spec objects. It also off-loads most of the boilerplate responsibilities of loaders back onto the import machinery. These changes allow the deprecation of several APIs in the import system and also addition of new methods to finders and loaders.
Footnotes |
| Shard | 16 (laksa) |
| Root Hash | 10954876678907435016 |
| Unparsed URL | org,python!docs,/3/reference/import.html s443 |