cpython/Doc/library/importlib.rst

:mod:`importlib` -- An implementation of :keyword:`import`
==========================================================

.. module:: importlib
   :synopsis: An implementation of the import machinery.

.. moduleauthor:: Brett Cannon <brett@python.org>
.. sectionauthor:: Brett Cannon <brett@python.org>

.. versionadded:: 3.1


Introduction
------------

The purpose of the :mod:`importlib` package is two-fold. One is to provide an
implementation of the :keyword:`import` statement (and thus, by extension, the
:func:`__import__` function) in Python source code. This provides an
implementation of :keyword:`import` which is portable to any Python
interpreter. This also provides a reference implementation which is easier to
comprehend than one implemented in a programming language other than Python.

Two, the components to implement :keyword:`import` are exposed in this
package, making it easier for users to create their own custom objects (known
generically as an :term:`importer`) to participate in the import process.
Details on custom importers can be found in :pep:`302`.

.. seealso::

    :ref:`import`
        The language reference for the :keyword:`import` statement.

    `Packages specification <http://www.python.org/doc/essays/packages.html>`__
        Original specification of packages. Some semantics have changed since
        the writing of this document (e.g. redirecting based on ``None``
        in :data:`sys.modules`).

    The :func:`.__import__` function
        The :keyword:`import` statement is syntactic sugar for this function.

    :pep:`235`
        Import on Case-Insensitive Platforms

    :pep:`263`
        Defining Python Source Code Encodings

    :pep:`302`
        New Import Hooks

    :pep:`328`
        Imports: Multi-Line and Absolute/Relative

    :pep:`366`
        Main module explicit relative imports

    :pep:`3120`
        Using UTF-8 as the Default Source Encoding

    :pep:`3147`
        PYC Repository Directories


Functions
---------

.. function:: __import__(name, globals=None, locals=None, fromlist=(), level=0)

    An implementation of the built-in :func:`__import__` function.

.. function:: import_module(name, package=None)

    Import a module. The *name* argument specifies what module to
    import in absolute or relative terms
    (e.g. either ``pkg.mod`` or ``..mod``). If the name is
    specified in relative terms, then the *package* argument must be set to
    the name of the package which is to act as the anchor for resolving the
    package name (e.g. ``import_module('..mod', 'pkg.subpkg')`` will import
    ``pkg.mod``).

    The :func:`import_module` function acts as a simplifying wrapper around
    :func:`importlib.__import__`. This means all semantics of the function are
    derived from :func:`importlib.__import__`, including requiring the package
    from which an import is occurring to have been previously imported
    (i.e., *package* must already be imported). The most important difference
    is that :func:`import_module` returns the most nested package or module
    that was imported (e.g. ``pkg.mod``), while :func:`__import__` returns the
    top-level package or module (e.g. ``pkg``).

.. function:: find_loader(name, path=None)

   Find the loader for a module, optionally within the specified *path*. If the
   module is in :attr:`sys.modules`, then ``sys.modules[name].__loader__`` is
   returned (unless the loader would be ``None``, in which case
   :exc:`ValueError` is raised). Otherwise a search using :attr:`sys.meta_path`
   is done. ``None`` is returned if no loader is found.

   A dotted name does not have its parent's implicitly imported as that requires
   loading them and that may not be desired. To properly import a submodule you
   will need to import all parent packages of the submodule and use the correct
   argument to *path*.

.. function:: invalidate_caches()

   Invalidate the internal caches of finders stored at
   :data:`sys.meta_path`. If a finder implements ``invalidate_caches()`` then it
   will be called to perform the invalidation.  This function may be needed if
   some modules are installed while your program is running and you expect the
   program to notice the changes.

   .. versionadded:: 3.3


:mod:`importlib.abc` -- Abstract base classes related to import
---------------------------------------------------------------

.. module:: importlib.abc
    :synopsis: Abstract base classes related to import

The :mod:`importlib.abc` module contains all of the core abstract base classes
used by :keyword:`import`. Some subclasses of the core abstract base classes
are also provided to help in implementing the core ABCs.

ABC hierarchy::

    object
     +-- Finder (deprecated)
     |    +-- MetaPathFinder
     |    +-- PathEntryFinder
     +-- Loader
          +-- ResourceLoader --------+
          +-- InspectLoader          |
               +-- ExecutionLoader --+
                                     +-- FileLoader
                                     +-- SourceLoader


.. class:: Finder

   An abstract base class representing a :term:`finder`.

   .. deprecated:: 3.3
      Use :class:`MetaPathFinder` or :class:`PathEntryFinder` instead.

   .. method:: find_module(fullname, path=None)

      An abstact method for finding a :term:`loader` for the specified
      module.  Originally specified in :pep:`302`, this method was meant
      for use in :data:`sys.meta_path` and in the path-based import subsystem.


.. class:: MetaPathFinder

   An abstract base class representing a :term:`meta path finder`. For
   compatibility, this is a subclass of :class:`Finder`.

   .. versionadded:: 3.3

   .. method:: find_module(fullname, path)

      An abstract method for finding a :term:`loader` for the specified
      module.  If this is a top-level import, *path* will be ``None``.
      Otherwise, this is a search for a subpackage or module and *path*
      will be the value of :attr:`__path__` from the parent
      package. If a loader cannot be found, ``None`` is returned.

   .. method:: invalidate_caches()

      An optional method which, when called, should invalidate any internal
      cache used by the finder. Used by :func:`importlib.invalidate_caches`
      when invalidating the caches of all finders on :data:`sys.meta_path`.


.. class:: PathEntryFinder

   An abstract base class representing a :term:`path entry finder`.  Though
   it bears some similarities to :class:`MetaPathFinder`, ``PathEntryFinder``
   is meant for use only within the path-based import subsystem provided
   by :class:`PathFinder`. This ABC is a subclass of :class:`Finder` for
   compatibility.

   .. versionadded:: 3.3

   .. method:: find_loader(fullname):

      An abstract method for finding a :term:`loader` for the specified
      module.  Returns a 2-tuple of ``(loader, portion)`` where ``portion``
      is a sequence of file system locations contributing to part of a namespace
      package. The loader may be ``None`` while specifying ``portion`` to
      signify the contribution of the file system locations to a namespace
      package. An empty list can be used for ``portion`` to signify the loader
      is not part of a package. If ``loader`` is ``None`` and ``portion`` is
      the empty list then no loader or location for a namespace package were
      found (i.e. failure to find anything for the module).

   .. method:: find_module(fullname):

      A concrete implementation of :meth:`Finder.find_module` which is
      equivalent to ``self.find_loader(fullname)[0]``.

   .. method:: invalidate_caches()

      An optional method which, when called, should invalidate any internal
      cache used by the finder. Used by :meth:`PathFinder.invalidate_caches`
      when invalidating the caches of all cached finders.


.. class:: Loader

    An abstract base class for a :term:`loader`.
    See :pep:`302` for the exact definition for a loader.

    .. method:: load_module(fullname)

        An abstract method for loading a module. If the module cannot be
        loaded, :exc:`ImportError` is raised, otherwise the loaded module is
        returned.

        If the requested module already exists in :data:`sys.modules`, that
        module should be used and reloaded.
        Otherwise the loader should create a new module and insert it into
        :data:`sys.modules` before any loading begins, to prevent recursion
        from the import. If the loader inserted a module and the load fails, it
        must be removed by the loader from :data:`sys.modules`; modules already
        in :data:`sys.modules` before the loader began execution should be left
        alone. The :func:`importlib.util.module_for_loader` decorator handles
        all of these details.

        The loader should set several attributes on the module.
        (Note that some of these attributes can change when a module is
        reloaded.)

        - :attr:`__name__`
            The name of the module.

        - :attr:`__file__`
            The path to where the module data is stored (not set for built-in
            modules).

        - :attr:`__path__`
            A list of strings specifying the search path within a
            package. This attribute is not set on modules.

        - :attr:`__package__`
            The parent package for the module/package. If the module is
            top-level then it has a value of the empty string. The
            :func:`importlib.util.set_package` decorator can handle the details
            for :attr:`__package__`.

        - :attr:`__loader__`
            The loader used to load the module.
            (This is not set by the built-in import machinery,
            but it should be set whenever a :term:`loader` is used.)

    .. method:: module_repr(module)

        An abstract method which when implemented calculates and returns the
        given module's repr, as a string.

        .. versionadded: 3.3


.. class:: ResourceLoader

    An abstract base class for a :term:`loader` which implements the optional
    :pep:`302` protocol for loading arbitrary resources from the storage
    back-end.

    .. method:: get_data(path)

        An abstract method to return the bytes for the data located at *path*.
        Loaders that have a file-like storage back-end
        that allows storing arbitrary data
        can implement this abstract method to give direct access
        to the data stored. :exc:`IOError` is to be raised if the *path* cannot
        be found. The *path* is expected to be constructed using a module's
        :attr:`__file__` attribute or an item from a package's :attr:`__path__`.


.. class:: InspectLoader

    An abstract base class for a :term:`loader` which implements the optional
    :pep:`302` protocol for loaders that inspect modules.

    .. method:: get_code(fullname)

        An abstract method to return the :class:`code` object for a module.
        ``None`` is returned if the module does not have a code object
        (e.g. built-in module).  :exc:`ImportError` is raised if loader cannot
        find the requested module.

        .. index::
           single: universal newlines; importlib.abc.InspectLoader.get_source method

    .. method:: get_source(fullname)

        An abstract method to return the source of a module. It is returned as
        a text string using :term:`universal newlines`, translating all
        recognized line separators into ``'\n'`` characters.  Returns ``None``
        if no source is available (e.g. a built-in module). Raises
        :exc:`ImportError` if the loader cannot find the module specified.

    .. method:: is_package(fullname)

        An abstract method to return a true value if the module is a package, a
        false value otherwise. :exc:`ImportError` is raised if the
        :term:`loader` cannot find the module.


.. class:: ExecutionLoader

    An abstract base class which inherits from :class:`InspectLoader` that,
    when implemented, helps a module to be executed as a script. The ABC
    represents an optional :pep:`302` protocol.

    .. method:: get_filename(fullname)

        An abstract method that is to return the value of :attr:`__file__` for
        the specified module. If no path is available, :exc:`ImportError` is
        raised.

        If source code is available, then the method should return the path to
        the source file, regardless of whether a bytecode was used to load the
        module.


.. class:: FileLoader(fullname, path)

   An abstract base class which inherits from :class:`ResourceLoader` and
   :class:`ExecutionLoader`, providing concreate implementations of
   :meth:`ResourceLoader.get_data` and :meth:`ExecutionLoader.get_filename`.

   The *fullname* argument is a fully resolved name of the module the loader is
   to handle. The *path* argument is the path to the file for the module.

   .. versionadded:: 3.3

   .. attribute:: name

      The name of the module the loader can handle.

   .. attribute:: path

      Path to the file of the module.

   .. method:: load_module(fullname)

      Calls super's ``load_module()``.

   .. method:: get_filename(fullname)

      Returns :attr:`path`.

   .. method:: get_data(path)

      Returns the open, binary file for *path*.


.. class:: SourceLoader

    An abstract base class for implementing source (and optionally bytecode)
    file loading. The class inherits from both :class:`ResourceLoader` and
    :class:`ExecutionLoader`, requiring the implementation of:

    * :meth:`ResourceLoader.get_data`
    * :meth:`ExecutionLoader.get_filename`
          Should only return the path to the source file; sourceless
          loading is not supported (see :class:`SourcelessLoader` if that
          functionality is required)

    The abstract methods defined by this class are to add optional bytecode
    file support. Not implementing these optional methods (or causing them to
    raise :exc:`NotImplementedError`) causes the loader to
    only work with source code. Implementing the methods allows the loader to
    work with source *and* bytecode files; it does not allow for *sourceless*
    loading where only bytecode is provided.  Bytecode files are an
    optimization to speed up loading by removing the parsing step of Python's
    compiler, and so no bytecode-specific API is exposed.

    .. method:: path_stats(path)

        Optional abstract method which returns a :class:`dict` containing
        metadata about the specifed path.  Supported dictionary keys are:

        - ``'mtime'`` (mandatory): an integer or floating-point number
          representing the modification time of the source code;
        - ``'size'`` (optional): the size in bytes of the source code.

        Any other keys in the dictionary are ignored, to allow for future
        extensions.

        .. versionadded:: 3.3

    .. method:: path_mtime(path)

        Optional abstract method which returns the modification time for the
        specified path.

        .. deprecated:: 3.3
           This method is deprecated in favour of :meth:`path_stats`.  You don't
           have to implement it, but it is still available for compatibility
           purposes.

    .. method:: set_data(path, data)

        Optional abstract method which writes the specified bytes to a file
        path. Any intermediate directories which do not exist are to be created
        automatically.

        When writing to the path fails because the path is read-only
        (:attr:`errno.EACCES`), do not propagate the exception.

    .. method:: compile_source(data, path)

        Create a code object from Python source.

        The *data* argument can be whatever the :func:`compile` function
        supports (i.e. string or bytes). The *path* argument should be
        the "path" to where the source code originated from, which can be an
        abstract concept (e.g. location in a zip file).

        .. versionadded:: 3.4

    .. method:: get_code(fullname)

        Concrete implementation of :meth:`InspectLoader.get_code`.

    .. method:: load_module(fullname)

        Concrete implementation of :meth:`Loader.load_module`.

    .. method:: get_source(fullname)

        Concrete implementation of :meth:`InspectLoader.get_source`.

    .. method:: is_package(fullname)

        Concrete implementation of :meth:`InspectLoader.is_package`. A module
        is determined to be a package if its file path (as provided by
        :meth:`ExecutionLoader.get_filename`) is a file named
        ``__init__`` when the file extension is removed **and** the module name
        itself does not end in ``__init__``.


:mod:`importlib.machinery` -- Importers and path hooks
------------------------------------------------------

.. module:: importlib.machinery
    :synopsis: Importers and path hooks

This module contains the various objects that help :keyword:`import`
find and load modules.

.. attribute:: SOURCE_SUFFIXES

   A list of strings representing the recognized file suffixes for source
   modules.

   .. versionadded:: 3.3

.. attribute:: DEBUG_BYTECODE_SUFFIXES

   A list of strings representing the file suffixes for non-optimized bytecode
   modules.

   .. versionadded:: 3.3

.. attribute:: OPTIMIZED_BYTECODE_SUFFIXES

   A list of strings representing the file suffixes for optimized bytecode
   modules.

   .. versionadded:: 3.3

.. attribute:: BYTECODE_SUFFIXES

   A list of strings representing the recognized file suffixes for bytecode
   modules. Set to either :attr:`DEBUG_BYTECODE_SUFFIXES` or
   :attr:`OPTIMIZED_BYTECODE_SUFFIXES` based on whether ``__debug__`` is true.

   .. versionadded:: 3.3

.. attribute:: EXTENSION_SUFFIXES

   A list of strings representing the recognized file suffixes for
   extension modules.

   .. versionadded:: 3.3

.. function:: all_suffixes()

   Returns a combined list of strings representing all file suffixes for
   modules recognized by the standard import machinery. This is a
   helper for code which simply needs to know if a filesystem path
   potentially refers to a module without needing any details on the kind
   of module (for example, :func:`inspect.getmodulename`)

   .. versionadded:: 3.3


.. class:: BuiltinImporter

    An :term:`importer` for built-in modules. All known built-in modules are
    listed in :data:`sys.builtin_module_names`. This class implements the
    :class:`importlib.abc.MetaPathFinder` and
    :class:`importlib.abc.InspectLoader` ABCs.

    Only class methods are defined by this class to alleviate the need for
    instantiation.


.. class:: FrozenImporter

    An :term:`importer` for frozen modules. This class implements the
    :class:`importlib.abc.MetaPathFinder` and
    :class:`importlib.abc.InspectLoader` ABCs.

    Only class methods are defined by this class to alleviate the need for
    instantiation.


.. class:: WindowsRegistryFinder

   :term:`Finder` for modules declared in the Windows registry.  This class
   implements the :class:`importlib.abc.Finder` ABC.

   Only class methods are defined by this class to alleviate the need for
   instantiation.

   .. versionadded:: 3.3


.. class:: PathFinder

   A :term:`Finder` for :data:`sys.path` and package ``__path__`` attributes.
   This class implements the :class:`importlib.abc.MetaPathFinder` ABC.

   Only class methods are defined by this class to alleviate the need for
   instantiation.

   .. classmethod:: find_module(fullname, path=None)

     Class method that attempts to find a :term:`loader` for the module
     specified by *fullname* on :data:`sys.path` or, if defined, on
     *path*. For each path entry that is searched,
     :data:`sys.path_importer_cache` is checked. If a non-false object is
     found then it is used as the :term:`finder` to look for the module
     being searched for. If no entry is found in
     :data:`sys.path_importer_cache`, then :data:`sys.path_hooks` is
     searched for a finder for the path entry and, if found, is stored in
     :data:`sys.path_importer_cache` along with being queried about the
     module. If no finder is ever found then ``None`` is both stored in
     the cache and returned.

   .. classmethod:: invalidate_caches()

     Calls :meth:`importlib.abc.PathEntryFinder.invalidate_caches` on all
     finders stored in :attr:`sys.path_importer_cache`.


.. class:: FileFinder(path, \*loader_details)

   A concrete implementation of :class:`importlib.abc.PathEntryFinder` which
   caches results from the file system.

   The *path* argument is the directory for which the finder is in charge of
   searching.

   The *loader_details* argument is a variable number of 2-item tuples each
   containing a loader and a sequence of file suffixes the loader recognizes.

   The finder will cache the directory contents as necessary, making stat calls
   for each module search to verify the cache is not outdated. Because cache
   staleness relies upon the granularity of the operating system's state
   information of the file system, there is a potential race condition of
   searching for a module, creating a new file, and then searching for the
   module the new file represents. If the operations happen fast enough to fit
   within the granularity of stat calls, then the module search will fail. To
   prevent this from happening, when you create a module dynamically, make sure
   to call :func:`importlib.invalidate_caches`.

   .. versionadded:: 3.3

   .. attribute:: path

      The path the finder will search in.

   .. method:: find_module(fullname)

      Attempt to find the loader to handle *fullname* within :attr:`path`.

   .. method:: invalidate_caches()

      Clear out the internal cache.

   .. classmethod:: path_hook(\*loader_details)

      A class method which returns a closure for use on :attr:`sys.path_hooks`.
      An instance of :class:`FileFinder` is returned by the closure using the
      path argument given to the closure directly and *loader_details*
      indirectly.

      If the argument to the closure is not an existing directory,
      :exc:`ImportError` is raised.


.. class:: SourceFileLoader(fullname, path)

   A concrete implementation of :class:`importlib.abc.SourceLoader` by
   subclassing :class:`importlib.abc.FileLoader` and providing some concrete
   implementations of other methods.

   .. versionadded:: 3.3

   .. attribute:: name

      The name of the module that this loader will handle.

   .. attribute:: path

      The path to the source file.

   .. method:: is_package(fullname)

      Return true if :attr:`path` appears to be for a package.

   .. method:: path_stats(path)

      Concrete implementation of :meth:`importlib.abc.SourceLoader.path_stats`.

   .. method:: set_data(path, data)

      Concrete implementation of :meth:`importlib.abc.SourceLoader.set_data`.


.. class:: SourcelessFileLoader(fullname, path)

   A concrete implementation of :class:`importlib.abc.FileLoader` which can
   import bytecode files (i.e. no source code files exist).

   Please note that direct use of bytecode files (and thus not source code
   files) inhibits your modules from being usable by all Python
   implementations or new versions of Python which change the bytecode
   format.

   .. versionadded:: 3.3

   .. attribute:: name

      The name of the module the loader will handle.

   .. attribute:: path

      The path to the bytecode file.

   .. method:: is_package(fullname)

      Determines if the module is a package based on :attr:`path`.

   .. method:: get_code(fullname)

      Returns the code object for :attr:`name` created from :attr:`path`.

   .. method:: get_source(fullname)

      Returns ``None`` as bytecode files have no source when this loader is
      used.


.. class:: ExtensionFileLoader(fullname, path)

   A concrete implementation of :class:`importlib.abc.InspectLoader` for
   extension modules.

   The *fullname* argument specifies the name of the module the loader is to
   support. The *path* argument is the path to the extension module's file.

   .. versionadded:: 3.3

   .. attribute:: name

      Name of the module the loader supports.

   .. attribute:: path

      Path to the extension module.

   .. method:: load_module(fullname)

      Loads the extension module if and only if *fullname* is the same as
      :attr:`name` or is ``None``.

   .. method:: is_package(fullname)

      Returns ``True`` if the file path points to a package's ``__init__``
      module based on :attr:`EXTENSION_SUFFIXES`.

   .. method:: get_code(fullname)

      Returns ``None`` as extension modules lack a code object.

   .. method:: get_source(fullname)

      Returns ``None`` as extension modules do not have source code.


:mod:`importlib.util` -- Utility code for importers
---------------------------------------------------

.. module:: importlib.util
    :synopsis: Utility code for importers

This module contains the various objects that help in the construction of
an :term:`importer`.

.. function:: resolve_name(name, package)

   Resolve a relative module name to an absolute one.

   If  **name** has no leading dots, then **name** is simply returned. This
   allows for usage such as
   ``importlib.util.resolve_name('sys', __package__)`` without doing a
   check to see if the **package** argument is needed.

   :exc:`ValueError` is raised if **name** is a relative module name but
   package is a false value (e.g. ``None`` or the empty string).
   :exc:`ValueError` is also raised a relative name would escape its containing
   package (e.g. requesting ``..bacon`` from within the ``spam`` package).

   .. versionadded:: 3.3

.. decorator:: module_for_loader

    A :term:`decorator` for a :term:`loader` method,
    to handle selecting the proper
    module object to load with. The decorated method is expected to have a call
    signature taking two positional arguments
    (e.g. ``load_module(self, module)``) for which the second argument
    will be the module **object** to be used by the loader.
    Note that the decorator will not work on static methods because of the
    assumption of two arguments.

    The decorated method will take in the **name** of the module to be loaded
    as expected for a :term:`loader`. If the module is not found in
    :data:`sys.modules` then a new one is constructed with its
    :attr:`__name__` attribute set to **name**, :attr:`__loader__` set to
    **self**, and :attr:`__package__` set if
    :meth:`importlib.abc.InspectLoader.is_package` is defined for **self** and
    does not raise :exc:`ImportError` for **name**. If a new module is not
    needed then the module found in :data:`sys.modules` will be passed into the
    method.

    If an exception is raised by the decorated method and a module was added to
    :data:`sys.modules` it will be removed to prevent a partially initialized
    module from being in left in :data:`sys.modules`. If the module was already
    in :data:`sys.modules` then it is left alone.

    Use of this decorator handles all the details of which module object a
    loader should initialize as specified by :pep:`302` as best as possible.

    .. versionchanged:: 3.3
       :attr:`__loader__` and :attr:`__package__` are automatically set
       (when possible).

.. decorator:: set_loader

    A :term:`decorator` for a :term:`loader` method,
    to set the :attr:`__loader__`
    attribute on loaded modules. If the attribute is already set the decorator
    does nothing. It is assumed that the first positional argument to the
    wrapped method (i.e. ``self``) is what :attr:`__loader__` should be set to.

   .. note::

      It is recommended that :func:`module_for_loader` be used over this
      decorator as it subsumes this functionality.


.. decorator:: set_package

    A :term:`decorator` for a :term:`loader` to set the :attr:`__package__`
    attribute on the module returned by the loader. If :attr:`__package__` is
    set and has a value other than ``None`` it will not be changed.
    Note that the module returned by the loader is what has the attribute
    set on and not the module found in :data:`sys.modules`.

    Reliance on this decorator is discouraged when it is possible to set
    :attr:`__package__` before importing. By
    setting it beforehand the code for the module is executed with the
    attribute set and thus can be used by global level code during
    initialization.

   .. note::

      It is recommended that :func:`module_for_loader` be used over this
      decorator as it subsumes this functionality.