Doc/library/importlib.rst - platform/external/python/cpython3 - Gitiles

 :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={}, locals={}, fromlist=list(), 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:: invalidate_caches()

    Invalidate the internal caches of the finders stored at
    :data:`sys.path_importer_cache`. If a finder implements
    :meth:`abc.Finder.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.


 .. class:: Finder

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

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

         An abstract method for finding a :term:`loader` for the specified
         module. If the :term:`finder` is found on :data:`sys.meta_path` and the
         module to be searched for is a subpackage or module then *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:`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.)


 .. 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.

     .. method:: get_source(fullname)

         An abstract method to return the source of a module. It is returned as
         a text string with universal newlines. 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:: 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.

     The abstract methods defined by this class are to add optional bytecode
     file support. Not implementing these optional methods 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(self, 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(self, 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(self, 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:: get_code(self, fullname)

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

     .. method:: load_module(self, fullname)

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

     .. method:: get_source(self, fullname)

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

     .. method:: is_package(self, fullname)

         Concrete implementation of :meth:`InspectLoader.is_package`. A module
         is determined to be a package if its file path is a file named
         ``__init__`` when the file extension is removed.


 .. class:: PyLoader

     An abstract base class inheriting from
     :class:`ExecutionLoader` and
     :class:`ResourceLoader` designed to ease the loading of
     Python source modules (bytecode is not handled; see
     :class:`SourceLoader` for a source/bytecode ABC). A subclass
     implementing this ABC will only need to worry about exposing how the source
     code is stored; all other details for loading Python source code will be
     handled by the concrete implementations of key methods.

     .. deprecated:: 3.2
         This class has been deprecated in favor of :class:`SourceLoader` and is
         slated for removal in Python 3.4. See below for how to create a
         subclass that is compatible with Python 3.1 onwards.

     If compatibility with Python 3.1 is required, then use the following idiom
     to implement a subclass that will work with Python 3.1 onwards (make sure
     to implement :meth:`ExecutionLoader.get_filename`)::

         try:
             from importlib.abc import SourceLoader
         except ImportError:
             from importlib.abc import PyLoader as SourceLoader


         class CustomLoader(SourceLoader):
             def get_filename(self, fullname):
                 """Return the path to the source file."""
                 # Implement ...

             def source_path(self, fullname):
                 """Implement source_path in terms of get_filename."""
                 try:
                     return self.get_filename(fullname)
                 except ImportError:
                     return None

             def is_package(self, fullname):
                 """Implement is_package by looking for an __init__ file
                 name as returned by get_filename."""
                 filename = os.path.basename(self.get_filename(fullname))
                 return os.path.splitext(filename)[0] == '__init__'


     .. method:: source_path(fullname)

         An abstract method that returns the path to the source code for a
         module. Should return ``None`` if there is no source code.
         Raises :exc:`ImportError` if the loader knows it cannot handle the
         module.

     .. method:: get_filename(fullname)

         A concrete implementation of
         :meth:`importlib.abc.ExecutionLoader.get_filename` that
         relies on :meth:`source_path`. If :meth:`source_path` returns
         ``None``, then :exc:`ImportError` is raised.

     .. method:: load_module(fullname)

         A concrete implementation of :meth:`importlib.abc.Loader.load_module`
         that loads Python source code. All needed information comes from the
         abstract methods required by this ABC. The only pertinent assumption
         made by this method is that when loading a package
         :attr:`__path__` is set to ``[os.path.dirname(__file__)]``.

     .. method:: get_code(fullname)

         A concrete implementation of
         :meth:`importlib.abc.InspectLoader.get_code` that creates code objects
         from Python source code, by requesting the source code (using
         :meth:`source_path` and :meth:`get_data`) and compiling it with the
         built-in :func:`compile` function.

     .. method:: get_source(fullname)

         A concrete implementation of
         :meth:`importlib.abc.InspectLoader.get_source`. Uses
         :meth:`importlib.abc.ResourceLoader.get_data` and :meth:`source_path`
         to get the source code.  It tries to guess the source encoding using
         :func:`tokenize.detect_encoding`.


 .. class:: PyPycLoader

     An abstract base class inheriting from :class:`PyLoader`.
     This ABC is meant to help in creating loaders that support both Python
     source and bytecode.

     .. deprecated:: 3.2
         This class has been deprecated in favor of :class:`SourceLoader` and to
         properly support :pep:`3147`. If compatibility is required with
         Python 3.1, implement both :class:`SourceLoader` and :class:`PyLoader`;
         instructions on how to do so are included in the documentation for
         :class:`PyLoader`. Do note that this solution will not support
         sourceless/bytecode-only loading; only source *and* bytecode loading.

     .. method:: source_mtime(fullname)

         An abstract method which returns the modification time for the source
         code of the specified module. The modification time should be an
         integer. If there is no source code, return ``None``. If the
         module cannot be found then :exc:`ImportError` is raised.

     .. method:: bytecode_path(fullname)

         An abstract method which returns the path to the bytecode for the
         specified module, if it exists. It returns ``None``
         if no bytecode exists (yet).
         Raises :exc:`ImportError` if the loader knows it cannot handle the
         module.

     .. method:: get_filename(fullname)

         A concrete implementation of
         :meth:`ExecutionLoader.get_filename` that relies on
         :meth:`PyLoader.source_path` and :meth:`bytecode_path`.
         If :meth:`source_path` returns a path, then that value is returned.
         Else if :meth:`bytecode_path` returns a path, that path will be
         returned. If a path is not available from both methods,
         :exc:`ImportError` is raised.

     .. method:: write_bytecode(fullname, bytecode)

         An abstract method which has the loader write *bytecode* for future
         use. If the bytecode is written, return ``True``. Return
         ``False`` if the bytecode could not be written. This method
         should not be called if :data:`sys.dont_write_bytecode` is true.
         The *bytecode* argument should be a bytes string or bytes array.


 :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.

 .. 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.Finder` 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.Finder` and :class:`importlib.abc.InspectLoader`
     ABCs.

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


 .. class:: PathFinder

     :term:`Finder` for :data:`sys.path`. This class implements the
     :class:`importlib.abc.Finder` ABC.

     This class does not perfectly mirror the semantics of :keyword:`import` in
     terms of :data:`sys.path`. No implicit path hooks are assumed for
     simplification of the class and its semantics. This implies that when
     ``None`` is found in :data:`sys.path_importer_cache` that it is simply
     ignored instead of implying a default finder.

     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 returned.


 :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`.

 .. 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. Otherwise 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`.

 .. 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.

 .. 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.
	: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={}, locals={}, fromlist=list(), 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:: invalidate_caches()

	Invalidate the internal caches of the finders stored at
	:data:`sys.path_importer_cache`. If a finder implements
	:meth:`abc.Finder.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.


	.. class:: Finder

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

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

	An abstract method for finding a :term:`loader` for the specified
	module. If the :term:`finder` is found on :data:`sys.meta_path` and the
	module to be searched for is a subpackage or module then 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:`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.)


	.. 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.

	.. method:: get_source(fullname)

	An abstract method to return the source of a module. It is returned as
	a text string with universal newlines. 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:: 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.

	The abstract methods defined by this class are to add optional bytecode
	file support. Not implementing these optional methods 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(self, 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(self, 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(self, 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:: get_code(self, fullname)

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

	.. method:: load_module(self, fullname)

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

	.. method:: get_source(self, fullname)

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

	.. method:: is_package(self, fullname)

	Concrete implementation of :meth:`InspectLoader.is_package`. A module
	is determined to be a package if its file path is a file named
	``__init__`` when the file extension is removed.


	.. class:: PyLoader

	An abstract base class inheriting from
	:class:`ExecutionLoader` and
	:class:`ResourceLoader` designed to ease the loading of
	Python source modules (bytecode is not handled; see
	:class:`SourceLoader` for a source/bytecode ABC). A subclass
	implementing this ABC will only need to worry about exposing how the source
	code is stored; all other details for loading Python source code will be
	handled by the concrete implementations of key methods.

	.. deprecated:: 3.2
	This class has been deprecated in favor of :class:`SourceLoader` and is
	slated for removal in Python 3.4. See below for how to create a
	subclass that is compatible with Python 3.1 onwards.

	If compatibility with Python 3.1 is required, then use the following idiom
	to implement a subclass that will work with Python 3.1 onwards (make sure
	to implement :meth:`ExecutionLoader.get_filename`)::

	try:
	from importlib.abc import SourceLoader
	except ImportError:
	from importlib.abc import PyLoader as SourceLoader


	class CustomLoader(SourceLoader):
	def get_filename(self, fullname):
	"""Return the path to the source file."""
	# Implement ...

	def source_path(self, fullname):
	"""Implement source_path in terms of get_filename."""
	try:
	return self.get_filename(fullname)
	except ImportError:
	return None

	def is_package(self, fullname):
	"""Implement is_package by looking for an __init__ file
	name as returned by get_filename."""
	filename = os.path.basename(self.get_filename(fullname))
	return os.path.splitext(filename)[0] == '__init__'


	.. method:: source_path(fullname)

	An abstract method that returns the path to the source code for a
	module. Should return ``None`` if there is no source code.
	Raises :exc:`ImportError` if the loader knows it cannot handle the
	module.

	.. method:: get_filename(fullname)

	A concrete implementation of
	:meth:`importlib.abc.ExecutionLoader.get_filename` that
	relies on :meth:`source_path`. If :meth:`source_path` returns
	``None``, then :exc:`ImportError` is raised.

	.. method:: load_module(fullname)

	A concrete implementation of :meth:`importlib.abc.Loader.load_module`
	that loads Python source code. All needed information comes from the
	abstract methods required by this ABC. The only pertinent assumption
	made by this method is that when loading a package
	:attr:`__path__` is set to ``[os.path.dirname(__file__)]``.

	.. method:: get_code(fullname)

	A concrete implementation of
	:meth:`importlib.abc.InspectLoader.get_code` that creates code objects
	from Python source code, by requesting the source code (using
	:meth:`source_path` and :meth:`get_data`) and compiling it with the
	built-in :func:`compile` function.

	.. method:: get_source(fullname)

	A concrete implementation of
	:meth:`importlib.abc.InspectLoader.get_source`. Uses
	:meth:`importlib.abc.ResourceLoader.get_data` and :meth:`source_path`
	to get the source code. It tries to guess the source encoding using
	:func:`tokenize.detect_encoding`.


	.. class:: PyPycLoader

	An abstract base class inheriting from :class:`PyLoader`.
	This ABC is meant to help in creating loaders that support both Python
	source and bytecode.

	.. deprecated:: 3.2
	This class has been deprecated in favor of :class:`SourceLoader` and to
	properly support :pep:`3147`. If compatibility is required with
	Python 3.1, implement both :class:`SourceLoader` and :class:`PyLoader`;
	instructions on how to do so are included in the documentation for
	:class:`PyLoader`. Do note that this solution will not support
	sourceless/bytecode-only loading; only source and bytecode loading.

	.. method:: source_mtime(fullname)

	An abstract method which returns the modification time for the source
	code of the specified module. The modification time should be an
	integer. If there is no source code, return ``None``. If the
	module cannot be found then :exc:`ImportError` is raised.

	.. method:: bytecode_path(fullname)

	An abstract method which returns the path to the bytecode for the
	specified module, if it exists. It returns ``None``
	if no bytecode exists (yet).
	Raises :exc:`ImportError` if the loader knows it cannot handle the
	module.

	.. method:: get_filename(fullname)

	A concrete implementation of
	:meth:`ExecutionLoader.get_filename` that relies on
	:meth:`PyLoader.source_path` and :meth:`bytecode_path`.
	If :meth:`source_path` returns a path, then that value is returned.
	Else if :meth:`bytecode_path` returns a path, that path will be
	returned. If a path is not available from both methods,
	:exc:`ImportError` is raised.

	.. method:: write_bytecode(fullname, bytecode)

	An abstract method which has the loader write bytecode for future
	use. If the bytecode is written, return ``True``. Return
	``False`` if the bytecode could not be written. This method
	should not be called if :data:`sys.dont_write_bytecode` is true.
	The bytecode argument should be a bytes string or bytes array.


	: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.

	.. 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.Finder` 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.Finder` and :class:`importlib.abc.InspectLoader`
	ABCs.

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


	.. class:: PathFinder

	:term:`Finder` for :data:`sys.path`. This class implements the
	:class:`importlib.abc.Finder` ABC.

	This class does not perfectly mirror the semantics of :keyword:`import` in
	terms of :data:`sys.path`. No implicit path hooks are assumed for
	simplification of the class and its semantics. This implies that when
	``None`` is found in :data:`sys.path_importer_cache` that it is simply
	ignored instead of implying a default finder.

	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 returned.


	: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`.

	.. 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. Otherwise 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`.

	.. 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.

	.. 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.