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={}, 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``).


: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.


.. 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_mtime(self, path)

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

    .. 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.

    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 an 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: Importers and path hooks

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 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 the execution of the code is possible. By
    setting it before the code for the module is executed it allows the
    attribute to be used at the global level of the module during
    initialization.