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
implementaiton of :keyword:`import` which is portable to any Python
interpreter. This also provides a reference implementation which is easier to
comprehend than one in a programming language other than Python.

Two, the components to implement :keyword:`import` can be 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 providing 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 :keyword:`None`
        in :data:`sys.modules`).

    The :func:`.__import__` function
        The built-in function for which the :keyword:`import` statement is
        syntactic sugar for.

    :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:`3128`
        Using UTF-8 as the Default Source Encoding


Functions
---------

.. function:: __import__(name, globals={}, locals={}, fromlist=list(), level=0)

    An implementation of the built-in :func:`__import__` function. See the
    built-in function's documentation for usage instructions.

.. 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 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:`__import__`. This means all semantics of the function are derived
    from :func:`__import__`, including requiring the package where an import is
    occuring from to already be imported (i.e., *package* must already be
    imported).

: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

    :term:`Importer` for built-in modules. All known built-in modules are
    listed in :data:`sys.builtin_module_names`.

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

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

        Class method that allows this class to be a :term:`finder` for built-in
        modules.

    .. classmethod:: load_module(fullname)

        Class method that allows this class to be a :term:`loader` for built-in
        modules.


.. class:: FrozenImporter

    :term:`Importer` for frozen modules.

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

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

        Class method that allows this class to be a :term:`finder` for frozen
        modules.

    .. classmethod:: load_module(fullname)

        Class method that allows this class to be a :term:`loader` for frozen
        modules.


.. class:: PathFinder

    :term:`Finder` for :data:`sys.path`.

    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 method 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* either 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 query for the module
        being searched for. For 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.