:mod:`pkgutil` --- Package extension utility
============================================

.. module:: pkgutil
   :synopsis: Utilities for the import system.

**Source code:** :source:`Lib/pkgutil.py`

--------------

This module provides utilities for the import system, in particular package
support.


.. function:: extend_path(path, name)

   Extend the search path for the modules which comprise a package.  Intended
   use is to place the following code in a package's :file:`__init__.py`::

      from pkgutil import extend_path
      __path__ = extend_path(__path__, __name__)

   This will add to the package's ``__path__`` all subdirectories of directories
   on ``sys.path`` named after the package.  This is useful if one wants to
   distribute different parts of a single logical package as multiple
   directories.

   It also looks for :file:`\*.pkg` files beginning where ``*`` matches the
   *name* argument.  This feature is similar to :file:`\*.pth` files (see the
   :mod:`site` module for more information), except that it doesn't special-case
   lines starting with ``import``.  A :file:`\*.pkg` file is trusted at face
   value: apart from checking for duplicates, all entries found in a
   :file:`\*.pkg` file are added to the path, regardless of whether they exist
   on the filesystem.  (This is a feature.)

   If the input path is not a list (as is the case for frozen packages) it is
   returned unchanged.  The input path is not modified; an extended copy is
   returned.  Items are only appended to the copy at the end.

   It is assumed that :data:`sys.path` is a sequence.  Items of :data:`sys.path`
   that are not strings referring to existing directories are ignored. Unicode
   items on :data:`sys.path` that cause errors when used as filenames may cause
   this function to raise an exception (in line with :func:`os.path.isdir`
   behavior).


.. class:: ImpImporter(dirname=None)

   :pep:`302` Importer that wraps Python's "classic" import algorithm.

   If *dirname* is a string, a :pep:`302` importer is created that searches that
   directory.  If *dirname* is ``None``, a :pep:`302` importer is created that
   searches the current :data:`sys.path`, plus any modules that are frozen or
   built-in.

   Note that :class:`ImpImporter` does not currently support being used by
   placement on :data:`sys.meta_path`.

   .. deprecated:: 3.3
      This emulation is no longer needed, as the standard import mechanism
      is now fully PEP 302 compliant and available in :mod:`importlib`.


.. class:: ImpLoader(fullname, file, filename, etc)

   :pep:`302` Loader that wraps Python's "classic" import algorithm.

   .. deprecated:: 3.3
      This emulation is no longer needed, as the standard import mechanism
      is now fully PEP 302 compliant and available in :mod:`importlib`.


.. function:: find_loader(fullname)

   Retrieve a :pep:`302` module loader for the given *fullname*.

   This is a backwards compatibility wrapper around
   :func:`importlib.util.find_spec` that converts most failures to
   :exc:`ImportError` and only returns the loader rather than the full
   :class:`ModuleSpec`.

   .. versionchanged:: 3.3
      Updated to be based directly on :mod:`importlib` rather than relying
      on the package internal PEP 302 import emulation.

   .. versionchanged:: 3.4
      Updated to be based on :pep:`451`

.. function:: get_importer(path_item)

   Retrieve a :pep:`302` importer for the given *path_item*.

   The returned importer is cached in :data:`sys.path_importer_cache` if it was
   newly created by a path hook.

   The cache (or part of it) can be cleared manually if a rescan of
   :data:`sys.path_hooks` is necessary.

   .. versionchanged:: 3.3
      Updated to be based directly on :mod:`importlib` rather than relying
      on the package internal PEP 302 import emulation.


.. function:: get_loader(module_or_name)

   Get a :pep:`302` "loader" object for *module_or_name*.

   If the module or package is accessible via the normal import mechanism, a
   wrapper around the relevant part of that machinery is returned.  Returns
   ``None`` if the module cannot be found or imported.  If the named module is
   not already imported, its containing package (if any) is imported, in order
   to establish the package ``__path__``.

   .. versionchanged:: 3.3
      Updated to be based directly on :mod:`importlib` rather than relying
      on the package internal PEP 302 import emulation.

   .. versionchanged:: 3.4
      Updated to be based on :pep:`451`


.. function:: iter_importers(fullname='')

   Yield :pep:`302` importers for the given module name.

   If fullname contains a '.', the importers will be for the package
   containing fullname, otherwise they will be all registered top level
   importers (i.e. those on both sys.meta_path and sys.path_hooks).

   If the named module is in a package, that package is imported as a side
   effect of invoking this function.

   If no module name is specified, all top level importers are produced.

   .. versionchanged:: 3.3
      Updated to be based directly on :mod:`importlib` rather than relying
      on the package internal PEP 302 import emulation.


.. function:: iter_modules(path=None, prefix='')

   Yields ``(module_finder, name, ispkg)`` for all submodules on *path*, or, if
   *path* is ``None``, all top-level modules on ``sys.path``.

   *path* should be either ``None`` or a list of paths to look for modules in.

   *prefix* is a string to output on the front of every module name on output.

   .. note::

      Only works for a :term:`finder` which defines an ``iter_modules()``
      method. This interface is non-standard, so the module also provides
      implementations for :class:`importlib.machinery.FileFinder` and
      :class:`zipimport.zipimporter`.

   .. versionchanged:: 3.3
      Updated to be based directly on :mod:`importlib` rather than relying
      on the package internal PEP 302 import emulation.


.. function:: walk_packages(path=None, prefix='', onerror=None)

   Yields ``(module_finder, name, ispkg)`` for all modules recursively on
   *path*, or, if *path* is ``None``, all accessible modules.

   *path* should be either ``None`` or a list of paths to look for modules in.

   *prefix* is a string to output on the front of every module name on output.

   Note that this function must import all *packages* (*not* all modules!) on
   the given *path*, in order to access the ``__path__`` attribute to find
   submodules.

   *onerror* is a function which gets called with one argument (the name of the
   package which was being imported) if any exception occurs while trying to
   import a package.  If no *onerror* function is supplied, :exc:`ImportError`\s
   are caught and ignored, while all other exceptions are propagated,
   terminating the search.

   Examples::

      # list all modules python can access
      walk_packages()

      # list all submodules of ctypes
      walk_packages(ctypes.__path__, ctypes.__name__ + '.')

   .. note::

      Only works for a :term:`finder` which defines an ``iter_modules()``
      method. This interface is non-standard, so the module also provides
      implementations for :class:`importlib.machinery.FileFinder` and
      :class:`zipimport.zipimporter`.

   .. versionchanged:: 3.3
      Updated to be based directly on :mod:`importlib` rather than relying
      on the package internal PEP 302 import emulation.


.. function:: get_data(package, resource)

   Get a resource from a package.

   This is a wrapper for the :pep:`302` loader :func:`get_data` API.  The
   *package* argument should be the name of a package, in standard module format
   (``foo.bar``).  The *resource* argument should be in the form of a relative
   filename, using ``/`` as the path separator.  The parent directory name
   ``..`` is not allowed, and nor is a rooted name (starting with a ``/``).

   The function returns a binary string that is the contents of the specified
   resource.

   For packages located in the filesystem, which have already been imported,
   this is the rough equivalent of::

      d = os.path.dirname(sys.modules[package].__file__)
      data = open(os.path.join(d, resource), 'rb').read()

   If the package cannot be located or loaded, or it uses a :pep:`302` loader
   which does not support :func:`get_data`, then ``None`` is returned.
