#7303: add documentation for useful pkgutil functions and classes.

Doc/library/pkgutil.rst

@@ -2,59 +2,185 @@
 ============================================
 
 .. module:: pkgutil
-   :synopsis: Utilities to support extension of packages.
+   :synopsis: Utilities for the import system.
 
-
-This module provides functions to manipulate packages:
+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`::
+   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.
+   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.)
+   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 ``sys.path`` is a sequence.  Items of ``sys.path`` that are
-   not strings referring to existing directories are ignored. Unicode items on
-   ``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).
+   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`.
+
+
+.. class:: ImpLoader(fullname, file, filename, etc)
+
+   :pep:`302` Loader that wraps Python's "classic" import algorithm.
+
+
+.. function:: find_loader(fullname)
+
+   Find a :pep:`302` "loader" object for *fullname*.
+
+   If *fullname* contains dots, path must be the containing package's
+   ``__path__``.  Returns ``None`` if the module cannot be found or imported.
+   This function uses :func:`iter_importers`, and is thus subject to the same
+   limitations regarding platform-specific special import locations such as the
+   Windows registry.
+
+
+.. 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.
+
+   If there is no importer, a wrapper around the basic import machinery is
+   returned.  This wrapper is never inserted into the importer cache (None is
+   inserted instead).
+
+   The cache (or part of it) can be cleared manually if a rescan of
+   :data:`sys.path_hooks` is necessary.
+
+
+.. 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__``.
+
+   This function uses :func:`iter_importers`, and is thus subject to the same
+   limitations regarding platform-specific special import locations such as the
+   Windows registry.
+
+
+.. 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 importers for :data:`sys.meta_path`,
+   :data:`sys.path`, and Python's "classic" import machinery, in that order.  If
+   the named module is in a package, that package is imported as a side effect
+   of invoking this function.
+
+   Non-:pep:`302` mechanisms (e.g. the Windows registry) used by the standard
+   import machinery to find files in alternative locations are partially
+   supported, but are searched *after* :data:`sys.path`.  Normally, these
+   locations are searched *before* :data:`sys.path`, preventing :data:`sys.path`
+   entries from shadowing them.
+
+   For this to cause a visible difference in behaviour, there must be a module
+   or package name that is accessible via both :data:`sys.path` and one of the
+   non-:pep:`302` file system mechanisms.  In this case, the emulation will find
+   the former version, while the builtin import mechanism will find the latter.
+
+   Items of the following types can be affected by this discrepancy:
+   ``imp.C_EXTENSION``, ``imp.PY_SOURCE``, ``imp.PY_COMPILED``,
+   ``imp.PKG_DIRECTORY``.
+
+
+.. function:: iter_modules(path=None, prefix='')
+
+   Yields ``(module_loader, 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.
+
+
+.. function:: walk_packages(path=None, prefix='', onerror=None)
+
+   Yields ``(module_loader, 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__ + '.')
+
 
 .. 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
+   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.
+   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()
+      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.
+   which does not support :func:`get_data`, then ``None`` is returned.