31.5. "pkgutil" — Package extension utility
*******************************************

New in version 2.3.

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

======================================================================

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

pkgutil.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
   "__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 "*.pkg" files beginning where "*" matches the
   *name* argument.  This feature is similar to "*.pth" files (see the
   "site" module for more information), except that it doesn’t
   special-case lines starting with "import".  A "*.pkg" file is
   trusted at face value: apart from checking for duplicates, all
   entries found in a "*.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 (Unicode or 8-bit) 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 "os.path.isdir()" behavior).

class pkgutil.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 "sys.path", plus any
   modules that are frozen or built-in.

   Note that "ImpImporter" does not currently support being used by
   placement on "sys.meta_path".

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

   **PEP 302** Loader that wraps Python’s “classic” import algorithm.

pkgutil.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 "iter_importers()", and is thus
   subject to the same limitations regarding platform-specific special
   import locations such as the Windows registry.

pkgutil.get_importer(path_item)

   Retrieve a **PEP 302** importer for the given *path_item*.

   The returned importer is cached in "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
   "sys.path_hooks" is necessary.

pkgutil.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 "iter_importers()", and is thus subject to the
   same limitations regarding platform-specific special import
   locations such as the Windows registry.

pkgutil.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
   "sys.meta_path", "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* "sys.path".
   Normally, these locations are searched *before* "sys.path",
   preventing "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 "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".

pkgutil.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.

pkgutil.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, "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__ + '.')

pkgutil.get_data(package, resource)

   Get a resource from a package.

   This is a wrapper for the **PEP 302** loader "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 "get_data()", then "None" is
   returned.

   New in version 2.6.
