31.2. "pkgutil" --- Package extension utility
*********************************************

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

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

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

class pkgutil.ModuleInfo(module_finder, name, ispkg)

   A namedtuple that holds a brief summary of a module's info.

   3.6 新版功能.

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 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** Finder that wraps Python's "classic" import algorithm.

   If *dirname* is a string, a **PEP 302** finder is created that
   searches that directory.  If *dirname* is "None", a **PEP 302**
   finder 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".

   3.3 版后已移除: This emulation is no longer needed, as the standard
   import mechanism is now fully PEP 302 compliant and available in
   "importlib".

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

   *Loader* that wraps Python's "classic" import algorithm.

   3.3 版后已移除: This emulation is no longer needed, as the standard
   import mechanism is now fully PEP 302 compliant and available in
   "importlib".

pkgutil.find_loader(fullname)

   Retrieve a module *loader* for the given *fullname*.

   This is a backwards compatibility wrapper around
   "importlib.util.find_spec()" that converts most failures to
   "ImportError" and only returns the loader rather than the full
   "ModuleSpec".

   在 3.3 版更改: Updated to be based directly on "importlib" rather
   than relying on the package internal PEP 302 import emulation.

   在 3.4 版更改: Updated to be based on **PEP 451**

pkgutil.get_importer(path_item)

   Retrieve a *finder* for the given *path_item*.

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

   在 3.3 版更改: Updated to be based directly on "importlib" rather
   than relying on the package internal PEP 302 import emulation.

pkgutil.get_loader(module_or_name)

   Get a *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__".

   在 3.3 版更改: Updated to be based directly on "importlib" rather
   than relying on the package internal PEP 302 import emulation.

   在 3.4 版更改: Updated to be based on **PEP 451**

pkgutil.iter_importers(fullname='')

   Yield *finder* objects for the given module name.

   If fullname contains a '.', the finders will be for the package
   containing fullname, otherwise they will be all registered top
   level finders (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 finders are produced.

   在 3.3 版更改: Updated to be based directly on "importlib" rather
   than relying on the package internal PEP 302 import emulation.

pkgutil.iter_modules(path=None, prefix='')

   Yields "ModuleInfo" 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.

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

   在 3.3 版更改: Updated to be based directly on "importlib" rather
   than relying on the package internal PEP 302 import emulation.

pkgutil.walk_packages(path=None, prefix='', onerror=None)

   Yields "ModuleInfo" 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.

   例如:

      # list all modules python can access
      walk_packages()

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

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

   在 3.3 版更改: Updated to be based directly on "importlib" rather
   than relying on the package internal PEP 302 import emulation.

pkgutil.get_data(package, resource)

   Get a resource from a package.

   This is a wrapper for the *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 *loader*
   which does not support "get_data", then "None" is returned.  In
   particular, the *loader* for *namespace packages* does not support
   "get_data".
