29.3. pkgutil — Package extension utility

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 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, ImportErrors 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.