cpython/Lib/importlib/__init__.py

"""A pure Python implementation of import."""
__all__ = ['__import__', 'import_module', 'invalidate_caches']

# Bootstrap help #####################################################

# Until bootstrapping is complete, DO NOT import any modules that attempt
# to import importlib._bootstrap (directly or indirectly). Since this
# partially initialised package would be present in sys.modules, those
# modules would get an uninitialised copy of the source version, instead
# of a fully initialised version (either the frozen one or the one
# initialised below if the frozen one is not available).
import _imp  # Just the builtin component, NOT the full Python module
import sys

try:
    import _frozen_importlib as _bootstrap
except ImportError:
    from . import _bootstrap
    _bootstrap._setup(sys, _imp)
else:
    # importlib._bootstrap is the built-in import, ensure we don't create
    # a second copy of the module.
    _bootstrap.__name__ = 'importlib._bootstrap'
    _bootstrap.__package__ = 'importlib'
    _bootstrap.__file__ = __file__.replace('__init__.py', '_bootstrap.py')
    sys.modules['importlib._bootstrap'] = _bootstrap

# To simplify imports in test code
_w_long = _bootstrap._w_long
_r_long = _bootstrap._r_long

# Fully bootstrapped at this point, import whatever you like, circular
# dependencies and startup overhead minimisation permitting :)

# Public API #########################################################

from ._bootstrap import __import__


def invalidate_caches():
    """Call the invalidate_caches() method on all meta path finders stored in
    sys.meta_path (where implemented)."""
    for finder in sys.meta_path:
        if hasattr(finder, 'invalidate_caches'):
            finder.invalidate_caches()


def find_loader(name, path=None):
    """Find the loader for the specified module.

    First, sys.modules is checked to see if the module was already imported. If
    so, then sys.modules[name].__loader__ is returned. If that happens to be
    set to None, then ValueError is raised. If the module is not in
    sys.modules, then sys.meta_path is searched for a suitable loader with the
    value of 'path' given to the finders. None is returned if no loader could
    be found.

    Dotted names do not have their parent packages implicitly imported. You will
    most likely need to explicitly import all parent packages in the proper
    order for a submodule to get the correct loader.

    """
    try:
        loader = sys.modules[name].__loader__
        if loader is None:
            raise ValueError('{}.__loader__ is None'.format(name))
        else:
            return loader
    except KeyError:
        pass
    except AttributeError:
        raise ValueError('{}.__loader__ is not set'.format(name))
    return _bootstrap._find_module(name, path)


def import_module(name, package=None):
    """Import a module.

    The 'package' argument is required when performing a relative import. It
    specifies the package to use as the anchor point from which to resolve the
    relative import to an absolute import.

    """
    level = 0
    if name.startswith('.'):
        if not package:
            raise TypeError("relative imports require the 'package' argument")
        for character in name:
            if character != '.':
                break
            level += 1
    return _bootstrap._gcd_import(name[level:], package, level)