pkgutil.py 文件源码

def 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.
    """
    try:
        importer = sys.path_importer_cache[path_item]
    except KeyError:
        for path_hook in sys.path_hooks:
            try:
                importer = path_hook(path_item)
                break
            except ImportError:
                pass
        else:
            importer = None
        sys.path_importer_cache.setdefault(path_item, importer)

    if importer is None:
        try:
            importer = ImpImporter(path_item)
        except ImportError:
            importer = None
    return importer