import sys

class VendorImporter:
    """
    A PEP 302 meta path importer for finding optionally-vendored
    or otherwise naturally-installed packages from root_name.
    """
    def __init__(self, root_name, vendored_names=(), vendor_pkg=None):
        self.root_name = root_name
        self.vendored_names = set(vendored_names)
        self.vendor_pkg = vendor_pkg or root_name.replace('extern', '_vendor')

    @property
    def search_path(self):
        yield self.vendor_pkg + '.'
        yield ''

    def find_module(self, fullname, path=None):
        root, base, target = fullname.partition(self.root_name + '.')
        if root:
            return
        if not any(map(target.startswith, self.vendored_names)):
            return
        return self

    def load_module(self, fullname):
        root, base, target = fullname.partition(self.root_name + '.')
        for prefix in self.search_path:
            try:
                __import__(prefix + target)
                mod = sys.modules[fullname] = sys.modules.pop(prefix + target)
                return mod
            except ImportError:
                pass
        else:
            raise ImportError(
                "The '{target}' package is required; "
                "normally this is bundled with this package so if you get "
                "this warning, consult the packager of your "
                "distribution.".format(**locals())
            )

    def install(self):
        if self not in sys.meta_path:
            sys.meta_path.append(self)

names = 'six',
VendorImporter(__name__, names).install()