def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def _invert_results(self, candidates):
        inverted_candidates = list(self.directory)

        for candidate in candidates:
            try:
                inverted_candidates.remove(candidate)
            except ValueError:
                pass

        return inverted_candidates