def _unregister_lookup(cls, lookup, lookup_name=None):
        """
        Remove given lookup from cls lookups. For use in tests only as it's
        not thread-safe.
        """
        if lookup_name is None:
            lookup_name = lookup.lookup_name
        del cls.class_lookups[lookup_name]
Remove given lookup from cls lookups. For use in tests only as it's
        not thread-safe.