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 serialize(self, value, display=False):
        lookup = {v: k for k, v in self.levels.items()}
        if value in lookup:
            return encode(lookup[value])
        return ""