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 __init__(self, masked_tokens=None, name='num_tokens', dtype=tf.int64):
    self._masked_tokens = masked_tokens or []
    super().__init__(name, dtype)