def unk_token(self):
        """ Unknown token (string). Log an error if used while not having been set. """
        if self._unk_token is None:
            logger.error("Using unk_token, but it is not set yet.")
        return self._unk_token
 Unknown token (string). Log an error if used while not having been set. 
xtune/src/transformers/tokenization_utils.py