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