def bos_token(self) -> str:
        """
        `str`: Beginning of sentence token. Log an error if used while not having been set.
        """
        if self._bos_token is None:
            if self.verbose:
                logger.error("Using bos_token, but it is not set yet.")
            return None
        return str(self._bos_token)

        `str`: Beginning of sentence token. Log an error if used while not having been set.
        
dpkd/transformers/src/transformers/tokenization_utils_base.py