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