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