src/transformers/models/mpnet/tokenization_mpnet_fast.py/MPNetTokenizerFast/mask_token
class MPNetTokenizerFast:    def mask_token(self) -> str:
        """
        :obj:`str`: Mask token, to use when training a model with masked-language modeling. Log an error if used while
        not having been set.

        MPNet tokenizer has a special mask token to be usable in the fill-mask pipeline. The mask token will greedily
        comprise the space before the `<mask>`.
        """
        if self._mask_token is None and self.verbose:
            logger.error("Using mask_token, but it is not set yet.")
            return None
        return str(self._mask_token)