def mask_token(self, value):
        """
        Overriding the default behavior of the mask token to have it eat the space before it.
        """
        # Mask token behave like a normal word, i.e. include the space before it
        # So we set lstrip to True
        value = AddedToken(value, lstrip=True, rstrip=False) if isinstance(value, str) else value
        self._mask_token = value

        Overriding the default behavior of the mask token to have it eat the space before it.
        
dpkd/transformers/src/transformers/models/deberta/tokenization_deberta_fast.py