code: def factorize(
              self,
              na_sentinel: int = -1,
      ) -> Tuple[np.ndarray, ABCExtensionArray]:
          from pandas.core.algorithms import _factorize_array
          arr, na_value = self._values_for_factorize()
          labels, uniques = _factorize_array(arr, na_sentinel=na_sentinel,
                                             na_value=na_value)
          uniques = self._from_factorized(uniques, self)
          return labels, uniques