code: def _values_for_factorize(self) -> Tuple[np.ndarray, Any]:
          
          return self.astype(object), np.nan
docs: Return an array and missing value suitable for factorization.
  
          Returns
          -------
          values : ndarray
  
              An array suitable for factorization. This should maintain order
              and be a supported dtype (Float64, Int64, UInt64, String, Object).
              By default, the extension array is cast to object dtype.
          na_value : object
              The value in `values` to consider missing. This will be treated
              as NA in the factorization routines, so it will be coded as
              `na_sentinal` and not included in `uniques`. By default,
              ``np.nan`` is used.
  
          Notes
          -----
          The values returned by this method are also used in
          :func:`pandas.util.hash_pandas_object`.