Applies a function to the contained :meth:`Result.Err` value.

        Args:
            op: The function to apply to the :meth:`Result.Err` value.

        Returns:
            A :class:`Result` with its error value as the function result
            if `self` is a :meth:`Result.Err` value, otherwise returns
            `self`.

        Examples:
            >>> Ok(1).map_err(lambda x: x * 2)
            Ok(1)
            >>> Err(1).map_err(lambda x: x * 2)
            Err(2)

def map_err(self, op: Callable[[E], F]) -> 'Union[Result[T, F], Result[T, E]]':
        """
        Applies a function to the contained :meth:`Result.Err` value.

        Args:
            op: The function to apply to the :meth:`Result.Err` value.

        Returns:
            A :class:`Result` with its error value as the function result
            if `self` is a :meth:`Result.Err` value, otherwise returns
            `self`.

        Examples:
            >>> Ok(1).map_err(lambda x: x * 2)
            Ok(1)
            >>> Err(1).map_err(lambda x: x * 2)
            Err(2)
        """
        return self if self._is_ok else cast(
            'Result[T, F]',
            self._type.Err(op(cast(E, self._val)))
        )