def _get_index_points(self, index_points=None):
if self._index_points is None and index_points is None:<EOL><INDENT>raise ValueError(<EOL>'<STR_LIT>'<EOL>'<STR_LIT>'<EOL>'<STR_LIT>'<EOL>'<STR_LIT>'<EOL>'<STR_LIT>'<EOL>'<STR_LIT>'<EOL>'<STR_LIT>'<EOL>'<STR_LIT>'<EOL>'<STR_LIT>')<EOL><DEDENT>return index_points if index_points is not None else self._index_points<EOL>
Return `index_points` if not None, else `self._index_points`.

        Args:
          index_points: if given, this is what is returned; else,
          `self._index_points`

        Returns:
          index_points: the given arg, if not None, else the class member
          `self._index_points`.

        Rases:
          ValueError: if `index_points` and `self._index_points` are both `None`.