Returns
        -------
        A tuple consisting of (max, min) of the index.

def _max_min(self):
        """
        Returns
        -------
        A tuple consisting of (max, min) of the index.
        """
        dtbl = self.datatable

        objs = object_session(self)
        if isinstance(dtbl, Table):
            return objs.query(func.max(dtbl.c.indx).label("max_indx"),
                              func.min(dtbl.c.indx).label("min_indx")).one()
        else:
            raise Exception("Symbol has no datatable")