<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _get_history_next(self):
 """ callback function for key down """

        if self._has_history:
            ret = self._input_history.return_history(1)
            self.string = ret
            self._curs_pos = len(ret)