<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def state_change_handler(self, evt):
 """StateChange event handler."""

        datapath = evt.datapath
        assert datapath is not None
        if datapath.id == self._querier.dpid:
            if evt.state == MAIN_DISPATCHER:
                self._querier.start_loop(datapath)
            elif evt.state == DEAD_DISPATCHER:
                self._querier.stop_loop()