loop 不能退出

jogger0703

class iothread(threading.Thread):
    def __init__(self, host, port):
        threading.Thread.__init__(self)
        self.host = host
        self.port = port
    def run(self):
        self.io = netio.netio(self.host, self.port)
        netio.netio_loop() #即asyncore.loop()

if __name__ == '__main__':
    io = iothread("172.16.103.174", 10031)
    io.start()
    for i in range(3):
        time.sleep(1)
        print 'sleep'

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问题，线程run之后，进入loop。主线程退出之后，线程2阻塞在loop中，有什么好的办法退出线程2么