python多线程编程(8):线程的合并和后台线程

线程的合并

python的Thread类中还提供了join()方法，使得一个线程可以等待另一个线程执行结束后再继续运行。这个方法还可以设定一个timeout参数，避免无休止的等待。因为两个线程顺序完成，看起来象一个线程，所以称为线程的合并。一个例子：

import threadingimport randomimport timeclass MyThread(threading.Thread):    def run(self):        wait_time=random.randrange(1,10)        print "%s will wait %d seconds" % (self.name, wait_time)        time.sleep(wait_time)        print "%s finished!" % self.nameif __name__=="__main__":    threads = []    for i in range(5):        t = MyThread()        t.start()        threads.append(t)    print 'main thread is waitting for exit...'            for t in threads:        t.join(1)            print 'main thread finished!'

执行结果：

Thread-1 will wait 3 seconds
Thread-2 will wait 4 seconds
Thread-3 will wait 1 seconds
Thread-4 will wait 5 seconds
Thread-5 will wait 3 seconds
main thread is waitting for exit...
Thread-3 finished!
Thread-1 finished!
Thread-5 finished!
main thread finished!
Thread-2 finished!
Thread-4 finished!

对于sleep时间过长的线程（这里是2和4），将不被等待。

后台线程

默认情况下，主线程在退出时会等待所有子线程的结束。如果希望主线程不等待子线程，而是在退出时自动结束所有的子线程，就需要设置子线程为后台线程(daemon)。方法是通过调用线程类的setDaemon()方法。如下：

import threadingimport randomimport timeclass MyThread(threading.Thread):    def run(self):        wait_time=random.randrange(1,10)        print "%s will wait %d seconds" % (self.name, wait_time)        time.sleep(wait_time)        print "%s finished!" % self.nameif __name__=="__main__":    print 'main thread is waitting for exit...'            for i in range(5):        t = MyThread()        t.setDaemon(True)        t.start()            print 'main thread finished!'

执行结果：

main thread is waitting for exit...
Thread-1 will wait 3 seconds
Thread-2 will wait 3 seconds
Thread-3 will wait 4 seconds
 Thread-4 will wait 7 seconds
 Thread-5 will wait 7 seconds
main thread finished!

可以看出，主线程没有等待子线程的执行，而直接退出。

小结

join()方法使得线程可以等待另一个线程的运行，而setDaemon()方法使得线程在结束时不等待子线程。join和setDaemon都可以改变线程之间的运行顺序。