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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 threading import random import time class 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.name if __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 threading import random import time class 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.name if __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。