from threading import Semaphore,Thread import time,random sm = Semaphore(5) def task(i): sm.acquire()#加锁 print("%s is doing" % i) time.sleep(random.randint(1,3)) print("\n%s is leaving" % i) sm.release()#解锁 if __name__ == "__main__": for i in range(20): p = Thread(target=task,args=(i,)) p.start()设置Semaphore 即允许同时运行的线程或者进程数，比如总共有十个线程要运行，Semaphore设置为5，则同时运行的为五个
结果：
0 is doing 1 is doing 2 is doing 3 is doing 4 is doing 1 is leaving 5 is doing 2 is leaving 6 is doing 3 is leaving 7 is doing 0 is leaving 8 is doing 5 is leaving 9 is doing 4 is leaving 10 is doing 6 is leaving 11 is doing 7 is leaving 12 is doing 9 is leaving 13 is doing 8 is leaving 14 is doing 13 is leaving 15 is doing 12 is leaving 10 is leaving 11 is leaving 16 is doing 17 is doing 18 is doing 14 is leaving 19 is doing 17 is leaving 15 is leaving 19 is leaving 16 is leaving 18 is leaving可以看出，只有一个线程运行结束了才能让另一个运行。
线程和进程里的Semaphore运用的方法一样。