Semaphore(信号量)

import threading, time


def run(n):
    semaphore.acquire()#信号量获取
    time.sleep(1)
    print("run the thread: %s\n" % n)
    semaphore.release()#信号量释放


if __name__ == ‘__main__‘:

    semaphore = threading.BoundedSemaphore(3)  # 声明信号量：最多允许3个线程同时运行
    for i in range(20):
        t = threading.Thread(target=run, args=(i,))
        t.start()

while threading.active_count() != 1:
    pass  # print threading.active_count()
else:
    print(‘----all threads done---‘)
#从运行结果的呈现可以看出，3个3个的执行，一次执行<=3个。