信号量

线程锁（也称互斥锁）是同一时间只允许一个线程在执行，信号量可以用来表示同一时间允许执行的线程的数目。

import threading
import time

def run(n):
    semaphore.acquire()
    time.sleep(1)
    print 'run the thread: {}
'.format(n)
    semaphore.release()

if __name__ == '__main__':
    semaphore = threading.BoundedSemaphore(5)
    for i in range(22):
        t = threading.Thread(target=run, args=(i,))
        t.start()

while threading.active_count() != 1:
    pass
else:
    print '---all threads done---'

结果可以看到是五个五个的一起执行

run the thread: 0

run the thread: 3
run the thread: 4
run the thread: 2
run the thread: 1




run the thread: 5

run the thread: 8
run the thread: 9
run the thread: 7
run the thread: 6




run the thread: 10

run the thread: 14
run the thread: 13
run the thread: 11
run the thread: 12

...