python多线程同步机制Semaphore

#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""
    Python 线程同步机制:Semaphore
"""

import time
import threading
import random

#  信号量同步基于内部计数器，每调用一次acquire()，计数器减1；每调用一次release()，计数器加1.当计数器为0时，acquire()调用被阻塞。
sema = threading.Semaphore(3)

def foo(param):
    with sema:
        print ‘%d acquire sema‘ % (param,)
        n = random.random() * 3
        time.sleep(5)
    print ‘%d releas sema‘ % (param,)


if __name__ == ‘__main__‘:
    threads = []
    for i in range(5):
        t = threading.Thread(target=foo, args=(i,))
        t.start()
        threads.append(t)

    print ‘------------‘
    for t in threads:
        t.join()

    print ‘main End‘