Python 信号量的使用

线程中，信号量主要是用来维持有限的资源，使得在一定时间使用该资源的线程只有指定的数量

# -*- coding:utf-8 -*-
""" Created by FizLin on 2017/07/23/-下午10:59
    mail: https://github.com/Fiz1994
    信号量

    maxconnections = 5
...
pool_sema = BoundedSemaphore(value=maxconnections)
Once spawned, worker threads call the semaphore’s acquire and release methods when they need to connect to the server:

pool_sema.acquire()
conn = connectdb()
... use connection ...
conn.close()
pool_sema.release()


"""
import threading
import time
import random

sites = ["https://www.baidu.com/", "https://github.com/Fiz1994", "https://stackoverflow.com/",
         "https://www.sogou.com/",
         "http://english.sogou.com/?b_o_e=1&ie=utf8&fr=common_index_nav&query="] * 20
sites_index = 0
maxconnections = 2
pool_sema = threading.BoundedSemaphore(value=maxconnections)


def test():
    with pool_sema:
        global sites_index, sites
        url = str(sites[sites_index])
        k = random.randint(10, 20)
        print("爬去： " + url + " 需要时间 : " + str(k))
        sites_index += 1
        # print(url)
        time.sleep(k)
        print('退出 ', url)


for i in range(100):
    threading.Thread(target=test).start()

可以发现该程序中，永远只有2个爬虫是处于活动状态