# -*- coding:utf-8 -*-""" Created by FizLin on 2017/07/23/-下午10:59 mail: https://github.com/Fiz1994 信號量 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 threadingimport timeimport randomsites = ["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="] * 20sites_index = 0maxconnections = 2pool_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 + " 需要時間 : " + str(k))  sites_index += 1  # print(url)  time.sleep(k)  print('退出 ', url)for i in range(100): threading.Thread(target=test).start()