python多线程实现异步

import timefrom threading import Threaddef long_io(cb):    def func(callback):        print("开始耗时操作io")        time.sleep(5)        print("io耗时操作完成")        res = "io res"        callback(res)    t1 = Thread(target=func, args=(cb,))    t1.start()def receive_msg(res):    print("获取结果回调结果")    print("io 结果:", res)def req_a():    print("开始处理请求a---")    long_io(receive_msg)    print("离开请求a")def req_b():    print("开始处理请求B")    time.sleep(2)    print("处理完请求B")def main():    req_a()    req_b()if __name__ == ‘__main__‘:    main()