python的线程join怎么用

import threadingimport timedef run():    time.sleep(2)    print('当前线程的名字是： ', threading.current_thread().name)    time.sleep(2)if __name__ == '__main__':    start_time = time.time()    print('这是主线程：', threading.current_thread().name)    thread_list = []    for i in range(5):        t = threading.Thread(target=run)        thread_list.append(t)    for t in thread_list:        t.setDaemon(True)        t.start()    for t in thread_list:        t.join()    print('主线程结束了！' , threading.current_thread().name)    print('一共用时：', time.time()-start_time)