How to wait until only the first thread is finished in Python

Question

The requirement is to start five threads, and wait only in the fastest thread. All five threads went to look for the same data 5 directions, and one is enough to continue the control flow.

Actually, I need to wait for the first two threads to return, to verify against each other. But I guess if I know how to wait for the fastest. I can figure out how to wait for the second-fastest.

很多人都在谈论join(timeout),但你事先并不知道哪一个要等(哪一个join提前申请).

Answer 1

使用队列:完成后每个线程将结果放在队列上,然后您只需要读取适当数量的结果并忽略其余部分:

#!python3.3
import queue    # For Python 2.x use 'import Queue as queue'
import threading, time, random

def func(id, result_queue):
    print("Thread", id)
    time.sleep(random.random() * 5)
    result_queue.put((id, 'done'))

def main():
    q = queue.Queue()
    threads = [ threading.Thread(target=func, args=(i, q)) for i in range(5) ]
    for th in threads:
        th.daemon = True
        th.start()

    result1 = q.get()
    result2 = q.get()

    print("Second result: {}".format(result2))

if __name__=='__main__':
    main()

文档Queue.get()(没有参数,相当于Queue.get(True, None):

Queue.get([block [,timeout]])

从队列中删除并返回一个项目.如果可选的args块为true且timeout为None(默认值),则在必要时阻止,直到某个项可用为止.如果timeout是一个正数,它会阻止最多超时秒,如果在该时间内没有可用的项,则会引发Empty异常.否则(块为假),如果一个项立即可用则返回一个项,否则引发Empty异常(在这种情况下忽略超时).

Answer 2

如果您的线程中有某种处理循环，以下代码将在使用threading.Event()终止时终止它们：

def my_thread(stop_event):
    while not stop_event.is_set():
        # do stuff in a loop

        # some check if stuff is complete
        if stuff_complete:
            stop_event.set()
            break

def run_threads():
    # create a thread event
    a_stop_event = threading.Event()

    # spawn the threads
    for x in range(5):
        t = threading.Thread(target=my_thread, args=[a_stop_event])
        t.start()

    while not a_stop_event.is_set():
        # wait for an event
        time.sleep(0.1)

    print "At least one thread is done"

如果您的流程“便宜”或单个请求-响应类型线程（例如异步 HTTP 请求），那么Duncan 的回答是一个很好的方法。

Answer 3

您可以为此使用事件。请参阅http://docs.python.org/2/library/threading.html#event-objects 这个想法是工作线程在完成时引发一个事件。主线程在继续之前等待此事件。工作线程可以设置一个（互斥的）变量来通过事件来标识自己。