如何让这个线程等待队列退出？

Question

I've got two threads in my application. One that puts values in a Queue , and another that pulls them from the Queue and processes them.

I am faced with a dilemma when shutting the application down. The thread that processes items in the Queue is stuck on:

item = request_queue.get() # this call blocks until an item is available

The only thing that will terminate the thread is if another item is added to the Queue - and since the main thread doesn't add anything (because it's shutting down), the application locks.

So... how can I instruct Queue.get() to somehow return even if there is nothing on the Queue ?

Answer 1

The answer it turns out is quite simple. Pick a value that would be invalid for the code that processes the Queue ( None is ideal for that) and push that into the Queue . Then have the Queue processing thread quit when it gets the value:

while True:

    item = request_queue.get()

    if item is None:
        break

    # process the Queue as per normal...

Answer 2

Since the blocking thread is not the main thread, you could also set .daemon = True .

import time
import threading
from Queue import Queue

def getter(q):
    while True:
        print 'getting...'
        print q.get(), 'gotten'

def putter(q):
    for i in range(10):
        time.sleep(0.5)
        q.put(i)
        time.sleep(0.5)

q = Queue()
get_thread = threading.Thread(target=getter, args=(q,))
get_thread.daemon = True
get_thread.start()

putter(q)

Answer 3

This problem can still occur if Queue.get() is called in the main thread - so the setDaemon(True) answer is not a universal solution.

For example this script cannot be stopped with Ctrl-C

#!/usr/bin/python
import Queue

theQ = Queue.Queue()
print "one thread getting from Queue"
print theQ.get()

Rather than putting a timeout on Queue.get() and dealing with exceptions, a simple solution is to do a wait loop until something is there. This script can be killed with Ctrl-C

#!/usr/bin/python
import Queue
theQ = Queue.Queue()
print "one thread getting from Queue"
while theQ.empty():
    time.sleep(0.01) # or whatever value is appropriate for your event arrivals
print theQ.get()