Python Queue 如何知道它将为空？

Question

I would like to understand how a queue knows that it wont receive any new items. In the following example the queue will indefintely wait when the tputter thread is not started (I assume because nothing was put to it so far). If the tputter is started it waits between 'puts' until something new is there and as soon as everything is finished it stops. But how does the tgetter know whether something new will end up in the queue or not?

import threading
import queue
import time

q = queue.Queue()

def getter():
    for i in range(5):
        print('worker:', q.get())
        time.sleep(2)


def putter():
    for i in range(5):
        print('putter: ', i)
        q.put(i)
        time.sleep(3)

tgetter = threading.Thread(target=getter)
tgetter.start()
tputter = threading.Thread(target=putter)
#tputter.start()

Answer 1

A common way to do this is to use the "poison pill" pattern. Basically, the producer and consumer agree on a special "poison pill" object that the producer can load into the queue, which will indicate that no more items are going to be sent, and the consumer can shut down.

So, in your example, it'd look like this:

import threading
import queue
import time

q = queue.Queue()
END = object()

def getter():
    while True:
        item = q.get()
        if item == END:
           break
        print('worker:', item)
        time.sleep(2)


def putter():
    for i in range(5):
        print('putter: ', i)
        q.put(i)
        time.sleep(3)
    q.put(END)

tgetter = threading.Thread(target=getter)
tgetter.start()
tputter = threading.Thread(target=putter)
#tputter.start()

This is a little contrived, since the producer is hard-coded to always send five items, so you have to imagine that the consumer doesn't know ahead of time how many items the producer will send.