python - Dumping a multiprocessing.Queue into a list

Question

I wish to dump a multiprocessing.Queue into a list. For that task I've written the following function:

import Queue

def dump_queue(queue):
    """
    Empties all pending items in a queue and returns them in a list.
    """
    result = []

    # START DEBUG CODE
    initial_size = queue.qsize()
    print("Queue has %s items initially." % initial_size)
    # END DEBUG CODE

    while True:
        try:
            thing = queue.get(block=False)
            result.append(thing)
        except Queue.Empty:

            # START DEBUG CODE
            current_size = queue.qsize()
            total_size = current_size + len(result)
            print("Dumping complete:")
            if current_size == initial_size:
                print("No items were added to the queue.")
            else:
                print("%s items were added to the queue." % 
                      (total_size - initial_size))
            print("Extracted %s items from the queue, queue has %s items 
            left" % (len(result), current_size))
            # END DEBUG CODE

            return result

But for some reason it doesn't work.

Observe the following shell session:

>>> import multiprocessing
>>> q = multiprocessing.Queue()
>>> for i in range(100):
...     q.put([range(200) for j in range(100)])
... 
>>> q.qsize()
100
>>> l=dump_queue(q)
Queue has 100 items initially.
Dumping complete:
0 items were added to the queue.
Extracted 1 items from the queue, queue has 99 items left
>>> l=dump_queue(q)
Queue has 99 items initially.
Dumping complete:
0 items were added to the queue.
Extracted 3 items from the queue, queue has 96 items left
>>> l=dump_queue(q)
Queue has 96 items initially.
Dumping complete:
0 items were added to the queue.
Extracted 1 items from the queue, queue has 95 items left
>>> 

What's happening here? Why aren't all the items being dumped?

Answer 1

Try this:

import Queue
import time

def dump_queue(queue):
    """
    Empties all pending items in a queue and returns them in a list.
    """
    result = []

    for i in iter(queue.get, 'STOP'):
        result.append(i)
    time.sleep(.1)
    return result

import multiprocessing
q = multiprocessing.Queue()
for i in range(100):
    q.put([range(200) for j in range(100)])
q.put('STOP')
l=dump_queue(q)
print len(l)

Multiprocessing queues have an internal buffer which has a feeder thread which pulls work off a buffer and flushes it to the pipe. If not all of the objects have been flushed, I could see a case where Empty is raised prematurely. Using a sentinel to indicate the end of the queue is safe (and reliable). Also, using the iter(get, sentinel) idiom is just better than relying on Empty.

I don't like that it could raise empty due to flushing timing (I added the time.sleep(.1) to allow a context switch to the feeder thread, you may not need it, it works without it - it's a habit to release the GIL).