Multiprocessing.Queue - I want to end.

[Luis Zarrabeitia]

Hi. I'm building a script that closely follows a producer-consumer model. In 
this case, the producer is disk-bound and the consumer is cpu-bound, so I'm 
using the multiprocessing module (python2.5 with the multiprocessing backport 
from google.code) to speed up the processing (two consumers, one per core, 
and one producer). The consumers are two multiprocessing.Process instances, 
the producer is the main script, and the data is sent using a 
multiprocessing.Queue instance (with bounded capacity).

The problem: when there is no more data to process, how can I signal the 
consumers to consume until the queue is empty and then stop consuming? I need 
them to do some clean-up work after they finish (and then I need the main 
script to summarize the results)

Currently, the script looks like this:

===
from multiprocessing import Queue, Process

def consumer(filename, queue):
    outfile = open(filename,'w')
    for data in iter(queue.get, None):
        process_data(data, outfile) # stores the result in the outfile
    outfile.close()
    cleanup_consumer(filename)

if __name__ == "__main__":
    queue = Queue(100)
    p1 = Process(target=consumer, args=("file1.txt", queue))
    p2 = Process(target=consumer, args=("file1.txt", queue))
    p1.start(); p2.start()
    for item in read_from_disk(): # this is the disk-bound operation
        queue.put(item)
    queue.put(None); queue.put(None)
    p1.join() # Wait until both consumers finish their work
    p2.join()
    # Tried to put this one before... but then the 'get' raises
    # an exception, even if there are still items to consume.
    queue.close() 
    summarize() # very fast, no need to parallelize this.
===

As you can see, I'm sending one 'None' per consumer, and hoping that no 
consumer will read more than one None. While this particular implementation 
ensures that, it is very fragile. Is there any way to signal the consumers? 
(or better yet, the queue itself, as it is shared by all consumers?) 
Should "close" work for this? (raise the exception when the queue is 
exhausted, not when it is closed by the producer).

-- 
Luis Zarrabeitia (aka Kyrie)
Fac. de Matemática y Computación, UH.
http://profesores.matcom.uh.cu/~kyrie