How to get the amount of "work" left to be done by a Python multiprocessing Pool?

Question

So far whenever I needed to use multiprocessing I have done so by manually creating a "process pool" and sharing a working Queue with all subprocesses.

For example:

from multiprocessing import Process, Queue


class MyClass:

    def __init__(self, num_processes):
        self._log         = logging.getLogger()
        self.process_list = []
        self.work_queue   = Queue()
        for i in range(num_processes):
            p_name = 'CPU_%02d' % (i+1)
            self._log.info('Initializing process %s', p_name)
            p = Process(target = do_stuff,
                        args   = (self.work_queue, 'arg1'),
                        name   = p_name)

This way I could add stuff to the queue, which would be consumed by the subprocesses. I could then monitor how far the processing was by checking the Queue.qsize():

    while True:
        qsize = self.work_queue.qsize()
        if qsize == 0:
            self._log.info('Processing finished')
            break
        else:
            self._log.info('%d simulations still need to be calculated', qsize)

Now I figure that multiprocessing.Pool could simplify a lot this code.

What I couldn't find out is how can I monitor the amount of "work" still left to be done.

Take the following example:

from multiprocessing import Pool


class MyClass:

    def __init__(self, num_processes):
        self.process_pool = Pool(num_processes)
        # ...
        result_list = []
        for i in range(1000):            
            result = self.process_pool.apply_async(do_stuff, ('arg1',))
            result_list.append(result)
        # ---> here: how do I monitor the Pool's processing progress?
        # ...?

Any ideas?