如何使用多处理map_async与回调函数

Question

我正在尝试使用multiprocessing.Pool的map_async。在这个最小的例子中。我应该增加一个classe属性，并且应该增加回调函数中的计数器。但是，似乎永远不会调用回调函数。

import random
from time import time
from multiprocessing import Pool


class BuyerAgent:
    def __init__(self):
        self.i = 0

class ZITraders:
    def __init__(self):
        self.max_number_of_trades = 5000

        self.buyers = [BuyerAgent()
                       for _ in xrange(5000)]


    def do_trading(self):
        counter = 0
        dd = 0

        def cb(datuum):
            dd = datuum
            counter += 1
            print 'cb'

        po = Pool()
        po.map_async(match, reverb(self.buyers, self.buyers, self.max_number_of_trades), callback=cb)
        po.close()
        po.join()
        print counter, dd


def reverb(buyers, sellers, num):
    for n in xrange(num):
        yield random.choice(buyers), random.choice(sellers)


def match(buyer, seller):
    print 'match'
    buyer.i += 1
    return buyer.i


if __name__ == '__main__':
    t = time()
    simulation = ZITraders()
    simulation.do_trading()
    print('time %2.2f' % (time() - t))

结果：

MacBook-Pro:zi taghawi$ python zitraders.py
0 0
time 0.13