def imap_unordered(pool, f, iterable):
    from collections import deque
    q = deque()
    # would be better to use the number of processes
    # actually in the pool
    MAX = multiprocessing.cpu_count() * 3
    for arg in iterable:
        while len(q) >= MAX:
            yield q.popleft().get()
        q.append(pool.apply_async(f, (arg,)))
    while q:
        yield q.popleft().get()

def ident(i):
    return i

if __name__ == "__main__":
    import multiprocessing
    N = 1000000
    pool = multiprocessing.Pool(3)
    total = 0
    for i in imap_unordered(pool, ident, xrange(N)):
        total += i
    pool.close()
    pool.join()
    print "got", total, "expected", N*(N-1)//2