from time import sleep

NSTAGES = 10

def init(*args):
    global qs, iolock
    qs, iolock = args

def worker(i):
    inq, outq = qs[i : i+2]
    if i == 0:
        assert inq is None
        # Note that there's no sleep in this branch.
        # Because outq has maxsize=1, the .put() blocks
        # until worker 1 has cleared the queue.
        for j in range(100):
            outq.put(j)
            with iolock:
                print("produced", j)
    else:
        while True:
            j = inq.get()
            if j is None:
                break
            sleep(1.0)
            outq.put(j + i * 10 ** (i + 3))
    outq.put(None)

if __name__ == "__main__":
    import multiprocessing as mp

    qs = [None] + [mp.Queue(maxsize=1) for i in range(NSTAGES)]
    iolock = mp.Lock()

    pool = mp.Pool(NSTAGES, initializer=init, initargs=(qs, iolock))
    pool.imap(worker, range(NSTAGES))
    while True:
        i = qs[-1].get()
        if i is None:
            break
        with iolock:
            print("got", i)
    pool.close()
    pool.join()