The code didn't work with imap because imap create a generator, so we can't access result outside the with statement.

    with Pool(os.cpu_count()) as p:
        result = p.imap(clean_up, k, 50)
    for r in result:
        print(r)

In https://docs.python.org/3.4/library/multiprocessing.html?highlight=process#using-a-pool-of-workers I found the correct example. I'm not sure should me add example or warning in imap function.