在python中使用多进程时子进程会做什么

Question

I'm now learning multiprocess library in python, so I design some experiments to see how does the overall process proceed. I'm confused about the experiment below. The first question is why the value of link_id that each subprocess uses is still 0. The second question is why line3-line6 has been conducted many times (9 times in my case), and it is not equal to the number of subprocesses I have. Python version: 3.7.4, Operation system: win10

from multiprocessing import Pool

link_id = 0
print('head is printing link, value {}, id {}'.format(link_id,id(link_id)))
node_id = 0
print('head is printing node, value {}, id {}'.format(node_id,id(node_id)))

def job(i):
    print('job {} is printing link, value {}, var id {}'.format(i,link_id,id(link_id)))    

def mainFunc():

    p = Pool()
    for i in range(20): p.apply_async(job, args=(i,))
    p.close()
    p.join()


if __name__ == '__main__':
    link_id = 10
    print('main is printing link, value {}, id {}'.format(link_id,id(link_id)))
    mainFunc()
    print('main is printing link, value {}, id {}'.format(link_id,id(link_id)))

Answer 1

Why is value of link_id that each subprocess uses 0 ?

Each subprocess is an instance of the python interpreter, each one will run/compile your script and have a separate reference to the module. __name__ == '__main__' will be False though as this is only True when you run the file directly, so link_id will be 0 for each of these subprocesses

Why is line3-line6 conducted many times

The answer to this is the same as the first, the lines will be run number_of_subprocesses + 1 times (the additional 1 is when you run the script). If you change the number of processes in the pool you should see the number of times the lines are run change