Loop Not Spending Value to Array in Python

Question

When running this code it just prints out a blank array at the end:

[]

So why is it not appending either the value a or the value b?

import multiprocessing as mu

array_values=[]

def a(array):
    array.append('a')

def b(array):
    array.append('b')

def runInParallel(*fns):
    z=0
    while z<6:
        if __name__=='__main__':
            proc = []
            for fn in fns:
                p = mu.Process(target=fn,args=(array_values,))
                p.start()
                proc.append(p)
            for p in proc:
                p.join()
        z+=1

runInParallel(a,b)
print(array_values)

DESIRED FINAL OUTPUT OF FUNCTION:

['a','b','a','b','a','b','a','b','a','b','a','b']

Thanks in advance!

wind · Accepted Answer · 2020-10-27 04:46:54Z

1

The reason it doesn't word is because multiprocessing doesn't use shared memory.

You can use the following code to get your desired output (it uses threading which uses shared memory):

import threading

array_values = []


def a(array):
    array.append('a')


def b(array):
    array.append('b')


def runInParallel(*fns):
    z = 0
    while z < 6:
        if __name__ == '__main__':
            proc = []
            for fn in fns:
                p = threading.Thread(target=fn, args=(array_values,))
                p.start()
                proc.append(p)
            for p in proc:
                p.join()
        z += 1


runInParallel(a, b)
print(array_values)

answered Oct 27, 2020 at 4:46

wind

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1 Comment

Philip Julian Over a year ago

thank you! Also thanks for giving me the insight on shared memory and are there any more big differences between multiprocessing and threading?

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