Problem

I was trying to use multiprocessing for a task which needed a shared array, when I encountered this error even after using multiprocessing.Array():
name ‘list2_array’ is not defined

Here is a simplified way to replicate it:

from multiprocessing import Pool, Array
from ctypes import c_char

def func(num):
    if num in list2_array:
        return num
    else:
        return num + b"Not Found"

if __name__ == "__main__":
    my_list = [b"1", b"2", b"4", b"8"]
    list2 = [b"1", b"4"]

    with Pool() as pool:
        list2_array = Array(c_char, list2)
        results = pool.map(func, my_list)

    print(results)

Expectation

In the example I tried to print all of the elements that was in both my_list and list2 using multiprocessing, the expected result was:

[b'1', b'2 Not Found', b'4', b'8 Not Found']

but a NameError was raised.

Thanks in advance!

>Solution :

Following on from my previous comment, here’s one way you could achieve this. The key point here is the use of starmap to pass multiple arguments to your sub-process:

from multiprocessing import Pool

def func(num, list2):
    if num in list2:
        return num
    else:
        return num + b" Not Found"

if __name__ == "__main__":
    my_list = [b"1", b"2", b"4", b"8"]
    list2 = [b"1", b"4"]

    with Pool() as pool:
        args = [(ml, list2) for ml in my_list]
        results = pool.starmap(func, args)

    print(results)

Output:

[b'1', b'2 Not Found', b'4', b'8 Not Found']