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!

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']