ModuleNotFoundError when importing functions and using ProcessPoolExecutor

Question

I am trying to execute a python script (here called file_2.py) in parallel using ProcessPoolExecutor. The script uses functions I have written in another file (here called file_1.py). Both of these files are in the same directory.

If I important the functions and run the script sequentially, all is fine. Moreover, when I copy the functions from file_2.py to file_1.py the script runs smoothly in parallel. However, when I try to import the functions in combination with using ProcessPoolExecutor, I get the following error:

ModuleNotFoundError: No module named 'file_1'.

I would be great if anyone knows how to fix this! Thanks in advance :)

Here is a simplified version of my files:

# file_1.py

def addition(a,b):
    return a+b

# file_2.py

import functools
import numpy as np
import concurrent.futures
from file_1 import addition

if __name__ == '__main__':
    lst = np.arange(10)
    cons = 100
    
    partial = functools.partial(addition, b=cons)
    with concurrent.futures.ProcessPoolExecutor() as exc: 
        res = list(exc.map(partial, lst))