Right way to exit from function called by multiprocessing.Pool?

Question

How to exit from a function called my multiprocessing.Pool

Here is an example of the code I am using, when I put a condition to exit from function worker when I use this as a script in terminal it halts and does not exit.

def worker(n):
    if n == 4:
        exit("wrong number")  # tried to use sys.exit(1) did not work
    return n*2

def caller(mylist, n=1):
    n_cores = n if n > 1 else multiprocessing.cpu_count()
    print(n_cores)
    pool = multiprocessing.Pool(processes=n_cores)
    result = pool.map(worker, mylist)
    pool.close()
    pool.join()
    return result

l = [2, 3, 60, 4]
myresult = caller(l, 4)

Answer 1

As I said, I don't think you can exit the process running the main script from a worker process.

You haven't explained exactly why you want to do this, so this answer is a guess, but perhaps raising a custom Exception and handling it in an explict except as shown below would be an acceptable way to workaround the limitation.

import multiprocessing
import sys

class WorkerStopException(Exception):
    pass

def worker(n):
    if n == 4:
        raise WorkerStopException()
    return n*2

def caller(mylist, n=1):
    n_cores = n if n > 1 else multiprocessing.cpu_count()
    print(n_cores)
    pool = multiprocessing.Pool(processes=n_cores)
    try:
        result = pool.map(worker, mylist)
    except WorkerStopException:
        sys.exit("wrong number")
    pool.close()
    pool.join()
    return result

if __name__ == '__main__':
    l = [2, 3, 60, 4]
    myresult = caller(l, 4)

Output displayed when run:

4
wrong number

(The 4 is the number of CPUs my system has.)