Python: multiprocessing - terminate other processes after one process finished

Question

I have some programm in which multiple processes try to finish some function. My aim now is to stop all the other processes after one process has successfully finished the function.

The python program shown below unfortunately waits until all the processes successfully solved the question given in find function. How can I fix my problem?

import multiprocessing
import random

FIND = 50
MAX_COUNT = 100000
INTERVAL = range(10)

def find(process, initial, return_dict):
    succ = False
    while succ == False:
        start=initial
        while(start <= MAX_COUNT):
            if(FIND == start):
                return_dict[process] = f"Found: {process}, start: {initial}"
                succ = True
                break;
            i = random.choice(INTERVAL)
            start = start + i
            print(start)

processes = []
manager = multiprocessing.Manager()
return_code = manager.dict()
for i in range(5):
    process = multiprocessing.Process(target=find, args=(f'computer_{i}', i, return_code))
    processes.append(process)
    process.start()

for process in processes:
    process.join()


print(return_code.values())

output can be for example:

['Found: computer_0, start: 0', 'Found: computer_4, start: 4', 'Found: computer_2, start: 2', 'Found: computer_1, start: 1', 'Found: computer_3, start: 3']

But this output shows me the program is waiting until all processes are finished ...

Answer 1

You can do this using multiprocessing.Queue and multiprocessing.Queue.get. How this works is that get by default blocks until there's something in the queue. So it will return the first result that gets appended to the queue, i.e. one of the processes finishing the search. After that, we can iterate over the processes and terminate each one (note that terminating a process doesn't kill child processes spawned by the process unless daemon is set to True).

import multiprocessing
import random
import time 

FIND = 50
MAX_COUNT = 100000
INTERVAL = range(10)

queue = multiprocessing.Queue(maxsize=1)

def find(process, initial):
    succ = False
    while succ == False:
        start=initial
        while(start <= MAX_COUNT):
            if(FIND == start):
                queue.put(f"Found: {process}, start: {initial}")
                break;
            i = random.choice(INTERVAL)
            start = start + i
            print(process, start)

processes = []
manager = multiprocessing.Manager()
for i in range(5):
    process = multiprocessing.Process(target=find, args=(f'computer_{i}', i))
    processes.append(process)
    process.start()

ret = queue.get()
for i in range(5):
    process = processes[i]
    process.terminate()
    print(f'terminated {i}')

print(ret)

You might also want to look into setting the daemon, which kills the processes after the main process exits.