multiprocessing

[Alivegamer]

So I'm trying to learn how to use multiprocessing and It looks like I have all the code correct but still returns "Done" and doesn't print hello and hi first.

import multiprocessing

def test(rt):
    print(rt)

def main(ar):
    print(ar)
    

if __name__ == '__main__':
    p1 = multiprocessing.Process(target=test, args=("Hello"))
    p2 = multiprocessing.Process(target=main, args=("hi"))

    p1.start()
    p2.start()

    p1.join()
    p2.join()
    print("Done")

[Axel_Erfurt]

This is how it works for me

import multiprocessing
 
def test(rt):
    print(rt)
 
def main(ar):
    print(ar)
     
 
if __name__ == '__main__':
    p1 = multiprocessing.Process(target=test("Hello"))
    p2 = multiprocessing.Process(target=main("hi"))
 
    p1.start()
    p2.start()
 
    p1.join()
    p2.join()
    print("Done")

[Gribouillis]

I think args should be a tuple like ("Hello",)

@Axel_Erfurt Your code amounts to passing target=None !!!

[Alivegamer]

This is how it works for me

import multiprocessing
 
def test(rt):
    print(rt)
 
def main(ar):
    print(ar)
     
 
if __name__ == '__main__':
    p1 = multiprocessing.Process(target=test("Hello"))
    p2 = multiprocessing.Process(target=main("hi"))
 
    p1.start()
    p2.start()
 
    p1.join()
    p2.join()
    print("Done")

Thank you that worked

[Alivegamer]

I think args should be a tuple like ("Hello",)

@Axel_Erfurt Your code amounts to passing target=None !!!

I've tried that it still didn't work but thank you for helping

[Gribouillis]

Thank you that worked

It does NOT work, your print were executed in the main process. It is the same as

>>> import multiprocessing
>>> p1 = multiprocessing.Process(target=None)
>>> p1.start()
>>> p1.join()
>>>

It does not complain, but it does nothing.

I've tried that it still didn't work but thank you for helping

This works, for me at least. I'm surprised it doesn't work for you.

import multiprocessing
 
def test(rt):
    print(rt)
 
def main(ar):
    print(ar)
     
 
if __name__ == '__main__':
    p1 = multiprocessing.Process(target=test, args=("Hello",))
    p2 = multiprocessing.Process(target=main, args=("hi",))
 
    p1.start()
    p2.start()
 
    p1.join()
    p2.join()
    print("Done")

[snippsat]

Also look into concurrent.futures.
It has a simpler/better interface and work the same whatever use multiple threads or multiple processes.

from concurrent.futures import ThreadPoolExecutor, ProcessPoolExecutor
from time import sleep

def test(rt):
    sleep(10)
    print(rt)

def main(ar):
    sleep(3)
    print(ar)

if __name__ == '__main__':
    executor = ProcessPoolExecutor(max_workers=4)
    executor.submit(test, 'hello')
    executor.submit(main, 'hi')

Set ThreadPoolExecutor and it will be threading instead.
Also set max_workers=1 then you see than need wait for hello before hi,also no parallelism.
In this Thread a more real world example,like getting images downloaded a lot faster.

[Alivegamer]

Thank you that worked

It does NOT work, your print were executed in the main process. It is the same as

>>> import multiprocessing
>>> p1 = multiprocessing.Process(target=None)
>>> p1.start()
>>> p1.join()
>>>

It does not complain, but it does nothing.

I've tried that it still didn't work but thank you for helping

This works, for me at least. I'm surprised it doesn't work for you.

import multiprocessing
 
def test(rt):
    print(rt)
 
def main(ar):
    print(ar)
     
 
if __name__ == '__main__':
    p1 = multiprocessing.Process(target=test, args=("Hello",))
    p2 = multiprocessing.Process(target=main, args=("hi",))
 
    p1.start()
    p2.start()
 
    p1.join()
    p2.join()
    print("Done")

Well i copied and paste your code to see if it's my errors but I still got the same problem where it will just print done. Plus I can see what you mean about it not working I've done ferther tests and your right

[snippsat]

Well i copied and paste your code to see if it's my errors but I still got the same problem where it will just print done

Remember always run from command line when test stuff like this,Editors/IDE can block process and make trouble.

G:\div_code\answer
# Look at code
λ cat multi_1.py
import multiprocessing

def test(rt):
    print(rt)

def main(ar):
    print(ar)


if __name__ == '__main__':
    p1 = multiprocessing.Process(target=test, args=("Hello",))
    p2 = multiprocessing.Process(target=main, args=("hi",))

    p1.start()
    p2.start()

    p1.join()
    p2.join()
    print("Done")

# Run code
G:\div_code\answer
λ python multi_1.py
Hello
hi
Done

[Alivegamer]

Well i copied and paste your code to see if it's my errors but I still got the same problem where it will just print done

Remember always run from command line when test stuff like this,Editors/IDE can block process and make trouble.

G:\div_code\answer
# Look at code
λ cat multi_1.py
import multiprocessing

def test(rt):
    print(rt)

def main(ar):
    print(ar)


if __name__ == '__main__':
    p1 = multiprocessing.Process(target=test, args=("Hello",))
    p2 = multiprocessing.Process(target=main, args=("hi",))

    p1.start()
    p2.start()

    p1.join()
    p2.join()
    print("Done")

# Run code
G:\div_code\answer
λ python multi_1.py
Hello
hi
Done

alright so when you said run things in cmd line and I ran the python in the cmd line it worked it's my IDLE thank you for your help!