Python Forum - multiprocessing

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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")

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")

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

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

(Mar-17-2022, 09:31 PM)Axel_Erfurt Wrote: [ -> ]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

(Mar-17-2022, 09:35 PM)Gribouillis Wrote: [ -> ]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

alivegamer Wrote: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.

alivegamer Wrote: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")

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.

(Mar-17-2022, 10:04 PM)Gribouillis Wrote: [ -> ]
alivegamer Wrote: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.

alivegamer Wrote: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

(Mar-18-2022, 12:01 AM)Alivegamer Wrote: [ -> ]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

(Mar-18-2022, 12:28 AM)snippsat Wrote: [ -> ]
(Mar-18-2022, 12:01 AM)Alivegamer Wrote: [ -> ]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!

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