How can I run processes in Python?

Question

I searched some examples in the official python page, others pages and also books, but processes doesn't work. In this example I only print a message that is passed as a parameter to each function. I don't know why, but processes don't work in Windows because I tried in 3 different computers(2 with Windows and other with Linux) and a web page to program with Python and process don't work in Windows

from multiprocessing import Process
from time import sleep

def worker(msg):
    
    for i in range(0, 10):
        
        print(msg, end='', flush=True) # <----- print msg

print('Starting')

t2 = Process(target=worker, args=('A',))
t3 = Process(target=worker, args=('B',))
t4 = Process(target=worker, args=('C',))

t2.start() # <---------------------------
t3.start() # <----- they don't work -----
t4.start() # <---------------------------

t2.join()
t3.join()
t4.join()

print('Done')

This is printed as output:

Starting

Done

Your edited example works for me - I get this output: https://i.imgur.com/jt0vDtJ.png — Nick ODell, Sep 18 '21 at 17:36
I tried this example in windows in 2 computers and a web page with python, I don't know why, but proccesses don't work in windows. — Br0k3nS0u1, Sep 18 '21 at 21:09
It works for me with Pycharm under Windows, but with the “if name main” thing. — quamrana, Sep 18 '21 at 21:12
Thank you guys, It is due to "if __name__ == '__main__'", but why I have to do it of this way? — Br0k3nS0u1, Sep 18 '21 at 21:26
Maybe [This](https://stackoverflow.com/a/20360812/13354437) could help — Almog-at-Nailo, Sep 19 '21 at 08:37

score 2 · Answer 1 · answered Sep 18 '21 at 17:16

2

You're not waiting for the child processes to stop; once the parent program ends, they're killed.

Add

t2.join()
t3.join()
t4.join()

after your print('Done') to give them a chance to do their thing.

Also, args must be a tuple of arguments, not a single string.

answered Sep 18 '21 at 17:16

AKX

152,115
15
115
172

I usually add: `print('Done')` to the very end of my scripts to show that it ended when I wanted it to end. Do you think the OP wanted this as well? – quamrana Sep 18 '21 at 17:18
If I use "join()" and I remove "sleep()" It still doesn't work. – Br0k3nS0u1 Sep 18 '21 at 17:21
1

@quamrana Perhaps it was "done starting"? :) – AKX Sep 19 '21 at 13:33

score 0 · Accepted Answer · edited Sep 19 '21 at 08:22

0

Thank to you guys I could solve this.

The error was due to that I didn't use if __name__ == "__main__":, but other error is that I used eclipse with pydev, I don't know why, but I can't do it with eclipse and pydev (I think is limited) until I tried it in sublime text 3 and pycharm.

edited Sep 19 '21 at 08:22

quamrana

37,849
12
53
71

answered Sep 18 '21 at 22:31

Br0k3nS0u1

51
5

How can I run processes in Python?

2 Answers2