CODEWITHSUNDEEP
python
PlasticDuck
PlasticDuck

Reputation: 105

Python multiprocessing returns nothing

Multiprocessing Process class example in python documentary seem to return None. I copy pasted the example and run it, but it returns only ">>>" instead of printing 'hello', name. Tested in 2.7.11 and 3.5.1. I must be missing something obvious here, but can't figure out what.

from multiprocessing import Process

def f(name):
    print('hello', name)

if __name__ == '__main__':
    p = Process(target=f, args=('bob',))
    p.start()
    p.join()

--Edit-- It works in Linux Ubuntu 14.04. Not in Windows 7. --Edit-- Seems that the main issue is that whether it is run from IDLE or from file. Works when run from file.

Upvotes: 0

Views: 619

Answers (2)

Hannes Ovrén
Hannes Ovrén

Reputation: 21831

Since you mention the >>> prompt, it sounds like you are running the example in the interactive interpreter? In that case, the if __name__ == "__main__" line will never evaluate to True and thus never run the code you want. You can read about this in the documentation.

Try either saving to a script and run that script, or just remove the if-clause:

from multiprocessing import Process

def f(name):
    print('hello', name)

p = Process(target=f, args=('bob',))
p.start()
p.join()

Upvotes: 1

Klaus D.
Klaus D.

Reputation: 14369

At first there is a large difference between returning and printing. Your code has no return so it is only printing.

An other process has a different stdout associated to which printed text will go. You will not see it until you explicitly connect it to the stdout of the main process or process it in an other way. The subprocess.Popen() allows to do that, multiprocessing.Process() does not as far as I am aware.

Upvotes: 0

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