Child processes created with python multiprocessing module won't print

Question

I have a problem with the code below, and with any code that uses the print function in the child processes: I can't see any printed statements, even if I use sys.std[err|out].write('worker') in place of print.

This is the code (from the official python documentation):

from multiprocessing import Process

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

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

The output is blank.

Note: the following code, using the threading module, works (prints hello bob):

import threading

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

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

Can you please point me to the solution? Thanks in advance.

Answer 1

Try this:

from multiprocessing import Process
import sys

def f(name):
    print 'hello', name
    sys.stdout.flush()

...

AFAIK the standard output of processed spawned by the multiprocessing module is buffered, hence you will see the output only if the buffer becomes full or you explicitly flush sys.stdout.

Answer 2

The docs for multiprocessing clearly explain why this won't work!

"Note: Functionality within this package requires that the _main_ method be importable by the children. This is covered in Programming guidelines however it is worth pointing out here. This means that some examples, such as the multiprocessing.Pool examples will not work in the interactive interpreter."