I was looking to implement multiprocess pool. It worked fine with the numpy function while with the user defined function it ran into error.

import numpy
>>> import multiprocessing
>>> P = multiprocessing.Pool(5)
>>> P.map(numpy.sqrt,range(50))
 [0.0, 1.0, 1.4142135623730951, 1.7320508075688772, 2.0, 2.2360679774997898,     2.4494897427831779, 2.6457513110645907, 2.8284271247461903, 3.0, 3.1622776601683795, 3.3166247903553998, 3.4641016151377544, 3.6055512754639891, 3.7416573867739413, 3.872983346207417, 4.0, 4.1231056256176606, 4.2426406871192848, 4.358898943540674, 4.4721359549995796, 4.5825756949558398, 4.6904157598234297, 4.7958315233127191, 4.8989794855663558, 5.0, 5.0990195135927845, 5.196152422706632, 5.2915026221291814, 5.3851648071345037, 5.4772255750516612, 5.5677643628300215, 5.6568542494923806, 5.7445626465380286, 5.8309518948453007, 5.9160797830996161, 6.0, 6.0827625302982193, 6.164414002968976, 6.2449979983983983, 6.324555320336759, 6.4031242374328485, 6.4807406984078604, 6.5574385243020004, 6.6332495807107996, 6.7082039324993694, 6.7823299831252681, 6.8556546004010439, 6.9282032302755088, 7.0]
>>> def f(x):
      return x*x

>>> P.map(f, range(50))
Exception in thread Thread-2:
Traceback (most recent call last):
File "C:\Python27\lib\threading.py", line 530, in __bootstrap_inner
self.run()
File "C:\Python27\lib\threading.py", line 483, in run
self.__target(*self.__args, **self.__kwargs)
File "C:\Python27\lib\multiprocessing\pool.py", line 285, in _handle_tasks
put(task)
TypeError: expected string or Unicode object, NoneType found

Multiprocessing works by firing up additional processes. Those processes won't have access to functions defined in the interactive interpreter.

Can you reproduce this problem in a standalone script? I suspect not, but if you can please post the script here.

Marking as "not a bug", but I haven't closed it yet in case a script reproducing the issue can be provided.

Yes actually it produce no erroe on standalone script. But the script
executes without any outputs. not even for numpy function. I used code
below in script.
import multiprocessing
import numpy
def f(x):
    return x*x

if __name__ = "__main__":
    p= multiprocessing.Pool(5)
    print p.map(numpy.sqrt,[1,2,3,4])
    print p.map(f,[1,2,3,4])
 On May 15, 2015 2:52 PM, "Paul Moore" <report@bugs.python.org> wrote:

>
> Paul Moore added the comment:
>
> Multiprocessing works by firing up additional processes. Those processes
> won't have access to functions defined in the interactive interpreter.
>
> Can you reproduce this problem in a standalone script? I suspect not, but
> if you can please post the script here.
>
> Marking as "not a bug", but I haven't closed it yet in case a script
> reproducing the issue can be provided.
>
> ----------
> nosy: +paul.moore
> resolution:  -> not a bug
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue24202>
> _______________________________________
>

OK, if it's not reproducible in a standalone script, I'll close this as it's expected behaviour.

Correcting the typo in your script (__name__ == "__main__"), I ran it and it worked as expected on my system:

>type multi.py
import multiprocessing
import numpy
def f(x):
    return x*x

if __name__ == "__main__":
    p= multiprocessing.Pool(5)
    print(p.map(numpy.sqrt,[1,2,3,4]))
    print(p.map(f,[1,2,3,4]))
PS 15:05 {00:00.089} C:\Work\Scratch
>py .\multi.py
[1.0, 1.4142135623730951, 1.7320508075688772, 2.0]
[1, 4, 9, 16]

I'm not sure why you weren't getting output, but it doesn't look like a Python issue.