Python's multiprocessing map_async generates error on Windows

Question

The code below works perfectly on Unix but generates a multiprocessing.TimeoutError on Windows 7 (both OS use python 2.7).

Any idea why? Thanks.

from multiprocessing import Pool

def increment(x):
    return x + 1

def decrement(x):
    return x - 1

pool = Pool(processes=2)
res1 = pool.map_async(increment, range(10))
res2 = pool.map_async(decrement, range(10))

print res1.get(timeout=1)
print res2.get(timeout=1)

Answer 1

You need to put your actual program logic in side a if __name__ == '__main__': block.

On Unixy systems, Python forks, producing multiple processes to work from. Windows doesn't have fork. Python has to launch a new interpreter and re-import all your modules instead. This means that each subprocess will reimport your main module. For the code you've written reimporting the module will cause each newly launched processes to launch processes of its own.

See: http://docs.python.org/library/multiprocessing.html#windows

EDIT this works for me:

from multiprocessing import Pool

def increment(x):
    return x + 1

def decrement(x):
    return x - 1

if __name__ == '__main__':
    pool = Pool(processes=2)
    res1 = pool.map_async(increment, range(10))
    res2 = pool.map_async(decrement, range(10))

    print res1.get(timeout=1)
    print res2.get(timeout=1)