yet another confusion with multiprocessing error, 'module' object has no attribute 'f'

Question

I know this has been answered before, but it seems that executing the script directly "python filename.py" does not work. I have Python 2.6.2 on SuSE Linux.

Code:

#!/usr/bin/python # -*- coding: utf-8 -*- from multiprocessing import Pool p = Pool(1) def f(x):     return x*x p.map(f, [1, 2, 3]) 

Command line:

> python example.py Process PoolWorker-1: Traceback (most recent call last): File "/usr/lib/python2.6/multiprocessing/process.py", line 231, in _bootstrap     self.run() File "/usr/lib/python2.6/multiprocessing/process.py", line 88, in run     self._target(*self._args, **self._kwargs) File "/usr/lib/python2.6/multiprocessing/pool.py", line 57, in worker     task = get() File "/usr/lib/python2.6/multiprocessing/queues.py", line 339, in get     return recv() AttributeError: 'module' object has no attribute 'f' 

Answer 1

Restructure your code so that the f() function is defined before you create instance of Pool. Otherwise the worker cannot see your function.

#!/usr/bin/python # -*- coding: utf-8 -*-  from multiprocessing import Pool  def f(x):     return x*x  p = Pool(1) p.map(f, [1, 2, 3]) 

Answer 2

This one works:

#!/usr/bin/python # -*- coding: utf-8 -*- from multiprocessing import Pool  def f(x):     return x*x  if __name__ == "__main__":     p = Pool(1)     p.map(f, [1, 2, 3]) 

I'm not 100% sure why your code does not work, but I guess the reason is that child processes launched by the multiprocessing module try to import the main module (to have access to the methods you defined), and the if __name__ == "__main__" stanza is required not to execute the initialization code where you set up your pool.