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from multiprocessing import Pool
def power (x, pwr=2):
 return x**pwr
import functools
run_test = functools.partial (power, pwr=3)
if __name__ == "__main__":
 pool = Pool()
 cases = [3,4,5]
 results = pool.map (run_test, cases)
TypeError: type 'partial' takes at least one argument
Traceback (most recent call last):
 File "/usr/lib/python2.5/site-packages/multiprocessing-2.6.1.1-py2.5-
 self.run()
 File "/usr/lib/python2.5/site-packages/multiprocessing-2.6.1.1-py2.5-
 self._target(*self._args, **self._kwargs)
 File "/usr/lib/python2.5/site-packages/multiprocessing-2.6.1.1-py2.5-
 task = get()
 File "/usr/lib/python2.5/site-packages/multiprocessing-2.6.1.1-py2.5-
 return recv()
TypeError: type 'partial' takes at least one argument

See this mail thread:
http://mail.python.org/pipermail/python-dev/2009-February/086034.html 

Rather than implement a multiprocessing-specific fix, it would certainly
be better to make functools.partial picklable, as pointed out in the ML
thread Jesse linked to.

> Antoine Pitrou <pitrou@free.fr> added the comment:
>
> Rather than implement a multiprocessing-specific fix, it would certainly
> be better to make functools.partial picklable, as pointed out in the ML
> thread Jesse linked to.
>
I would tend to agree

I agree that this is a nice feature, however it requires adding a 
getstate/setstate within the functools C code. 
I would need a patch which does this, and adds tests to the 
functools/pickle test suite for this. Also, the feature will only be 
accepted for py3k (3.1) currently. Features such as this will not be 
backported to the 2.x branch

Jack offered to take a peek

Fixed rev 70931. Happy pickling!

I think this bug still exists in Python 2.6.4, and I haven't tested 2.6.5, but since 2.6.4 was released 6 months after this bug was closed I assume it's still an issue. Running ndbecker's test program as is produces the following output on Solaris 10:
Process PoolWorker-1:
Process PoolWorker-2:
Traceback (most recent call last):
Traceback (most recent call last):
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/process.py", line 232, in _bootstrap
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/process.py", line 232, in _bootstrap
Process PoolWorker-3:
Traceback (most recent call last):
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/process.py", line 232, in _bootstrap
 self.run()
 self.run()
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/process.py", line 88, in run
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/process.py", line 88, in run
 self._target(*self._args, **self._kwargs)
 self._target(*self._args, **self._kwargs)
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/pool.py", line 57, in worker
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/pool.py", line 57, in worker
 self.run()
 task = get()
 task = get()
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/queues.py", line 352, in get
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/process.py", line 88, in run
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/queues.py", line 352, in get
 self._target(*self._args, **self._kwargs)
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/pool.py", line 57, in worker
 return recv()
 return recv()
TypeError: type 'partial' takes at least one argument
TypeError: type 'partial' takes at least one argument
 task = get()
 File "/opt/app/g++lib6/python-2.6/lib/python2.6/multiprocessing/queues.py", line 352, in get
 return recv()
TypeError: type 'partial' takes at least one argument

The bug still exists in 2.6.6
Process PoolWorker-2:
Traceback (most recent call last):
Process PoolWorker-1:
Traceback (most recent call last):
 File "/usr/lib/python2.6/multiprocessing/process.py", line 232, in _bootstrap
 File "/usr/lib/python2.6/multiprocessing/process.py", line 232, in _bootstrap
 self.run()
 self.run()
 File "/usr/lib/python2.6/multiprocessing/process.py", line 88, in run
 File "/usr/lib/python2.6/multiprocessing/process.py", line 88, in run
 self._target(*self._args, **self._kwargs)
 self._target(*self._args, **self._kwargs)
 File "/usr/lib/python2.6/multiprocessing/pool.py", line 57, in worker
 File "/usr/lib/python2.6/multiprocessing/pool.py", line 57, in worker
 task = get()
 task = get()
 File "/usr/lib/python2.6/multiprocessing/queues.py", line 352, in get
 File "/usr/lib/python2.6/multiprocessing/queues.py", line 352, in get
 return recv()
 return recv()
TypeError: type 'partial' takes at least one argument
TypeError: type 'partial' takes at least one argument

The bug was fixed in 2.7, and not backported to 2.6.
2.6 is now in security-fix-only mode, if possible you should upgrade to 2.7.

Unfortunately, upgrading to 2.7 is not an option for me, since my application works on Debian stable which only includes 2.6. For now I'm settled with the following workaround: http://paste.pocoo.org/show/351774/