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/