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When trying to send an object via a Queue that can't be pickled, one gets a quite unhelpful traceback:
Traceback (most recent call last):
 File "/usr/lib/python2.6/multiprocessing/queues.py", line 242, in _feed
 send(obj)
PicklingError: Can't pickle <type 'module'>: attribute lookup __builtin__.module failed
I have no idea where I am sending this. It would be helpful to get the call trace to the call to Queue.put.
My workaround was to create a Queue via this function MyQueue:
def MyQueue():
 import cPickle
 def myput(obj, *args, **kwargs):
 cPickle.dumps(obj)
 return orig_put(obj, *args, **kwargs)
 q = Queue()
 orig_put, q.put = q.put, myput
 return q
That way I get the pickle exception in the caller to put and was able to find out the offending code.