Python multiprocessing PicklingError: Can't pickle <type 'function'>

I am sorry that I can't reproduce the error with a simpler example, and my code is too complicated to post. If I run the program in IPython shell instead of the regular Python, things work out well. I looked up some previous notes on this problem. They were all caused by using pool to call function defined within a class function. But this is not the case for me. Exception in thread Thread-3: Trac

[incep]

"In particular, functions are only picklable if they are defined at the top-level of a module."