如何在Python中的multiproccesing中排除“AttributeError: __exit__”?

I tried to rewrite some csv-reading code to be able to run it on multiple cores in Python 3.2.2. I tried to use the Pool object of multiprocessing, which I adapted from working examples (and already worked for me for another part of my project). I ran into an error message I found hard to decipher and troubleshoot. What's a good way to troubleshoot this? Thanks!I tried to rewrite some csv-reading code to be