A couple of years ago I developed a Python interface for a Raspberry Pi talking to a robot being driven by an Arduino. I had gotten the idea for the library from somewhere but the library that gave me the idea was incomplete so I took the idea an implemented my own solution.
Best posts made by Will Kostelecky
Python Library for Arduino Dock and I2C
RE: Trying to use I2C with Python but Library not Found [RESOLVED]
If anyone else runs into this problem...I think, though I am not sure, that my problem was that I had not upgraded my onion to the latest firmware? Once I did so things worked as documented here:
I reset to factory settings just to be sure and everything still works.