A couple of people at PyCon Au mentioned running into this kind of issue with Python 3. It relates to the fact that:
1. String formatting is *coercive* by default
2. Absolutely everything, including bytes objects can be coerced to a string, due to the repr() fallback
So it's relatively easy to miss a decode or encode operation, and end up interpolating an unwanted "b" prefix and some quotes.
For existing versions, I think the easiest answer is to craft a regex that matches bytes object repr's and advise people to check that it *doesn’t* match their formatted strings in their unit tests.
For 3.4+ a non-coercive string interpolation format code may be desirable.
Sent from my phone, thus the relative brevity :)