E.g., wherever the back-end takes a string value, but requires that it be one of a set of known values? Likewise for integers which map protobuf enum values.

@dhermes suggested using https://pypi.python.org/pypi/enum.

I favor using the PEP-435-compatible https://pypi.python.org/pypi/enum34/, which backports the enum from the Python 3.4 stdlib.