I'm just trying to figure out what breaks if you have every rule code be equivalent to the 'prefix' of the offset. The only place I see is with is_subperiod/is_superperiod (everywhere else is either using as an error message or effectively only using the prefix) and I'm concerned that this area may not be well tested and I'd hesitate to do it so close to a release. Let's go with your suggestion of defining some attribute on the class for the mapping, and then warn that rule_code will change in 0.14.