Looks nice! I've not really figured out how this interacts in the integer case, since it's typed as str choices.

Also, looks like the test case needs some tweaking?... https://travis-ci.org/encode/orm/jobs/654198625?utm_medium=notification&utm_source=github_status

@tomchristie My mypy does complain that it expects a str in choices. However, the tests show that boolean and integer choices are handled okay. Maybe the choices class types can be relaxed a bit, or maybe something else is happening here (the class maybe converts everything to string before comparing, which can have undesirable results, or it might in some cases require to use string only functions, breaking the functionality.)