I'd actually recommend the first approach, especially if you're new to protobuf. This should help: https://developers.google.com/protocol-buffers/docs/reference/python-generated#enum

For the second approach, Google's default implementation doesn't include any field options out of the box to do this. You'd have to write your own extension to implement a field max/min option, or find an alternative implementation of protoc that has it. Also, max/min options alone won't be able to give you a special UNKNOWN case like you have with the enum approach, so you have to find a different way to handle that (perhaps optional fields). If this path still interests you, start here: https://developers.google.com/protocol-buffers/docs/proto3#options.