The main problem with such approach is that `class _RealShipOpts:` is very ugly to write unit tests for. You need to import a private entity in tests. I would slightly change the presented approach, and move the "public" `ShippingOptions`, `shipFast`, etc., into a new module that is a public API, for my users to use something like `from my_lib.shipping.api import ShippingOptions`.

That way, I can use "normal" naming in `class RealShipOpts:...`, and be explicit that it's not really public for the end users (they should use the `.api` module instead).