I think it would have been better to say "internal library code for foundational projects." That code that takes advantage of all the freedom in Python to mess

I think it would have been better to say "internal library code for foundational projects." That code that takes advantage of all the freedom in Python to mess with types, generate classes on the fly, use type annotations at runtime, add and remove attributes dynamically. Code that is basically a notch away from `eval` (or sometimes literately for dataclasses). There's no way you'll ever be able to fully type this stuff outside of Any Any Any Any. But to be useful, the boundary between your library and it's user should ideally be fully typed so the type checker can help your users use the library correctly.