The problem I see with proxy objects for functions/coroutines/generators is that it *doesn't* match how locals() currently behaves in the absence of a tracing function - that gives you a "single shared snapshot" behaviour, where writes to the result of locals() *don't* affect the original namespace.

I agree that replacing frame.f_locals with a write-through proxy would be a good way to get rid of PyFrame_LocalsToFast, though (and thus fix the bug this issue covers).

The point where we disagree is that I think we should replace the tracing-or-not distinction with a locals()-or-frame.f_locals distinction, not get rid of the distinction entirely.