Comment by tln
14 days ago
How does that work in practice? Python stores local variables in the stack frame with a compiler-calculated index. Do you have a growable stack frame? Add stack frames for every loop iteration?
I don't think the Python compiler is to know if a loop variable is closed over and then adjust the code generation. So this change would likely introduce a lot of complexity, or be a pessimisation in the normal case.
I think the simple, although less helpful, semantics are clearly better here. FWIW I don't think this comes up in practice very much. Maybe because callbacks in Python APIs aren't common (and its not due to this).
See my other reply about how [x for x in y] now works. It'd be a variation on that.
I agree the current semantics are better by the measure of performance of the simplest possible implementation. Though that's not Python's main design goal.
it comes up often enough for me to be annoying. The defaulted argument works, but it is an ugly hack. It is not obvious to me that the surprising semantics are clearly better, but python has a long history to preferring implementation simplicity to user ergonomics.