Pass variable as final about typing HOT 4 OPEN

Yes, but that's not what Final means. Final is closer to const, i.e. you can't reassign the name to something else, but it doesn't prevent mutations.

x: Final[dict[str, int]] = {}
x["five"] = 5   # this is allowed
x.clear()  # this is also allowed
x = {}  # but this is forbidden

But tracking mutations statically in Python is tricky if not impossible (especially in pyi files), hence the additional need for telling the type checker which methods will cause an object to be mutated. So this is a massive change and will require a PEP, it's not as simple as allowing Final in function signatures.

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There's also this existing discussion on the Python Discourse: https://discuss.python.org/t/extend-the-typing-final-type-qualifier-to-support-function-arguments/41916

I personally don't think Final is a good equivalent for the absence of Rust's mut. They actually mean quite different things and an Immutable modifier would be difficult to support from a type checker's perspective unless you treat all method calls as mutations by default and then also add an @idempotent decorator to mark methods that are fine to call on Immutable references to objects.

Also you are using str in your example which is an immutable type in Python, so your example never modifies changeable or unchangeable regardless, it will always create new str instances, even for +=.

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