Feature proposal: Warn when narrowing to a mutable type

Feature

I often encourage people to use "immutable" types for function parameters, as a way to indicate that the passed data structures won't be mutated by the function. Of course that isn't a guarantee, but a nice indication that can make it easier to reason about code like this:

vals = [1, 2, 3]
operation(vals)
print(vals)  # What is vals here?

If operation is typed with Sequence we can usually assume that vals isn't mutated by it. However, right now it wouldn't be a type error for operation to be implemented like this, which breaks that indication.

def operation(vals: Sequence[int]) -> None:
    assert isinstance(vals, list)
    vals += [3, 4, 5]

Here's the above example on mypy playground.

Pitch

I think it would be a valuable feature for mypy to warn when it narrows from a known immutable type such as Sequence to a known mutable type such as list.

Inizia con l’esempio mypy playground collegato e verifica come viene attualmente gestito il narrowing da Sequence a list. Il lavoro è completato quando mypy è in grado di identificare questo narrowing da un tipo immutabile noto a un tipo mutabile noto ed emettere l’avviso proposto senza modificare il comportamento degli altri narrowing.