Support option for type inference to default to immutable types when possible

[justinaustin]

Feature

Add an option for mypy, maybe something like --infer-immutable-when-possible, that would cause mypy to infer immutable version of types (like Sequence, Mapping, FrozenSet) instead of mutable (List, Dict, Set)

With this feature turned on the below code:

x = [1, 2, 3]
x.append(4)

mypy would give error: "Sequence[int]" has no attribute "append" as it would infer x to be Sequence[int] instead of List[int]

Pitch

Many developers support the idea of immutability by default for a number of reasons, and adding this option would be a nice way for them to opt-in to some version of this with mypy. Currently if I want my variable x = [1, 2, 3] to be Sequence[int] instead of List[int], I need to explicitly add a Sequence[int] type annotation, which is easy to forget. If I had this feature option turned on, I would instead need to explicitly opt-in to mutability with x: List[int] = [1, 2, 3]

I'd love to find out if this feature is something that would be 1. feasible and 2. welcome in the project. If so, I could take a shot with implementing it!

[eyaler]

Sequence does not support add for concatenation, so such inference would prevent more than just mutation. In this case, will the type checker know that the inference is not possible? Also how does the inference option relate to variance? e.g. I imagine callable return types should not change from List to Sequence.

[hauntsaninja]

There haven't been any other requests for this feature. I think it wouldn't be usable in most Python programs, and could interact surprisingly with narrowing. I think #5756 would be the good, explicit way to do this.