narrowing to generic shouldn't introduce new `Any`

[beauxq]

Bug Report

If I don't put any Any (and don't import anything with Any), there shouldn't be any Any .

But if I use isinstance to narrow to a generic, a new Any is introduced.

To Reproduce

--strict

def f(x: object) -> None:
    if isinstance(x, Iterable):
        reveal_type(x)  # `Iterable[Any]`
        # an `Any` is introduced, when I didn't put any `Any` in my code
        # should be `Iterable[object]`

        for y in x:
            reveal_type(y)  # `Any` should be `object`
            print(y + 3)  # not safe - should report type error

mypy playground

This is with --strict It's probably ok to put this Any there in the default mode.

Also, even in --strict, if narrowing from Any, then it's ok to narrow the type variable to Any.

def g(x: Any) -> None:
    if isinstance(x, Iterable):
        reveal_type(x)
        # `Iterable[Any]` is ok here, because we narrowed from `Any`

        for y in x:
            reveal_type(y)  # `Any`
            print(y + 3)  # silenced by `Any`


# current behavior here is good - want to make sure it's not lost
def h(x: Iterable[int] | None) -> None:
    if isinstance(x, Iterable):
        reveal_type(x)  # `Iterable[int]`

        for y in x:
            reveal_type(y)  # `int`
            print(y + 3)

Your Environment

• Mypy version used: 1.8.0
• Mypy command-line flags: --strict
• Python version used: 3.12

[chepner]

From https://docs.python.org/3/library/typing.html#user-defined-generic-types: "Using a generic class without specifying type parameters assumes Any for each position."

isinstance(x, Iterable) is equivalent to isinstance(x, Iterable[Any]).

[beauxq]

That is not something that should be followed if we want type safety, and mypy already recognizes this. With --strict this reports an error:

x: Iterable = []