Why does the "is" keyword not narrow the type

[lovetox]

class A:
    def __init__(self, test: int) -> None:
        pass

class B:
    def __init__(self, test: str) -> None:
        pass

cls_list = {'a': A, 'b': B}

cls_ = cls_list['a']

if cls_ is A:
    cls_(1)

This does not narrow the type to A, pyright still thinks cls_ can be A | B

[erictraut]

The type of cls_ prior to the if statement is A | B. That means cls_ could be A, B, or any subclass of either type.

It's not safe to narrow in this case because B is not final. There could be another type C that is a subclass of both A and B (and therefore a subclass of B). If you mark B as @final, the narrowing will work as you expect.

[lovetox]

I cannot follow your argument.
The is keyword tests for identity.
How does it matter that other objects (Subclass of A and B) could exist, when they would not have the same identity as A.
The condition tells me that it must be A, not a subclass. It would evaluate to False if i compare cls_ with any subclass of A or B or both.

[erictraut]

Ah, you're correct. Since the RHS is a specific class A, as opposed to a variable of type type[A], we can safely narrow the type here. I'll include a fix in the next release.