Missing reportUnnecessaryComparison when comparing objects of different types

[tkukushkin]

Hello! I've noticed, that pyright doesn't report reportUnnecessaryComparison when I compare two objects of different types with is operator:

class Foo:
    pass

def test(x: list[Foo], y: Foo) -> None:
    if x is y:
        pass

I understand why this might not work with ==, but I think this should work with is. It works with int for example.

[erictraut]

What you're suggesting isn't safe. It's possible to create a class that subclasses from both Foo and list.

class Bar(Foo, list):
    pass

b = Bar()
test(b, b)

The reportUnnecessaryComparison check must be rather conservative to avoid false positive errors. This is true even for is and is not narrowing.

[tkukushkin]

But is pyright able to understand that Foo is not list as it seems it does with int and list[int]?

[erictraut]

I'm not sure what you mean. Could you provide a code sample?

[tkukushkin]

def test(x: list[int], y: int) -> None:
    if x is y:
        pass

Gives Condition will always evaluate to False since the types "list[int]" and "int" have no overlap (reportUnnecessaryComparison)

Is it so difficult to derive that Foo doesn't inherit from list? Or am I looking at the problem too narrowly and realising that types don't overlap in the general case is too difficult?

[erictraut]

There are some heuristics involved in pyright's check here. It assumes that if the types are both built-in types like int and list, then it's not possible to create a subclass of the two types. Indeed, if you try to create a class that subclasses from both int and list, you will see that the runtime doesn't allow this. Compare this to my example above where it is possible to create a subclass of both Foo and list.

All else equal, when we need to choose between a potential false positive and a potential false negative, we tend to opt for the false negative. That informs the heuristics in this check.

In any case, pyright is working as intended here.

[tkukushkin]

Got it, thanks!