type-comparison (E721) doesn't work when comparing against an actual type

[DetachHead]

foo = 1
if type(foo) is int: # no error
    ...
if type(foo) is type(1): # error
    ...

the pylint rule unidiomatic-typecheck catches both cases

[KotlinIsland]

@charliermarsh Could you please clarify if the change here applies to only builtin types? And if so why, because in pylint it applies to any type, not just builtins:

class A: ...
a = A()
type(a) == A  # C0123: Use isinstance() rather than type() for a typecheck. (unidiomatic-typecheck)

[charliermarsh]

I believe that allowing arbitrary expressions on the right-hand side will lead to false positives for legitimate usages, as in: PyCQA/pycodestyle#882 (comment).

[KotlinIsland]

Oh, is this because Ruff isn't able to tell if the expression on the rhs is a type or not?

[charliermarsh]

Yes that's correct. E.g., this is contrived but seems reasonable?

type_of_b = type(b)
type(a) is type_of_b

Pylint flags this under unidiomatic-typecheck, but it seems fine to me.

[KotlinIsland]

Yes that's correct.

If Ruff can't determine if the rhs is a type then I understand that it's better to be less strict here.

E.g., this is contrived but seems reasonable?

type_of_b = type(b)
type(a) is type_of_b

Pylint flags this under unidiomatic-typecheck, but it seems fine to me.

I would write:

type_of_b = type(b)
isinstance(a, type_of_b)

Assuming the context is that I'm just dealing with some kind of usual operation where what I really want is a compatible subtype, and that I don't want to be checking for the exact same type.

Isn't the whole point of the inspection to encourage isinstance over ==/is for types because isinstance is aware of subtypes but ==/is isn't?

class A: ...
class B(A): ...
b = B()
isinstace(b, A)  # True
type(b) == A  # False

[hauntsaninja]

Would you accept a PR with the naive autofix for this?

[charliermarsh]

Yeah, we can make it a suggested fix.

(Also open to further refinements on the rule logic itself, I’m not certain that our criteria is optimal given above discussion.)