isinstance with generic type

[jhance]

For example

class A(Generic[T]):
   pass

class B(Generic[T], A[T]):
   pass

def f(x: A[T]):
    if isinstance(x, B[T]):
        ...

is apparently illegal and I'm not sure what replacement I'm supposed to use. It tells me to use a # type comment which isn't really applicable here.

Removing the [T] causes mypy to be unable to infer that x is in fact a B[T].

[JukkaL]

Hmm this looks like a mypy bug. It should work like this:

class A(Generic[T]):
   pass

class B(Generic[T], A[T]):
   pass

def f(x: A[T]):
    if isinstance(x, B):
        # type of B is B[T] here
        ...

[JukkaL]

And if you try to use isinstance(x, B[T]), mypy should give a useful error message.

[jhance]

That seems kind of limited but I guess its the best we can do given we can't modify the behavior of isinstance itself to respect generics...

I don't like it because it could be both an A[T] and a B[U] with multiinheritance...

[JukkaL]

We are stuck with isinstance semantics here. However, isinstance doesn't seem to be used that often with generic types, at least in the code I've looked at.

[jhance]

The issue is there logically isn't anything we can infer here, as far as I can tell. Since isinstance does not respect generics, we can't either. The best we can do is give an error message instructing the user that they need to cast, or to assume that they want to use the same type variables (but this could be very misleading or, even worse, give an incorrect type).

The use-case I can think of for isinstance with generic types is if you have an AST (something like Node) but you want the option to parameterize the AST (so that different passes can add different data - this type of data wold be the annotation, and then the type-checker would enforce whether or not that pass has occurred yet, which could potentially be quite useful).

[elazarg]

Related #3037

[ilevkivskyi]

On current master it behaves like this:

class A(Generic[T]):
   pass

class B(A[T]):
   pass

x: A[int]
if isinstance(x, B):
    reveal_type(x)  # Revealed type is '__main__.B[Any]'
isinstance(x, B[int])  # Parameterized generics cannot be used with class or instance checks

It looks like the issue is fixed only partially. I think the revealed type should be B[int] (even if it is not 100% safe).

[sobolevn]

Are there any known work-arounds?

I mean: are there any reliable ways to ensure that x is instance of List[int] or MyCustomGeneric[str]?

Things I have tried:

• isinstance, results in TypeError
• pytypes, does not work: Cannot use typing.Generic with pytypes Stewori/pytypes#82
• typeguard, does not work: custom generics are not supported
• typing_inspect, does not have this feature

Is there anything I am missing?

[Stewori]

For the record, typechecking of generics does work in pytypes (Python 3.7+ support is almost done but not released as of this writing). The issue linked above rather required type inference which is beyond the scope of pytypes. See the specific example that was requested and the answers for clarification.

[AlexWaygood]

In 2022, the way to achieve "slightly unsafe" type-narrowing of generics in this fashion is to use typing.TypeGuard (or typing_extensions.TypeGuard on Python <=3.9).