Using `isinstance(obj, A | B)` incorrectly requires generic parameters for classes

[bryanforbes]

Describe the bug
Given the following code:

from __future__ import annotations

from typing import Any, Generic, TypeVar

T = TypeVar('T')


class Thing(Generic[T]):
    prop: T


class OtherThing:
    ...


def func(thing: Any) -> None:
    if isinstance(thing, Thing | OtherThing):
        ...

An error is reported at line 17:

Expected type arguments for generic class "Thing"

Expected behavior
No errors are reported

VS Code extension or command-line
Are you running pyright as a VS Code extension, a language server in another editor, or the command-line tool?
Command line

Which version?
1.1.314

[erictraut]

Thanks for the bug report. This will be addressed in the next release.

[Hootner]

It seems that you are reporting that an error is being reported at line 17, although there shouldn't be one. The error message mentions "Expected type arguments for generic class 'Thing'". You are running Pyright version 1.1.314 as a command-line tool. I understand the issue and will try to help you out.

The error you are encountering is because you are missing the type argument for Thing at line 17. Since Thing is a generic class, it should take a type argument like Thing[int] or Thing[str] when used. The problem is occurring because isinstance is used with a union of both Thing and OtherThing as the second argument. Instead of using Thing | OtherThing, you need to replace it with the appropriate type argument for Thing.

One way to correct the code is to use a type variable for the type of thing when using the union:

from __future__ import annotations

from typing import Any, Generic, TypeVar

T = TypeVar('T')
U = TypeVar('U', bound='Thing')  # Added a bound to specifically target the Thing class


class Thing(Generic[T]):
    prop: T


class OtherThing:
    ...


def func(thing: Any) -> None:
    if isinstance(thing, U | OtherThing):  # Replaced Thing with U
        ...

By adding the bound U, you tell the type checker that U should be an instance of a subclass or a subclass of an instance of Thing. This way, you preserve the generic nature of Thing and resolve the error.

[erictraut]

This is included in pyright 1.1.315, which I just published. It will also be included in this week's insiders release of pylance.

[mgmira]

I'm still being able to reproduce this in specific conditions, with latest version of pyright

# pyright: strict

from __future__ import annotations

from typing import Any


class RestApi:
    @staticmethod
    def output_json(data: int | str | dict[str, Any]):
        if isinstance(data, dict | str):
            return data

[erictraut]

Yeah, sorry about that. There was a second (infrequent) code path that needed to be updated as well. It will be addressed in the next release.