Correctly type-hinting a Descriptor

[d-tw]

Hi,

I'm trying to type-hint a Descriptor protocol, but encountering some issues with variance.

In the following code snippet, the descriptor is type-hinted such that when it is accessed on an instance, it returns Optional[ValueType], and when it is accessed on a class, it returns FieldType. In the example, I've used trivial types for ValueType and FieldType.

pyright (version 1.1.143) complains that the ValueType type variable should be contravariant, but ValueType shouldn't be contravariant as it is used as a return type (any pyright raises this exact error if I mark ValueType as being contravariant).

Is there an issue with my type-hinting? Is this an issue with pyright? I'm not sure how to understand or fix the issue.

1 - Invariant ValueType

"""Sample."""

from typing import Optional, Protocol, Type, TypeVar, Union, overload

ValueType = TypeVar('ValueType')
FieldType = TypeVar('FieldType', covariant=True)
T = TypeVar('T')


class MyDescriptor(Protocol[ValueType, FieldType]):
    @overload
    def __get__(self, instance: None, owner: type) -> FieldType:
        ...

    @overload
    def __get__(self, instance: T, owner: Type[T]) -> Optional[ValueType]:
        ...

    def __get__(self, instance: Optional[T], owner: Type[T]) -> Union[ValueType, FieldType]:
        ...

    def __set__(self, instance: object, value: Optional[ValueType]) -> None:
        ...


class MyClass:
    foo: MyDescriptor[int, float]


def my_fn(c: MyClass):
    reveal_type(MyClass.foo)  # noqa: F821
    reveal_type(c.foo)  # noqa: F821

Throws the following error:

Found 1 source file
.../sample.py
  .../sample.py:10:7 - error: Type variable "ValueType" used in generic protocol "MyDescriptor" should be contravariant (reportInvalidTypeVarUse)
 .../sample.py:31:17 - info: Type of "MyClass.foo" is "float"
  .../sample.py:32:17 - info: Type of "c.foo" is "int | None"
1 error, 0 warnings, 2 infos 
Completed in 0.682sec

2 - Contravariant ValueType

"""Sample."""

from typing import Optional, Protocol, Type, TypeVar, Union, overload

ValueType = TypeVar('ValueType', contravariant=True)
FieldType = TypeVar('FieldType', covariant=True)
T = TypeVar('T')


class MyDescriptor(Protocol[ValueType, FieldType]):
    @overload
    def __get__(self, instance: None, owner: type) -> FieldType:
        ...

    @overload
    def __get__(self, instance: T, owner: Type[T]) -> Optional[ValueType]:
        ...

    def __get__(self, instance: Optional[T], owner: Type[T]) -> Union[ValueType, FieldType]:
        ...

    def __set__(self, instance: object, value: Optional[ValueType]) -> None:
        ...


class MyClass:
    foo: MyDescriptor[int, float]


def my_fn(c: MyClass):
    reveal_type(MyClass.foo)  # noqa: F821
    reveal_type(c.foo)  # noqa: F821

Throws the following error:

Found 1 source file
../sample.py
  ../sample.py:16:55 - error: Contravariant type variable cannot be used in return type
    TypeVar "ValueType@MyDescriptor" is contravariant (reportGeneralTypeIssues)
  ../sample.py:19:65 - error: Contravariant type variable cannot be used in return type
    TypeVar "ValueType@MyDescriptor" is contravariant (reportGeneralTypeIssues)
  ../sample.py:31:17 - info: Type of "MyClass.foo" is "float"
  ../sample.py:32:17 - info: Type of "c.foo" is "int | None"
2 errors, 0 warnings, 2 infos 
Completed in 0.591sec

[erictraut]

Thanks for the bug report. Your second example (where ValueType is contravariant) should type check without errors. The check for a contravariant type variable used as a return type should ignore cases when it is part of a union. I've addressed the bug, and the fix will appear in the next release. In the meantime, you can use a # type: ignore comment to suppress the errors.