Shared via mypy Playground

main.py

from typing import Any, cast, Generic, overload, Self, TypeVar

T = TypeVar('T')

class CustomProperty(property, Generic[T]):
    @overload  # type: ignore[override]
    def __get__(self, instance: None, owner: type[Any] | None, /) -> Self: ...

    @overload
    def __get__(self, instance: Any, owner: type[Any] | None, /) -> T: ...

    def __get__(self, instance: Any, owner: type[Any] | None = None) -> T | Self:
        return cast(T | Self, super().__get__(instance, owner))

    def a_function(self) -> T: ...

def custom_property(type: type[T]) -> CustomProperty[T]:
    return CustomProperty(type)


module_level = custom_property(int)
reveal_type(module_level)                               # CustomProperty[int]
reveal_type(module_level.a_function())                  # int

class ClassWithCustomProperty:
    property_holder = custom_property(int)
    reveal_type(property_holder)                        # CustomProperty[int]

reveal_type(ClassWithCustomProperty.property_holder)    # CustomProperty[int]
reveal_type(ClassWithCustomProperty().property_holder)  # int