Upcasting

[dbrattli]

Describe the bug

Would it be possible to get upcasting using a function to type check its arguments? Or more generally that pyright checks that arguments with Type annotation of a generic type e.g Type[T] are compatible with other arguments where the generic type T is also used.

To Reproduce

from typing import TypeVar, Type
T = TypeVar("T")

class Base:
    pass

class Derived(Base):
    pass

class Other:
    pass

def upcast(type: Type[T], expr: T) -> T:
    return expr

d = Derived()
x = upcast(Other, d)

Would it be possible to get pyright to type check the last line and give error? Currently it will not detect that the function upcast is given two incompatible arguments. Here is how it looks in vscode today:

upcast(Base, d)  # should not show error
upcast(Other, d)  # should show error

Currently no error will show even if Other is not a super type of the type of d:

VS Code extension or command-line

vscode extension: 1.1.94

[erictraut]

Pyright is doing the right thing here. It's correctly "solving" for the type of TypeVar T, and there are no type violations, so it's not displaying any errors.

I might be able to suggest a solution if I better understood what you're trying to do here. A derived class is always compatible with its base class, so there's no need to explicitly upcast.

If you want the static type checker to test whether an expression is of a type that is derived from Base, just do the following:

# Will not show an error
y: Base = d

# Will show an error
z: Other = d

Does that address your use case?

[dbrattli]

I was trying to implement the upcast and downcast from F#. Upcast is perhaps not needed as you say, but good for symmetry. After reading python/typing#565 I thought it should be possible to implement this using type annotations, and sort of expected pyright to give an error for upcast(Other, d) when d is not derived from Other.

[erictraut]

Yeah, there's no need for an upcast call. Upcasting is implicit in Python, as it is in most languages.