_TSource in Option should be covariant

[denghz]

Describe the bug _TSource type in Option should be covariant.

class A:
  pass
class B(A):
  pass

x: Option[A] = Some(B())

Now since the _TSource is invariant, pyright will complain that B is not the same as A, but actually Option type should be covariant. Is Some[Dog] a Some[Animal]? I think you'll agree that the answer is yes. Also Option type is not actually mutable, so there is no way to assign a Some[Dog] to a variable typed Some[Aninmal] and then make it become Some[Cat]

Additional context Add any other context about the problem here.

• OS Linux
• Expression version 4.2.4
• Python version 3.11

[brendanmaguire]

I didn't see the above failing the type check in either mypy or pyright but the following did fail:

from expression import Some, Option

class A:
    pass

class B(A):
    pass

def test_covariance() -> None:
    x: Option[B] = Some(B())
    y: Option[A] = x

[brendanmaguire]

This isn't so easy to fix because covariant types can't be passed as parameters to other functions.

[phi-friday]

It also looks good to use infer_variance, which eliminates the need to consider covariant. ex:

from typing_extensions import TypeVar

_T = TypeVar("_T", infer_variance=True)

This parameter is supported by typing-extensions>=4.4.0 and is the default behavior in python>=3.12.

[dbrattli]

This looks really interesting. Will try. Look forward to when we can eventually be >= Python 3.12 😀

[brendanmaguire]

My understanding of infer_variance is that it only aids in not having to specify the variance of the TypeVar. See the related spec docs:

type checker should use inference to determine whether the type variable is invariant, covariant or contravariant

So I don't think it helps here. The problem still exists that covariant types can't be passed as parameters to other functions.

[brendanmaguire]

I locally updated typing-extensions to 4.8.0 and tried with infer_variance. My above code example still fails the type checker.

[dbrattli]

I've tried to make some progress on this one by making methods that takes _TSource static methods and use another type when generating e.g:

    @staticmethod
    def Some(value: _T1) -> Option[_T1]:
        """Create a Some option."""
        return Option(some=value)

Then for some more tricky methods like default_value I return a union with the default arg:

def default_value(self, value: _T1) -> _TSource | _T1:    

I'm just unsure if the rest is ok, or false negatives. E.g:

def default_with(self, getter: Callable[[], _TSource]) -> _TSource:

It type checks, but is it ok to use _TSource in an argument that is a callable that returns _TSource? I could do the same trick as with default_value but then there's all the other methods like map, bind, but they are probably ok since they return a differnet type in the callable?

[ShravanSunder]

I locally updated typing-extensions to 4.8.0 and tried with infer_variance. My above code example still fails the type checker.

From what i understand, I don't think we can have input paramters in python type hints as covariant. i think they usually are contravariant while returns are covariant. The type hints don't seem to let you know this and just has a generic error.