TypeVar not compatible with object

[Dreamsorcerer]

In order to keep code type safe and declare that my code will work with any type, I have used object. But, when trying to use a TypeVar in another part of the code, it complains that it is incompatible with object.

My code looks roughly like:

jobs: Set[Job[object]] = set()

async def spawn(self, coro: Coroutine[object, object, _T]) -> Job[_T]:
    jobs.add(job)

I'm getting:

error: Argument 1 to "add" of "set" has incompatible type "Job[_T]"; expected "Job[object]"  [arg-type]

I don't think it's an invariance problem..

[JelleZijlstra]

Your code sample is incomplete, but that does look like an invariance problem. If Job is invariant in its type parameter, a Job[_T] is not compatible with a Job[object].

[Dreamsorcerer]

One day I'll get my head round covariance properly. :P

After making the TypeVar covariant, I still have one more error in:

_T = TypeVar("_T", covariant=True)
task: asyncio.Task[_T]
_failed_tasks: asyncio.Queue[Optional[asyncio.Task[object]]] = asyncio.Queue()
_failed_tasks.put_nowait(task)  #  Argument 1 to "put_nowait" of "Queue" has incompatible type "Task[_T]"; expected "Optional[Task[object]]"  [arg-type]

That seems pretty much the same to me as the previous problem. I'll have the real code up in a PR in a couple of days, if it's useful to look at.

[JelleZijlstra]

The variance you set there doesn't do anything. What matters is how Task is declared in the stubs, and the TypeVar is invariant there: https://github.com/python/typeshed/blob/6ca80d340ef16714e61284df8338dac91479aa79/stdlib/asyncio/tasks.pyi#L268

[Dreamsorcerer]

OK, is that correct though?

I'm looking at the list example at: https://mypy.readthedocs.io/en/stable/generics.html#variance-of-generics
But, I'm not seeing a reason that Task would need to be invariant. It's not a mutable container, and I can't think of anything else that would break it.

[JelleZijlstra]

It does seem like we could make it covariant. We'd also have to make the base class asyncio.Future covariant in its type parameter.

[Dreamsorcerer]

python/typeshed#8781

[hauntsaninja]

Hmm asyncio.Future is mutable, if you made it covariant set_result would be unsound