TypeVar for self breaks super

[esoma]

from typing import TypeVar

T = TypeVar('T')

class X:
    def __init__(self: T):
        super().__init__() # error: Argument 2 for "super" not an instance of argument 1

Possibly related to python/typing#909? But it seems the actual super call is handled specially so, possibly not?

It would seem to me if self is a TypeVar we can safely say that it must be compatible with super's arg 2...which seems like a simple fix. If T is bound to an incompatible type we'll get errors elsewhere to cover that situation (error: The erased type of self "__main__.A" is not a supertype of its class "__main__.X").

[sobolevn]

@esoma yes, I also think that this should be supported. We can also check corner cases for type vars with bounds and explicit values.

Would you like to work on this? 🙂

[esoma]

@sobolevn Yeah, I'll look into it.

[esoma]

Found a test that works this by declaring:

T = TypeVar('T', bound='X', covariant=True)

So maybe that's just the solution -- it works for my particular use case at least.

[ethanscorey]

Thanks for opening this issue — I've run into the same problem. Just wanted to chime in to say that I tried your solution of adding covariant=True to my TypeVar declaration, but it didn't work for me.

from typing import TypeVar

from django.db import models


ModelType = TypeVar("ModelType", bound=models.Model, covariant=True)


class MyModel(models.Model):
    def save(self: ModelType, *args, **kwargs) -> None:
        ...
        super().save(*args, **kwargs)

[esoma]

@ethanscorey
The bound target needs to be the subclass, not the base class:

ModelType = TypeVar("ModelType", bound='MyModel', covariant=True)

Unfortunately that means for every subclass you'll need a unique TypeVar.

[erictraut]

This appears to have been fixed. Mypy 1.5 doesn't generate any errors for the sample at the top of this issue.

[hauntsaninja]

Fixed in #13488