Default of TypeVar isn't respected for assignments.

[s3rius]

Hello, here's an issue I stumbled upon.

from typing import Generic, Type
from typing_extensions import TypeVar


class Interface: ...


_T = TypeVar("_T", bound=Interface, default=Interface)


class MyGeneric(Generic[_T]):
    def __init__(self) -> None:
        super().__init__()
        self._value: Type[_T] = Interface

In this example, I expect this code to be valid, because I have a default value for _T and don't have it in constructor.
Also, while playing around I noticed that default doesn't work with default argument in __init__ either.

from typing import Generic, Type
from typing_extensions import TypeVar


class Interface: ...


_T = TypeVar("_T", bound=Interface, default=Interface)


class MyGeneric(Generic[_T]):
    def __init__(self, my_val: Type[_T] = Interface) -> None:
        super().__init__()
        self._value = my_val

I'd expect this to be valid code because we define the default for the Type[_T] argument before the object is initialized, so it should fall back to the default of TypeVar in that case. At least, that's what I would expect.

Is this intended behavior?

[brianschubert]

I think mypy is correct to reject this. The assignment self._value: Type[_T] = Interface isn't sound, _T since _T may be bound to a subtype of Interface. Consider for example:

class FooInterface(Interface):
    @staticmethod
    def foo() -> None: ...

class FooGeneric(MyGeneric[FooInterface]): ...

x = FooGeneric()
x._value.foo()  # boom! 

[s3rius]

Thank you. That's a good point. But in that case, how can I implement such a logic, that would force subclasses to obey type they declare?

[A5rocks]

You might be interested in #3737 regarding the second example. It's not completely clear what you want from the first example so I can't offer a work around.

But for the second you could try an overload and self-types I think?

[sterliakov]

This is very similar to #3737 and #18812, the latter exactly about typevar default and defaults in generic classes.

force subclasses to obey type they declare

Do you need to provide any default implementation? I'd recommend going a level deeper and providing the default implementation that depends on _T in a separate subclass. Remove Interface default argument, let subclasses take care of that. Something like (untested)

from typing import Generic
from typing_extensions import TypeVar

class Interface: ...

_T = TypeVar("_T", bound=Interface, default=Interface)

class MyGeneric(Generic[_T]):
    _value: type[_T]

class FooInterface(Interface):
    foo: int
    
class FooGeneric(MyGeneric[FooInterface]):
    def __init__(self) -> None:
        super().__init__()
        self._value = FooInterface