Infer model field types without plugin

[finlayacourt]

I think it would be possible for the types of model fields to be infered without the need for the mypy plugin. This would significantly improve typechecking using pyright. It would require:

• defining unique type vars with default values for each field type
• defining a new type var that represents nullability
• defining additional overloads for the __set__ and __get__ methods

Here's a minimal example of just the Field and IntegerField types

_ST = TypeVar("_ST", contravariant=True)
_GT = TypeVar("_GT", covariant=True)
_NT = TypeVar("_NT", Literal[True], Literal[False], default=Literal[False])

class Field(RegisterLookupMixin, Generic[_ST, _GT, _NT]):
    def __init__(self, null: _NT = False) -> None: ...
    @overload
    def __set__(self: Field[_ST, _GT, Literal[False]], instance: Any, value: _ST) -> None: ...
    @overload
    def __set__(self: Field[_ST, _GT, Literal[True]], instance: Any, value: _ST | None) -> None: ...
    @overload
    def __get__(self, instance: None, owner: Any) -> _FieldDescriptor[Self]: ...
    @overload
    def __get__(self: Field[_ST, _GT, Literal[False]], instance: Model, owner: Any) -> _GT: ...
    @overload
    def __get__(self: Field[_ST, _GT, Literal[True]], instance: Model, owner: Any) -> _GT | None: ...
    @overload
    def __get__(self, instance: Any, owner: Any) -> Self: ...

_ST_1 = TypeVar("_ST_1", bound=float | int | str | Combinable, default=float | int | str | Combinable)
_GT_1 = TypeVar("_GT_1", bound=int, default=int)

class IntegerField(Field[_ST_1, _GT_1, _NT]): ...

I don't have much experience with mypy. Does this seem like a good idea?

[sobolevn]

It might be :)
We need to test this out.

[finlayacourt]

Realised it's already been discussed here. Seems like this might not be supported in mypy yet!