mypy plugin weirdness

mypy.ini

[mypy]
plugins = mypy_plugin.py

[mypy-django.*]
ignore_missing_imports = True

mypy_plugin.py

from __future__ import annotations

from typing import Callable

from mypy.plugin import AttributeContext
from mypy.plugin import Plugin
from mypy.types import CallableType
from mypy.types import Instance
from mypy.types import Type
from mypy.types import TypeType
from mypy.types import UnionType


def _get_objects_type(ctx: AttributeContext) -> Type:
    def _type_to_res(target_cls: Instance) -> Type:
        # TODO: this should check if `target_cls` is a `Model`
        real_type = target_cls.type.get('objects')
        # could not find the `objects` definition.
        # probably not our custom model -- probably a django model
        if real_type is None or not isinstance(real_type.type, Instance):
            return ctx.default_attr_type
        else:
            return Instance(real_type.type.type, (target_cls,))

    # early-phase parses will treat these as `() -> TheType`
    if isinstance(ctx.type, CallableType) and isinstance(ctx.type.ret_type, Instance):
        return _type_to_res(ctx.type.ret_type)
    # later-phase parses will treat these as `type[TheType]`
    elif isinstance(ctx.type, TypeType) and isinstance(ctx.type.item, Instance):
        return _type_to_res(ctx.type.item)
    # handle unions
    elif isinstance(ctx.type, UnionType):
        object_types = []
        for tp in ctx.type.items:
            if isinstance(tp, TypeType) and isinstance(tp.item, Instance):
                object_types.append(_type_to_res(tp.item))
            else:
                ctx.api.fail('objects plugin bug 001', ctx.context)
                return ctx.default_attr_type
        return UnionType(object_types)
    else:
        ctx.api.fail('objects plugin bug 002', ctx.context)
        return ctx.default_attr_type


class CustomPlugin(Plugin):
    def get_class_attribute_hook(
        self,
        fullname: str,
    ) -> Callable[[AttributeContext], Type] | None:
        if fullname.endswith('.objects'):
            return _get_objects_type
        else:
            return None


def plugin(version: str) -> type[Plugin]:
    return CustomPlugin

output-with-init-commented.txt

$ mypy t.py
t.py:34: note: Revealed type is "t.BaseManager[t.Model1]"
t.py:35: note: Revealed type is "t.Model1"
t.py:36: note: Revealed type is "t.CustomManager[t.CustomModel]"
t.py:37: note: Revealed type is "t.CustomModel"
t.py:38: note: Revealed type is "t.CustomModel"
Success: no issues found in 1 source file

output-with-init.txt

$ mypy t.py
t.py:34: note: Revealed type is "t.BaseManager[t.Model1]"
t.py:35: note: Revealed type is "t.Model1"
t.py:36: error: Access to generic instance variables via class is ambiguous
t.py:36: note: Revealed type is "t.BaseManager[t.CustomModel]"
t.py:37: error: Access to generic instance variables via class is ambiguous
t.py:37: note: Revealed type is "t.CustomModel"
t.py:38: error: Access to generic instance variables via class is ambiguous
t.py:38: note: Revealed type is "Any"
Found 3 errors in 1 file (checked 1 source file)

t.py

from __future__ import annotations

from typing import Any
from typing import Generic
from typing import TypeVar

from django.db.models.manager import BaseManager as DjangoBaseManager

M = TypeVar('M')


class BaseManager(DjangoBaseManager, Generic[M]):
    # uncomment this line for some reason to break everything
    # def __init__(self) -> None: ...
    def get(self, id: int) -> M: ...


class BaseModel:
    objects: BaseManager[Any] = BaseManager()


class Model1(BaseModel):
    ...


class CustomManager(BaseManager['CustomModel']):
    def m(self) -> CustomModel: ...


class CustomModel(BaseModel):
    objects = CustomManager()


reveal_type(Model1.objects)                 # should be `BaseModel[Model1]`
reveal_type(Model1.objects.get(1))          # should be `Model1`
reveal_type(CustomModel.objects)            # should be `CustomManager`
reveal_type(CustomModel.objects.get(1))     # should be `CustomModel`
reveal_type(CustomModel.objects.m())        # should be `CustomModel`