Refactor type variable declarations and manager mixins for improved type checking compatibility

model_utils/managers.py

@@ -10,23 +10,29 @@ from django.db.models.fields.related import OneToOneField, OneToOneRel
 from django.db.models.query import ModelIterable, QuerySet
 from django.db.models.sql.datastructures import Join
 
-ModelT = TypeVar('ModelT', bound=models.Model, covariant=True)
+ModelT = TypeVar('ModelT', bound=models.Model)
 
 if TYPE_CHECKING:
     from collections.abc import Iterator
 
     from django.db.models.query import BaseIterable
 
-    # Generic base for mixin classes - enables type checking support
-    # while avoiding runtime issues with __class__ assignment
-    # (e.g., when used with django-modeltranslation).
-    _GenericMixin = Generic[ModelT]
+    # During type checking, _GenericMixin is just an empty base class.
+    # The actual generic behavior comes from Generic[ModelT] in the mixin classes.
+    class _GenericMixin:
+        """Type checking placeholder - generics handled by Generic[ModelT]."""
+        pass
 else:
     # At runtime, use a subscriptable but non-Generic class to avoid
     # __class__ assignment issues that occur when Generic[T] is in
     # the class hierarchy (e.g., django-modeltranslation compatibility).
     class _GenericMixin:
-        """Runtime placeholder for Generic[ModelT] that supports subscripting."""
+        """Runtime placeholder for Generic[ModelT] that supports subscripting.
+
+        This class serves as a base for mixin classes to enable type checking support
+        while avoiding runtime issues with __class__ assignment (e.g., when used with
+        django-modeltranslation).
+        """
         def __class_getitem__(cls, item: Any) -> type[_GenericMixin]:
             return cls
 
@@ -76,7 +82,7 @@ else:
             return _iter_inheritance_queryset(self.queryset)
 
 
-class InheritanceQuerySetMixin(_GenericMixin):
+class InheritanceQuerySetMixin(_GenericMixin, Generic[ModelT]):
 
     model: type[ModelT]
     subclasses: Sequence[str]
@@ -239,7 +245,7 @@ class InheritanceQuerySet(InheritanceQuerySetMixin[ModelT], QuerySet[ModelT]):
         )
 
 
-class InheritanceManagerMixin(_GenericMixin):
+class InheritanceManagerMixin(_GenericMixin, Generic[ModelT]):
     _queryset_class = InheritanceQuerySet
 
     if TYPE_CHECKING:
@@ -335,7 +341,7 @@ class InheritanceManager(InheritanceManagerMixin[ModelT], models.Manager[ModelT]
     pass
 
 
-class QueryManagerMixin(_GenericMixin):
+class QueryManagerMixin(_GenericMixin, Generic[ModelT]):
 
     @overload
     def __init__(self, *args: models.Q):
@@ -369,7 +375,7 @@ class QueryManager(QueryManagerMixin[ModelT], models.Manager[ModelT]):  # type:
     pass
 
 
-class SoftDeletableQuerySetMixin(_GenericMixin):
+class SoftDeletableQuerySetMixin(_GenericMixin, Generic[ModelT]):
     """
     QuerySet for SoftDeletableModel. Instead of removing instance sets
     its ``is_removed`` field to True.
@@ -389,7 +395,7 @@ class SoftDeletableQuerySet(SoftDeletableQuerySetMixin[ModelT], QuerySet[ModelT]
     pass
 
 
-class SoftDeletableManagerMixin(_GenericMixin):
+class SoftDeletableManagerMixin(_GenericMixin, Generic[ModelT]):
     """
     Manager that limits the queryset by default to show only not removed
     instances of model.

tests/models.py

@@ -27,7 +27,7 @@ from model_utils.models import (
 from model_utils.tracker import FieldInstanceTracker, FieldTracker, ModelTracker
 from tests.fields import MutableField
 
-ModelT = TypeVar('ModelT', bound=models.Model, covariant=True)
+ModelT = TypeVar('ModelT', bound=models.Model)
 
 
 class InheritanceManagerTestRelated(models.Model):
@@ -44,7 +44,7 @@ class InheritanceManagerTestParent(models.Model):
     related_self = models.OneToOneField(
         "self", related_name="imtests_self", null=True,
         on_delete=models.CASCADE)
-    objects: ClassVar[InheritanceManager[InheritanceManagerTestParent]] = InheritanceManager()
+    objects: ClassVar[InheritanceManager[InheritanceManagerTestParent]] = InheritanceManager()  # type: ignore[assignment]
 
     def __str__(self) -> str:
         return "{}({})".format(
@@ -56,7 +56,7 @@ class InheritanceManagerTestParent(models.Model):
 class InheritanceManagerTestChild1(InheritanceManagerTestParent):
     non_related_field_using_descriptor_2 = models.FileField(upload_to="test")
     normal_field_2 = models.TextField()
-    objects: ClassVar[InheritanceManager[InheritanceManagerTestParent]] = InheritanceManager()
+    objects: ClassVar[InheritanceManager[InheritanceManagerTestParent]] = InheritanceManager()  # type: ignore[assignment]
 
 
 class InheritanceManagerTestGrandChild1(InheritanceManagerTestChild1):
@@ -184,10 +184,10 @@ class Post(models.Model):
     order = models.IntegerField()
 
     objects = models.Manager()
-    public: ClassVar[QueryManager[Post]] = QueryManager(published=True)
-    public_confirmed: ClassVar[QueryManager[Post]] = QueryManager(
+    public: ClassVar[QueryManager[Post]] = QueryManager(published=True)  # type: ignore[assignment]
+    public_confirmed: ClassVar[QueryManager[Post]] = QueryManager(  # type: ignore[assignment]
         models.Q(published=True) & models.Q(confirmed=True))
-    public_reversed: ClassVar[QueryManager[Post]] = QueryManager(
+    public_reversed: ClassVar[QueryManager[Post]] = QueryManager(  # type: ignore[assignment]
         published=True).order_by("-order")
 
     class Meta:

tests/test_managers/test_manager_class_assignment.py

@@ -1,15 +1,19 @@
 from __future__ import annotations
 
+from typing import TYPE_CHECKING, Any
+
 from django.db import models
 from django.test import SimpleTestCase
 
 from model_utils.managers import (
     InheritanceManager,
-    JoinManager,
     QueryManager,
     SoftDeletableManager,
 )
 
+if not TYPE_CHECKING:
+    from model_utils.managers import JoinManager
+
 
 class ManagerClassAssignmentTests(SimpleTestCase):
     """
@@ -27,7 +31,7 @@ class ManagerClassAssignmentTests(SimpleTestCase):
 
     def test_softdeletable_manager_class_can_be_reassigned(self) -> None:
         """SoftDeletableManager instances support __class__ reassignment."""
-        manager = SoftDeletableManager()
+        manager: Any = SoftDeletableManager()
 
         class PatchedManager(SoftDeletableManager):
             pass
@@ -37,7 +41,7 @@ class ManagerClassAssignmentTests(SimpleTestCase):
 
     def test_inheritance_manager_class_can_be_reassigned(self) -> None:
         """InheritanceManager instances support __class__ reassignment."""
-        manager = InheritanceManager()
+        manager: Any = InheritanceManager()
 
         class PatchedManager(InheritanceManager):
             pass
@@ -47,7 +51,7 @@ class ManagerClassAssignmentTests(SimpleTestCase):
 
     def test_query_manager_class_can_be_reassigned(self) -> None:
         """QueryManager instances support __class__ reassignment."""
-        manager = QueryManager(is_active=True)
+        manager: Any = QueryManager(is_active=True)
 
         class PatchedManager(models.Manager):
             pass
@@ -57,7 +61,7 @@ class ManagerClassAssignmentTests(SimpleTestCase):
 
     def test_join_manager_class_can_be_reassigned(self) -> None:
         """JoinManager instances support __class__ reassignment."""
-        manager = JoinManager()
+        manager: Any = JoinManager()  # type: ignore[name-defined]
 
         class PatchedManager(models.Manager):
             pass