Fix compatibility issues with django-modeltranslation

[Hopiu]

Problem

When using django-model-utils together with django-modeltranslation, registering a model that uses SoftDeletableModel (or other models using manager mixins) causes a TypeError during app initialization:

This occurs because django-modeltranslation patches manager classes by reassigning manager.__class__ to a dynamically created subclass. Python's __class__ assignment has strict layout compatibility requirements, and the Generic[ModelT] base class in our manager mixins causes a layout mismatch.

Fixes #636

Solution

Move the Generic[ModelT] inheritance behind TYPE_CHECKING so it's only present during static type analysis, not at runtime. At runtime, a simple placeholder class _GenericMixin is used instead.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.

[Copilot]

Pull request overview

This PR fixes compatibility issues between django-model-utils and django-modeltranslation by addressing a Python runtime limitation with __class__ assignment when Generic[T] is in the class hierarchy. The fix moves Generic[ModelT] inheritance behind TYPE_CHECKING so it's only present during static type analysis, while using a runtime placeholder class _GenericMixin that supports subscripting but avoids layout incompatibility issues.

• Introduces a _GenericMixin class that replaces Generic[ModelT] at runtime while maintaining type-checking support
• Updates all manager mixin classes to inherit from _GenericMixin instead of Generic[ModelT]
• Adds comprehensive tests to verify __class__ reassignment works correctly for all affected managers

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
model_utils/managers.py	Implements the _GenericMixin pattern and updates all mixin classes (InheritanceQuerySetMixin, InheritanceManagerMixin, QueryManagerMixin, SoftDeletableQuerySetMixin, SoftDeletableManagerMixin) to use it
tests/test_managers/test_manager_class_assignment.py	Adds regression tests that verify __class__ assignment works for SoftDeletableManager, InheritanceManager, QueryManager, and JoinManager
CHANGES.rst	Documents the fix with reference to issue #636
AUTHORS.rst	Adds contributor credit for Benedikt Willi

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[Copilot]

The formatting of the issue reference is inconsistent with other entries in this file. Other entries use the format (GH-#636) at the end of the line, but this entry spans multiple lines with the reference at the end. Consider reformatting to match the style of other entries, such as placing it all on one line or breaking it differently to maintain consistency.

Suggested change

	- Fix compatibility with django-modeltranslation: manager mixins no longer
	inherit from `Generic[T]` at runtime, preventing `TypeError` on `__class__`
	assignment (GH-#636)
	- Fix compatibility with django-modeltranslation: manager mixins no longer inherit from `Generic[T]` at runtime, preventing `TypeError` on `__class__` assignment (GH-#636)