Permit override_settings to work with test class mixins that don't inherit from unittest.TestCase

[Chris Jerdonek]

Currently, the @override_settings decorator raises a ValueError if it's used to decorate a class that doesn't inherit from SimpleTestCase:
​https://github.com/django/django/blob/56f9579105c324ff15250423bf9f8bdf1634cfb4/django/test/utils.py#L519-L521

However, this prevents the decorator from being useable as a test-class mixin for a number of SimpleTestCase subclasses. When creating shared test-class functionality, it's often better to use a mixin rather than a concrete TestCase class. This way the mixin won't have its setUp methods run unnecessarily on a test-case class with no tests, and it won't count towards the parallel test runner's test-case class count, etc.

The check could instead be done lazily, e.g. inside setUpClass(), while still having the same desired effect.

I noticed this when seeing if I could convert ​AuthViewsTestCase into a mixin. This AuthViewsTestCase change could be done in a nicer way once this ticket is implemented, perhaps even as part of this ticket.

[Chris Jerdonek]

PS - should this be a TypeError instead of ValueError?

[Chris Jerdonek]

I realize now that the "non-lazy" check could be kept by modifying it to be done only when the class inherits from unittest.TestCase (i.e. when the class is a concrete test class):

--- a/django/test/utils.py
+++ b/django/test/utils.py
@@ -515,7 +515,7 @@ class override_settings(TestContextDecorator):
 
     def decorate_class(self, cls):
         from django.test import SimpleTestCase
-        if not issubclass(cls, SimpleTestCase):
+        if issubclass(cls, TestCase) and not issubclass(cls, SimpleTestCase):
             raise ValueError(
                 "Only subclasses of Django SimpleTestCase can be decorated "
                 "with override_settings")

[Chris Jerdonek]

This way the mixin won't have its setUp methods run unnecessarily on a test-case class with no tests, and it won't count towards the parallel test runner's test-case class count, etc.

Investigating further, it turns out neither of these seem to happen in practice, since test classes without tests get filtered out. Nevertheless, being able to apply the decorator to test mixins would still be a useful enhancement.