Fixed #33346 -- Fixed SimpleTestCase.assertFormsetError() crash on a formset named "form".

Thanks OutOfFocus4 for the report.

Regression in 456466d932.

django/test/testcases.py

@@ -572,7 +572,7 @@ class SimpleTestCase(unittest.TestCase):
         # Search all contexts for the error.
         found_formset = False
         for i, context in enumerate(contexts):
-            if formset not in context:
+            if formset not in context or not hasattr(context[formset], 'forms'):
                 continue
             found_formset = True
             for err in errors:

docs/releases/4.0.1.txt

@@ -9,4 +9,6 @@ Django 4.0.1 fixes several bugs in 4.0.
 Bugfixes
 ========
 
-* ...
+* Fixed a regression in Django 4.0 that caused a crash of
+  :meth:`~django.test.SimpleTestCase.assertFormsetError` on a formset named
+  ``form`` (:ticket:`33346`).

tests/test_utils/tests.py

@@ -1512,6 +1512,16 @@ class AssertFormsetErrorTests(SimpleTestCase):
         ])
         self.assertFormsetError(response, 'formset', None, None, 'error')
 
+    def test_formset_named_form(self):
+        formset = TestFormset.invalid()
+        # The mocked context emulates the template-based rendering of the
+        # formset.
+        response = mock.Mock(context=[
+            {'form': formset},
+            {'form': formset.management_form},
+        ])
+        self.assertFormsetError(response, 'form', 0, 'field', 'invalid value')
+
 
 class FirstUrls:
     urlpatterns = [path('first/', empty_response, name='first')]