[4.0.x] Fixed #33130 -- Restored form errors to be a dict.

Regression in 456466d932.

Backport of 7fe9b6f6df from main

django/forms/utils.py

@@ -1,5 +1,5 @@
 import json
-from collections import UserDict, UserList
+from collections import UserList
 
 from django.conf import settings
 from django.core.exceptions import ValidationError
@@ -84,7 +84,7 @@ class RenderableErrorMixin(RenderableMixin):
         return self.render(self.template_name_ul)
 
 
-class ErrorDict(UserDict, RenderableErrorMixin):
+class ErrorDict(dict, RenderableErrorMixin):
     """
     A collection of errors that knows how to display itself in various formats.
 
@@ -94,8 +94,8 @@ class ErrorDict(UserDict, RenderableErrorMixin):
     template_name_text = 'django/forms/errors/dict/text.txt'
     template_name_ul = 'django/forms/errors/dict/ul.html'
 
-    def __init__(self, data=None, renderer=None):
+    def __init__(self, *args, renderer=None, **kwargs):
-        super().__init__(data)
+        super().__init__(*args, **kwargs)
         self.renderer = renderer or get_default_renderer()
 
     def as_data(self):

tests/forms_tests/tests/test_forms.py

@@ -67,6 +67,7 @@ class FormsTestCase(SimpleTestCase):
 
         self.assertTrue(p.is_bound)
         self.assertEqual(p.errors, {})
+        self.assertIsInstance(p.errors, dict)
         self.assertTrue(p.is_valid())
         self.assertHTMLEqual(p.errors.as_ul(), '')
         self.assertEqual(p.errors.as_text(), '')

tests/forms_tests/tests/test_utils.py

@@ -1,4 +1,5 @@
 import copy
+import json
 
 from django.core.exceptions import ValidationError
 from django.forms.utils import ErrorDict, ErrorList, flatatt
@@ -162,3 +163,48 @@ class FormsUtilsTestCase(SimpleTestCase):
         e = ErrorList(['Invalid username.'])
         self.assertTrue(hasattr(ErrorList, '__html__'))
         self.assertEqual(str(e), e.__html__())
+
+    def test_error_dict_is_dict(self):
+        self.assertIsInstance(ErrorDict(), dict)
+
+    def test_error_dict_is_json_serializable(self):
+        init_errors = ErrorDict([
+            ('__all__', ErrorList([
+                ValidationError('Sorry this form only works on leap days.')
+            ])),
+            ('name', ErrorList([ValidationError('This field is required.')])),
+        ])
+        min_value_error_list = ErrorList([
+            ValidationError('Ensure this value is greater than or equal to 0.')
+        ])
+        e = ErrorDict(
+            init_errors,
+            date=ErrorList([
+                ErrorDict({
+                    'day': min_value_error_list,
+                    'month': min_value_error_list,
+                    'year': min_value_error_list,
+                }),
+            ]),
+        )
+        e['renderer'] = ErrorList([
+            ValidationError(
+                'Select a valid choice. That choice is not one of the '
+                'available choices.'
+            ),
+        ])
+        self.assertJSONEqual(json.dumps(e), {
+            '__all__': ['Sorry this form only works on leap days.'],
+            'name': ['This field is required.'],
+            'date': [
+                {
+                    'day': ['Ensure this value is greater than or equal to 0.'],
+                    'month': ['Ensure this value is greater than or equal to 0.'],
+                    'year': ['Ensure this value is greater than or equal to 0.'],
+                },
+            ],
+            'renderer': [
+                'Select a valid choice. That choice is not one of the '
+                'available choices.'
+            ],
+        })