Fixed #33307 -- Allowed Form.add_error() with ValidationError dict.

Form.add_error() raised a TypeError when called with a field name and a
ValidationError that has an error_dict with a single entry for the same
field. This was confusing because the error message referred to "multiple
fields" when only one field was involved.

Modified Form.add_error() to allow this specific use case. Also ensured
that unbound forms do not raise an AttributeError when add_error() is
called.

django/forms/forms.py

@@ -285,7 +285,11 @@ class BaseForm(RenderableFormMixin):
             error = ValidationError(error)
 
         if hasattr(error, "error_dict"):
-            if field is not None:
+            if field is None:
+                error = error.error_dict
+            # Raise an error if the ValidationError contains more than one
+            # item or if the only item's key doesn't match the form field.
+            elif len(error.error_dict) != 1 or field not in error.error_dict:
                 raise TypeError(
                     "The argument `field` must be `None` when the `error` "
                     "argument contains errors for multiple fields."
@@ -312,7 +316,7 @@ class BaseForm(RenderableFormMixin):
                         field_id=self[field].auto_id,
                     )
             self._errors[field].extend(error_list)
-            if field in self.cleaned_data:
+            if hasattr(self, "cleaned_data") and field in self.cleaned_data:
                 del self.cleaned_data[field]
 
     def has_error(self, field, code=None):

tests/forms_tests/tests/test_forms.py

@@ -1710,6 +1710,34 @@ aria-describedby="id_birthday_error">
             },
         )
 
+    def test_add_error_validation_error_dict(self):
+        class FooForm(Form):
+            the_field = CharField(max_length=100)
+
+        exc = ValidationError({"the_field": "Something is wrong with the field."})
+        form = FooForm()
+        form.add_error("the_field", exc)
+        self.assertEqual(form.errors, {"the_field": ["Something is wrong with the field."]})
+
+    def test_add_error_validation_error_dict_multiple_fields(self):
+        class FooForm(Form):
+            the_field = CharField(max_length=100)
+            another_field = CharField(max_length=100)
+
+        exc = ValidationError(
+            {
+                "the_field": "Something is wrong with the field.",
+                "another_field": "Something is wrong with this other field.",
+            }
+        )
+        form = FooForm()
+        msg = (
+            "The argument `field` must be `None` when the `error` argument "
+            "contains errors for multiple fields."
+        )
+        with self.assertRaisesMessage(TypeError, msg):
+            form.add_error("the_field", exc)
+
     def test_has_error(self):
         class UserRegistration(Form):
             username = CharField(max_length=10)