Fixed a regression in forms changed_data

Thanks Loic Bistuer for spotting the regression and the initial
patch. Refs #16612.

django/forms/forms.py

@@ -345,8 +345,13 @@ def changed_data(self):
                 else:
                     initial_prefixed_name = self.add_initial_prefix(name)
                     hidden_widget = field.hidden_widget()
-                    initial_value = field.to_python(hidden_widget.value_from_datadict(
-                        self.data, self.files, initial_prefixed_name))
+                    try:
+                        initial_value = field.to_python(hidden_widget.value_from_datadict(
+                            self.data, self.files, initial_prefixed_name))
+                    except ValidationError:
+                        # Always assume data has changed if validation fails.
+                        self._changed_data.append(name)
+                        continue
                 if hasattr(field.widget, '_has_changed'):
                     warnings.warn("The _has_changed method on widgets is deprecated,"
                         " define it at field level instead.",

tests/forms_tests/tests/forms.py

@@ -1201,6 +1201,32 @@ class UserRegistration(Form):
 <option value="w">whiz</option>
 </select></li>""")
 
+    def test_changed_data(self):
+        class Person(Form):
+            first_name = CharField(initial='Hans')
+            last_name = CharField(initial='Greatel')
+            birthday = DateField(initial=datetime.date(1974, 8, 16))
+
+        p = Person(data={'first_name': 'Hans', 'last_name': 'Scrmbl',
+                         'birthday': '1974-08-16'})
+        self.assertTrue(p.is_valid())
+        self.assertNotIn('first_name', p.changed_data)
+        self.assertIn('last_name', p.changed_data)
+        self.assertNotIn('birthday', p.changed_data)
+
+        # Test that field raising ValidationError is always in changed_data
+        class PedanticField(forms.Field):
+            def to_python(self, value):
+                raise ValidationError('Whatever')
+
+        class Person2(Person):
+            pedantic = PedanticField(initial='whatever', show_hidden_initial=True)
+
+        p = Person2(data={'first_name': 'Hans', 'last_name': 'Scrmbl',
+                         'birthday': '1974-08-16', 'initial-pedantic': 'whatever'})
+        self.assertFalse(p.is_valid())
+        self.assertIn('pedantic', p.changed_data)
+
     def test_boundfield_values(self):
         # It's possible to get to the value which would be used for rendering
         # the widget for a field by using the BoundField's value method.