[1.7.x] Fixed #22510 -- Harden field removal to only None.

Refs #8620.

If we allow any value to remove form fields then we get name clashes
with method names, media classes etc. There was a backwards
incompatibility introduced meaning ModelForm subclasses with declared
fields called media or clean would lose those fields.

Field removal is now only permitted by using the sentinel value None.
The docs have been slightly reworded to refer to removal of fields
rather than shadowing.

Thanks to gcbirzan for the report and initial patch, and several of the
core team for opinions.

Backport of 9fb0f5dddc from master

django/forms/forms.py

@@ -92,8 +92,8 @@ class DeclarativeFieldsMetaclass(MediaDefiningClass):
                 declared_fields.update(base.declared_fields)
 
             # Field shadowing.
-            for attr in base.__dict__.keys():
-                if attr in declared_fields:
+            for attr, value in base.__dict__.items():
+                if value is None and attr in declared_fields:
                     declared_fields.pop(attr)
 
         new_class.base_fields = declared_fields

docs/ref/forms/api.txt

@@ -964,10 +964,20 @@ classes::
 
 .. versionadded:: 1.7
 
-* It's possible to opt-out from a ``Field`` inherited from a parent class by
-  shadowing it. While any non-``Field`` value works for this purpose, it's
-  recommended to use ``None`` to make it explicit that a field is being
-  nullified.
+* It's possible to declaratively remove a ``Field`` inherited from a parent
+  class by setting the name to be ``None`` on the subclass. For example::
+
+    >>> from django import forms
+
+    >>> class ParentForm(forms.Form):
+    ...     name = forms.CharField()
+    ...     age = forms.IntegerField()
+
+    >>> class ChildForm(ParentForm):
+    ...     name = None
+
+    >>> ChildForm().fields.keys()
+    ... ['age']
 
 .. _form-prefix:
 

docs/releases/1.7.txt

@@ -544,8 +544,8 @@ Forms
   inheriting from both ``Form`` and ``ModelForm`` simultaneously have been
   removed as long as ``ModelForm`` appears first in the MRO.
 
-* It's now possible to opt-out from a ``Form`` field declared in a parent class
-  by shadowing it with a non-``Field`` value.
+* It's now possible to remove a field from a ``Form`` when subclassing by
+  setting the name to ``None``.
 
 * The new :meth:`~django.forms.Form.add_error()` method allows adding errors
   to specific form fields.

docs/topics/forms/modelforms.txt

@@ -674,10 +674,8 @@ There are a couple of things to note, however.
 
 .. versionadded:: 1.7
 
-* It's possible to opt-out from a ``Field`` inherited from a parent class by
-  shadowing it. While any non-``Field`` value works for this purpose, it's
-  recommended to use ``None`` to make it explicit that a field is being
-  nullified.
+* It's possible to declaratively remove a ``Field`` inherited from a parent class by
+  setting the name to be ``None`` on the subclass.
 
   You can only use this technique to opt out from a field defined declaratively
   by a parent class; it won't prevent the ``ModelForm`` metaclass from generating

tests/model_forms/tests.py

@@ -2267,7 +2267,7 @@ class ModelFormInheritanceTests(TestCase):
 
         self.assertEqual(list(ModelForm().fields.keys()), ['name', 'age'])
 
-    def test_field_shadowing(self):
+    def test_field_removal(self):
         class ModelForm(forms.ModelForm):
             class Meta:
                 model = Writer
@@ -2290,6 +2290,24 @@ class ModelFormInheritanceTests(TestCase):
         self.assertEqual(list(type(str('NewForm'), (ModelForm, Form, Mixin), {})().fields.keys()), ['name', 'age'])
         self.assertEqual(list(type(str('NewForm'), (ModelForm, Form), {'age': None})().fields.keys()), ['name'])
 
+    def test_field_removal_name_clashes(self):
+        """Regression test for https://code.djangoproject.com/ticket/22510."""
+
+        class MyForm(forms.ModelForm):
+            media = forms.CharField()
+
+            class Meta:
+                model = Writer
+                fields = '__all__'
+
+        class SubForm(MyForm):
+            media = None
+
+        self.assertIn('media', MyForm().fields)
+        self.assertNotIn('media', SubForm().fields)
+        self.assertTrue(hasattr(MyForm, 'media'))
+        self.assertTrue(hasattr(SubForm, 'media'))
+
 
 class StumpJokeForm(forms.ModelForm):
     class Meta: