Fixed django#8620 -- Updated the Form metaclass to support excluding …

…fields by shadowing them.

django/forms/forms.py

@@ -90,6 +90,11 @@ def __new__(mcs, name, bases, attrs):
             if hasattr(base, 'declared_fields'):
                 declared_fields.update(base.declared_fields)
 
+            # Field shadowing.
+            for attr in base.__dict__.keys():
+                if attr in declared_fields:
+                    declared_fields.pop(attr)
+
         new_class.base_fields = declared_fields
         new_class.declared_fields = declared_fields
 

docs/ref/forms/api.txt

@@ -854,6 +854,13 @@ classes::
     <li>Instrument: <input type="text" name="instrument" /></li>
     <li>Haircut type: <input type="text" name="haircut_type" /></li>
 
+.. 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.
+
 .. _form-prefix:
 
 Prefixes for forms

docs/releases/1.7.txt

@@ -293,6 +293,9 @@ 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.
+
 Internationalization
 ^^^^^^^^^^^^^^^^^^^^
 

docs/topics/forms/modelforms.txt

@@ -651,6 +651,18 @@ There are a couple of things to note, however.
   because these classes rely on different metaclasses and a class can only have
   one metaclass.
 
+.. 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.
+
+  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
+  a default field. To opt-out from default fields, see
+  :ref:`controlling-fields-with-fields-and-exclude`.
+
 .. _modelforms-factory:
 
 ModelForm factory function
@@ -749,6 +761,8 @@ instances of the model, you can specify an empty QuerySet::
    >>> AuthorFormSet(queryset=Author.objects.none())
 
 
+.. _controlling-fields-with-fields-and-exclude:
+
 Controlling which fields are used with ``fields`` and ``exclude``
 -----------------------------------------------------------------
 

tests/model_forms/tests.py

@@ -1815,3 +1815,26 @@ class Meta:
                 fields = '__all__'
 
         self.assertEqual(list(ModelForm().fields.keys()), ['name', 'age'])
+
+    def test_field_shadowing(self):
+        class ModelForm(forms.ModelForm):
+            class Meta:
+                model = Writer
+                fields = '__all__'
+
+        class Mixin(object):
+            age = None
+
+        class Form(forms.Form):
+            age = forms.IntegerField()
+
+        class Form2(forms.Form):
+            foo = forms.IntegerField()
+
+        self.assertEqual(list(ModelForm().fields.keys()), ['name'])
+        self.assertEqual(list(type(str('NewForm'), (Mixin, Form), {})().fields.keys()), [])
+        self.assertEqual(list(type(str('NewForm'), (Form2, Mixin, Form), {})().fields.keys()), ['foo'])
+        self.assertEqual(list(type(str('NewForm'), (Mixin, ModelForm, Form), {})().fields.keys()), ['name'])
+        self.assertEqual(list(type(str('NewForm'), (ModelForm, Mixin, Form), {})().fields.keys()), ['name'])
+        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'])