complain when a class variable shadows a name in __slots__ (closes #12766)

Lib/test/test_descr.py

@@ -4253,6 +4253,14 @@ order (MRO) for bases """
         foo = Foo()
         str(foo)
 
+    def test_slot_shadows_class(self):
+        with self.assertRaises(ValueError) as cm:
+            class X:
+                __slots__ = ["foo"]
+                foo = None
+        m = str(cm.exception)
+        self.assertEqual("'foo' in __slots__ conflicts with class variable", m)
+
 class DictProxyTests(unittest.TestCase):
     def setUp(self):
         class C(object):

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.3 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #12766: Raise an ValueError when creating a class with a class variable
+  that conflicts with a name in __slots__.
+
 - Issue #12266: Fix str.capitalize() to correctly uppercase/lowercase
   titlecased and cased non-letter characters.
 

Objects/typeobject.c

@@ -2094,6 +2094,12 @@ type_new(PyTypeObject *metatype, PyObject *args, PyObject *kwds)
             if (!tmp)
                 goto bad_slots;
             PyList_SET_ITEM(newslots, j, tmp);
+            if (PyDict_GetItem(dict, tmp)) {
+                PyErr_Format(PyExc_ValueError,
+                             "%R in __slots__ conflicts with class variable",
+                             tmp);
+                goto bad_slots;
+            }
             j++;
         }
         assert(j == nslots - add_dict - add_weak);