diff -r ad855c779bf3 Lib/test/test_class.py
--- a/Lib/test/test_class.py	Thu Dec 03 17:32:05 2015 -0800
+++ b/Lib/test/test_class.py	Fri Dec 04 06:11:21 2015 -0600
@@ -568,5 +568,17 @@
         a = A(hash(A.f)^(-1))
         hash(a.f)
 
+    def testSetattrWrapperNameIntern(self):
+        ''' issue 25794: __setattr__ should intern the attribute name'''
+        class Test(object):
+            pass
+
+        def add(self, other):
+            return 1
+
+        name = '__{}__'.format('add') # not interned
+        type.__setattr__(Test, name, add)
+        Test() + 1
+
 if __name__ == '__main__':
     unittest.main()
diff -r ad855c779bf3 Objects/typeobject.c
--- a/Objects/typeobject.c	Thu Dec 03 17:32:05 2015 -0800
+++ b/Objects/typeobject.c	Fri Dec 04 06:11:21 2015 -0600
@@ -5379,7 +5379,16 @@
         return NULL;
     if (!hackcheck(self, func, "__setattr__"))
         return NULL;
+    if (!PyUnicode_Check(name)) {
+        PyErr_Format(PyExc_TypeError,
+                     "attribute name must be string, not '%.200s'",
+                     name->ob_type->tp_name);
+        return NULL;
+    }
+    Py_INCREF(name);
+    PyUnicode_InternInPlace(&name);
     res = (*func)(self, name, value);
+    Py_DECREF(name);
     if (res < 0)
         return NULL;
     Py_INCREF(Py_None);
@@ -5398,7 +5407,16 @@
     name = PyTuple_GET_ITEM(args, 0);
     if (!hackcheck(self, func, "__delattr__"))
         return NULL;
+    if (!PyUnicode_Check(name)) {
+        PyErr_Format(PyExc_TypeError,
+                     "attribute name must be string, not '%.200s'",
+                     name->ob_type->tp_name);
+        return NULL;
+    }
+    Py_INCREF(name);
+    PyUnicode_InternInPlace(&name);
     res = (*func)(self, name, NULL);
+    Py_DECREF(name);
     if (res < 0)
         return NULL;
     Py_INCREF(Py_None);