Issue #28214: Now __set_name__ is looked up on the class instead of the

instance.

Lib/test/test_subclassinit.py

@@ -148,6 +148,18 @@ class Test(unittest.TestCase):
             class A:
                 d = Descriptor()
 
+    def test_set_name_lookup(self):
+        resolved = []
+        class NonDescriptor:
+            def __getattr__(self, name):
+                resolved.append(name)
+
+        class A:
+            d = NonDescriptor()
+
+        self.assertNotIn('__set_name__', resolved,
+                         '__set_name__ is looked up in instance dict')
+
     def test_set_name_init_subclass(self):
         class Descriptor:
             def __set_name__(self, owner, name):

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.7.0 alpha 1
 Core and Builtins
 -----------------
 
+- Issue #28214: Now __set_name__ is looked up on the class instead of the
+  instance.
+
 - Issue #27955: Fallback on reading /dev/urandom device when the getrandom()
   syscall fails with EPERM, for example when blocked by SECCOMP.
 

Objects/typeobject.c

@@ -6990,19 +6990,21 @@ update_all_slots(PyTypeObject* type)
 static int
 set_names(PyTypeObject *type)
 {
-    PyObject *key, *value, *tmp;
+    PyObject *key, *value, *set_name, *tmp;
     Py_ssize_t i = 0;
 
     while (PyDict_Next(type->tp_dict, &i, &key, &value)) {
-        if (PyObject_HasAttr(value, _PyUnicode_FromId(&PyId___set_name__))) {
-            tmp = PyObject_CallMethodObjArgs(
-                value, _PyUnicode_FromId(&PyId___set_name__),
-                type, key, NULL);
+        set_name = lookup_maybe(value, &PyId___set_name__);
+        if (set_name != NULL) {
+            tmp = PyObject_CallFunctionObjArgs(set_name, type, key, NULL);
+            Py_DECREF(set_name);
             if (tmp == NULL)
                 return -1;
             else
                 Py_DECREF(tmp);
         }
+        else if (PyErr_Occurred())
+            return -1;
     }
 
     return 0;