Address SF patch #480716 as well as related issues.

SF patch #480716 by Greg Chapman fixes the problem that super's
__get__ method always returns an instance of super, even when the
instance whose __get__ method is called is an instance of a subclass
of super.

Other issues fixed:

- super(C, C()).__class__ would return the __class__ attribute of C()
  rather than the __class__ attribute of the super object.  This is
  confusing.  To fix this, I decided to change the semantics of super
  so that it only applies to code attributes, not to data attributes.
  After all, overriding data attributes is not supported anyway.

- While super(C, x) carefully checked that x is an instance of C,
  super(C).__get__(x) made no such check, allowing for a loophole.
  This is now fixed.

Lib/test/test_descr.py

@@ -1572,7 +1572,57 @@ def supers():
         def meth(self, a):
             return "D(%r)" % a + super(D, self).meth(a)
 
-    verify (D().meth(4) == "D(4)C(4)B(4)A(4)")
+    vereq(D().meth(4), "D(4)C(4)B(4)A(4)")
+
+    # Test for subclassing super
+
+    class mysuper(super):
+        def __init__(self, *args):
+            return super(mysuper, self).__init__(*args)
+
+    class E(D):
+        def meth(self, a):
+            return "E(%r)" % a + mysuper(E, self).meth(a)
+
+    vereq(E().meth(5), "E(5)D(5)C(5)B(5)A(5)")
+
+    class F(E):
+        def meth(self, a):
+            s = self.__super
+            return "F(%r)[%s]" % (a, s.__class__.__name__) + s.meth(a)
+    F._F__super = mysuper(F)
+
+    vereq(F().meth(6), "F(6)[mysuper]E(6)D(6)C(6)B(6)A(6)")
+
+    # Make sure certain errors are raised
+
+    try:
+        super(D, 42)
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "shouldn't allow super(D, 42)"
+
+    try:
+        super(D, C())
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "shouldn't allow super(D, C())"
+
+    try:
+        super(D).__get__(12)
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "shouldn't allow super(D).__get__(12)"
+
+    try:
+        super(D).__get__(C())
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "shouldn't allow super(D).__get__(C())"
 
 def inherits():
     if verbose: print "Testing inheritance from basic types..."

Objects/typeobject.c

@@ -3929,7 +3929,7 @@ super_getattro(PyObject *self, PyObject *name)
 			else
 				continue;
 			res = PyDict_GetItem(dict, name);
-			if (res != NULL) {
+			if (res != NULL  && !PyDescr_IsData(res)) {
 				Py_INCREF(res);
 				f = res->ob_type->tp_descr_get;
 				if (f != NULL) {
@@ -3944,6 +3944,21 @@ super_getattro(PyObject *self, PyObject *name)
 	return PyObject_GenericGetAttr(self, name);
 }
 
+static int
+supercheck(PyTypeObject *type, PyObject *obj)
+{
+	if (!PyType_IsSubtype(obj->ob_type, type) &&
+	    !(PyType_Check(obj) &&
+	      PyType_IsSubtype((PyTypeObject *)obj, type))) {
+		PyErr_SetString(PyExc_TypeError,
+			"super(type, obj): "
+			"obj must be an instance or subtype of type");
+		return -1;
+	}
+	else
+		return 0;
+}
+
 static PyObject *
 super_descr_get(PyObject *self, PyObject *obj, PyObject *type)
 {
@@ -3955,14 +3970,25 @@ super_descr_get(PyObject *self, PyObject *obj, PyObject *type)
 		Py_INCREF(self);
 		return self;
 	}
-	new = (superobject *)PySuper_Type.tp_new(&PySuper_Type, NULL, NULL);
-	if (new == NULL)
-		return NULL;
-	Py_INCREF(su->type);
-	Py_INCREF(obj);
-	new->type = su->type;
-	new->obj = obj;
-	return (PyObject *)new;
+	if (su->ob_type != &PySuper_Type)
+		/* If su is an instance of a subclass of super,
+		   call its type */
+		return PyObject_CallFunction((PyObject *)su->ob_type,
+					     "OO", su->type, obj);
+	else {
+		/* Inline the common case */
+		if (supercheck(su->type, obj) < 0)
+			return NULL;
+		new = (superobject *)PySuper_Type.tp_new(&PySuper_Type,
+							 NULL, NULL);
+		if (new == NULL)
+			return NULL;
+		Py_INCREF(su->type);
+		Py_INCREF(obj);
+		new->type = su->type;
+		new->obj = obj;
+		return (PyObject *)new;
+	}
 }
 
 static int
@@ -3976,15 +4002,8 @@ super_init(PyObject *self, PyObject *args, PyObject *kwds)
 		return -1;
 	if (obj == Py_None)
 		obj = NULL;
-	if (obj != NULL &&
-	    !PyType_IsSubtype(obj->ob_type, type) &&
-	    !(PyType_Check(obj) &&
-	      PyType_IsSubtype((PyTypeObject *)obj, type))) {
-		PyErr_SetString(PyExc_TypeError,
-			"super(type, obj): "
-			"obj must be an instance or subtype of type");
+	if (obj != NULL && supercheck(type, obj) < 0)
 		return -1;
-	}
 	Py_INCREF(type);
 	Py_XINCREF(obj);
 	su->type = type;