(Merge 3.1) Issue #9756: When calling a method descriptor or a slot wrapper

descriptor, the check of the object type doesn't read the __class__ attribute
anymore.  Fix a crash if a class override its __class__ attribute (e.g. a proxy
of the str type).

Lib/test/test_descr.py

@@ -4235,6 +4235,22 @@ order (MRO) for bases """
         with self.assertRaises(AttributeError):
             del X.__abstractmethods__
 
+    def test_proxy_call(self):
+        class FakeStr:
+            __class__ = str
+
+        fake_str = FakeStr()
+        # isinstance() reads __class__
+        self.assertTrue(isinstance(fake_str, str))
+
+        # call a method descriptor
+        with self.assertRaises(TypeError):
+            str.split(fake_str)
+
+        # call a slot wrapper descriptor
+        with self.assertRaises(TypeError):
+            str.__add__(fake_str, "abc")
+
 
 class DictProxyTests(unittest.TestCase):
     def setUp(self):

Misc/NEWS

@@ -10,6 +10,11 @@ What's New in Python 3.2.1?
 Core and Builtins
 -----------------
 
+- Issue #9756: When calling a method descriptor or a slot wrapper descriptor,
+  the check of the object type doesn't read the __class__ attribute anymore.
+  Fix a crash if a class override its __class__ attribute (e.g. a proxy of the
+  str type).
+
 - Issue #10914: Initialize correctly the filesystem codec when creating a new
   subinterpreter to fix a bootstrap issue with codecs implemented in Python, as
   the ISO-8859-15 codec.

Objects/descrobject.c

@@ -226,7 +226,8 @@ methoddescr_call(PyMethodDescrObject *descr, PyObject *args, PyObject *kwds)
         return NULL;
     }
     self = PyTuple_GET_ITEM(args, 0);
-    if (!PyObject_IsInstance(self, (PyObject *)PyDescr_TYPE(descr))) {
+    if (!_PyObject_RealIsSubclass((PyObject *)Py_TYPE(self),
+                                  (PyObject *)PyDescr_TYPE(descr))) {
         PyErr_Format(PyExc_TypeError,
                      "descriptor '%V' "
                      "requires a '%.100s' object "
@@ -284,7 +285,8 @@ wrapperdescr_call(PyWrapperDescrObject *descr, PyObject *args, PyObject *kwds)
         return NULL;
     }
     self = PyTuple_GET_ITEM(args, 0);
-    if (!PyObject_IsInstance(self, (PyObject *)PyDescr_TYPE(descr))) {
+    if (!_PyObject_RealIsSubclass((PyObject *)Py_TYPE(self),
+                                  (PyObject *)PyDescr_TYPE(descr))) {
         PyErr_Format(PyExc_TypeError,
                      "descriptor '%V' "
                      "requires a '%.100s' object "
@@ -1065,7 +1067,8 @@ PyWrapper_New(PyObject *d, PyObject *self)
 
     assert(PyObject_TypeCheck(d, &PyWrapperDescr_Type));
     descr = (PyWrapperDescrObject *)d;
-    assert(PyObject_IsInstance(self, (PyObject *)PyDescr_TYPE(descr)));
+    assert(_PyObject_RealIsSubclass((PyObject *)Py_TYPE(self),
+                                    (PyObject *)PyDescr_TYPE(descr)));
 
     wp = PyObject_GC_New(wrapperobject, &wrappertype);
     if (wp != NULL) {