when __getattr__ is a descriptor, call it correctly; fixes #4230

patch from Ziga Seilnacht

Lib/test/test_descr.py

@@ -4289,6 +4289,46 @@ order (MRO) for bases """
         c[1:2] = 3
         self.assertEqual(c.value, 3)
 
+    def test_getattr_hooks(self):
+        # issue 4230
+
+        class Descriptor(object):
+            counter = 0
+            def __get__(self, obj, objtype=None):
+                def getter(name):
+                    self.counter += 1
+                    raise AttributeError(name)
+                return getter
+
+        descr = Descriptor()
+        class A(object):
+            __getattribute__ = descr
+        class B(object):
+            __getattr__ = descr
+        class C(object):
+            __getattribute__ = descr
+            __getattr__ = descr
+
+        self.assertRaises(AttributeError, getattr, A(), "attr")
+        self.assertEquals(descr.counter, 1)
+        self.assertRaises(AttributeError, getattr, B(), "attr")
+        self.assertEquals(descr.counter, 2)
+        self.assertRaises(AttributeError, getattr, C(), "attr")
+        self.assertEquals(descr.counter, 4)
+
+        import gc
+        class EvilGetattribute(object):
+            # This used to segfault
+            def __getattr__(self, name):
+                raise AttributeError(name)
+            def __getattribute__(self, name):
+                del EvilGetattribute.__getattr__
+                for i in range(5):
+                    gc.collect()
+                raise AttributeError(name)
+
+        self.assertRaises(AttributeError, getattr, EvilGetattribute(), "attr")
+
 
 class DictProxyTests(unittest.TestCase):
     def setUp(self):

Misc/NEWS

@@ -12,6 +12,8 @@ What's New in Python 2.7 alpha 1
 Core and Builtins
 -----------------
 
+- Issue #4230: If ``__getattr__`` is a descriptor, it now functions correctly.
+
 - Issue #4048: The parser module now correctly validates relative imports.
 
 - Issue #4225: ``from __future__ import unicode_literals`` didn't work in an

Objects/typeobject.c

@@ -5390,6 +5390,24 @@ slot_tp_getattro(PyObject *self, PyObject *name)
 			   "(O)", name);
 }
 
+static PyObject *
+call_attribute(PyObject *self, PyObject *attr, PyObject *name)
+{
+	PyObject *res, *descr = NULL;
+	descrgetfunc f = Py_TYPE(attr)->tp_descr_get;
+
+	if (f != NULL) {
+		descr = f(attr, self, (PyObject *)(Py_TYPE(self)));
+		if (descr == NULL)
+			return NULL;
+		else
+			attr = descr;
+	}
+	res = PyObject_CallFunctionObjArgs(attr, name, NULL);
+	Py_XDECREF(descr);
+	return res;
+}
+
 static PyObject *
 slot_tp_getattr_hook(PyObject *self, PyObject *name)
 {
@@ -5409,24 +5427,39 @@ slot_tp_getattr_hook(PyObject *self, PyObject *name)
 		if (getattribute_str == NULL)
 			return NULL;
 	}
+	/* speed hack: we could use lookup_maybe, but that would resolve the
+	   method fully for each attribute lookup for classes with
+	   __getattr__, even when the attribute is present. So we use
+	   _PyType_Lookup and create the method only when needed, with
+	   call_attribute. */
 	getattr = _PyType_Lookup(tp, getattr_str);
 	if (getattr == NULL) {
 		/* No __getattr__ hook: use a simpler dispatcher */
 		tp->tp_getattro = slot_tp_getattro;
 		return slot_tp_getattro(self, name);
 	}
+	Py_INCREF(getattr);
+	/* speed hack: we could use lookup_maybe, but that would resolve the
+	   method fully for each attribute lookup for classes with
+	   __getattr__, even when self has the default __getattribute__
+	   method. So we use _PyType_Lookup and create the method only when
+	   needed, with call_attribute. */
 	getattribute = _PyType_Lookup(tp, getattribute_str);
 	if (getattribute == NULL ||
 	    (Py_TYPE(getattribute) == &PyWrapperDescr_Type &&
 	     ((PyWrapperDescrObject *)getattribute)->d_wrapped ==
 	     (void *)PyObject_GenericGetAttr))
 		res = PyObject_GenericGetAttr(self, name);
-	else
-		res = PyObject_CallFunctionObjArgs(getattribute, self, name, NULL);
+	else {
+		Py_INCREF(getattribute);
+		res = call_attribute(self, getattribute, name);
+		Py_DECREF(getattribute);
+	}
 	if (res == NULL && PyErr_ExceptionMatches(PyExc_AttributeError)) {
 		PyErr_Clear();
-		res = PyObject_CallFunctionObjArgs(getattr, self, name, NULL);
+		res = call_attribute(self, getattr, name);
 	}
+	Py_DECREF(getattr);
 	return res;
 }