diff -r 2bf154ca43c6 Lib/test/test_weakref.py
--- a/Lib/test/test_weakref.py	Sat Apr 06 01:15:30 2013 +0200
+++ b/Lib/test/test_weakref.py	Sat Apr 06 13:52:33 2013 +0100
@@ -802,6 +802,29 @@
         del root
         gc.collect()
 
+    def test_callback_attribute(self):
+        x = Object(1)
+        callback = lambda ref: None
+        ref1 = weakref.ref(x, callback)
+        self.assertIs(ref1.__callback__, callback)
+
+        ref2 = weakref.ref(x)
+        self.assertIsNone(ref2.__callback__)
+
+    def test_callback_attribute_after_deletion(self):
+        x = Object(1)
+        ref = weakref.ref(x, self.callback)
+        self.assertIsNotNone(ref.__callback__)
+        del x
+        self.assertIsNone(ref.__callback__)
+
+    def test_set_callback_attribute(self):
+        x = Object(1)
+        callback = lambda ref: None
+        ref1 = weakref.ref(x, callback)
+        with self.assertRaises(AttributeError):
+            ref1.__callback__ = lambda ref: None
+
 
 class SubclassableWeakrefTestCase(TestBase):
 
diff -r 2bf154ca43c6 Objects/weakrefobject.c
--- a/Objects/weakrefobject.c	Sat Apr 06 01:15:30 2013 +0200
+++ b/Objects/weakrefobject.c	Sat Apr 06 13:52:33 2013 +0100
@@ -209,6 +209,24 @@
                                 PyWeakref_GET_OBJECT(other), op);
 }
 
+/* Getter for __callback__ attribute. */
+static PyObject *
+weakref_get_callback(PyWeakReference* self)
+{
+    if (self->wr_callback == NULL) {
+        Py_RETURN_NONE;
+    }
+    else {
+        Py_INCREF(self->wr_callback);
+        return self->wr_callback;
+    }
+}
+
+static PyGetSetDef weakref_getset[] = {
+    {"__callback__", (getter)weakref_get_callback, (setter)NULL},
+    {NULL} /* Sentinel */
+};
+
 /* Given the head of an object's list of weak references, extract the
  * two callback-less refs (ref and proxy).  Used to determine if the
  * shared references exist and to determine the back link for newly
@@ -370,7 +388,7 @@
     0,                          /*tp_iternext*/
     0,                          /*tp_methods*/
     0,                          /*tp_members*/
-    0,                          /*tp_getset*/
+    weakref_getset,             /*tp_getset*/
     0,                          /*tp_base*/
     0,                          /*tp_dict*/
     0,                          /*tp_descr_get*/