A small change to the C API for weakly-referencable types: Such types must now initialize the extra field used by the weak-ref machinery to NULL themselves, to avoid having to require PyObject_INIT() to check if the type supports weak references and do it there. This causes less work to be done for all objects (the type object does not need to be consulted to check for the Py_TPFLAGS_HAVE_WEAKREFS bit).

--- a/Objects/classobject.c	Thu Mar 22 18:05:30 2001 +0000
+++ b/Objects/classobject.c	Thu Mar 22 18:26:47 2001 +0000
@@ -453,6 +453,7 @@
 		Py_DECREF(dict);
 		return NULL;
 	}
+	inst->in_weakreflist = NULL;
 	Py_INCREF(klass);
 	inst->in_class = (PyClassObject *)klass;
 	inst->in_dict = dict;