I had the inheritance cycle stuff backwards. Oops!

Lib/test/test_descr.py

@@ -3452,6 +3452,7 @@ def mutable_bases():
         pass
     d = D()
     e = E()
+    D.__bases__ = (C,)
     D.__bases__ = (C2,)
     vereq(d.meth(), 1)
     vereq(e.meth(), 1)
@@ -3492,6 +3493,13 @@ def mutable_bases():
         # actually, we'll have crashed by here...
         raise TestFailed, "shouldn't be able to create inheritance cycles"
 
+    try:
+        D.__bases__ = (E,)
+    except TypeError:
+        pass
+    else:
+        raise TestFailed, "shouldn't be able to create inheritance cycles"
+
     # let's throw a classic class into the mix:
     class Classic:
         def meth2(self):

Objects/typeobject.c

@@ -208,10 +208,12 @@ type_set_bases(PyTypeObject *type, PyObject *value, void *context)
 				type->tp_name, ob->ob_type->tp_name);
 			return -1;
 		}
-		if (PyType_IsSubtype(type, (PyTypeObject*)ob)) {
-			PyErr_SetString(PyExc_TypeError,
-		"a __bases__ item causes an inheritance cycle");
-			return -1;
+		if (PyType_Check(ob)) {
+			if (PyType_IsSubtype((PyTypeObject*)ob, type)) {
+				PyErr_SetString(PyExc_TypeError,
+			"a __bases__ item causes an inheritance cycle");
+				return -1;
+			}
 		}
 	}