diff --git a/Lib/inspect.py b/Lib/inspect.py
index 1f49bcb..4874904 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -553,7 +553,7 @@
 def walktree(classes, children, parent):
     """Recursive helper function for getclasstree()."""
     results = []
-    classes.sort(lambda a, b: cmp(a.__name__, b.__name__))
+    classes.sort(key=lambda c: c.__name__)
     for c in classes:
         results.append((c, c.__bases__))
         if c in children: