diff --git a/Lib/inspect.py b/Lib/inspect.py
index 03347b9..dd89932 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -29,6 +29,7 @@
 __date__ = '1 Jan 2001'
 
 import sys, os, types, string, re, dis, imp, tokenize, linecache
+from operator import attrgetter
 
 # ----------------------------------------------------------- type-checking
 def ismodule(object):
@@ -567,7 +568,7 @@
 def walktree(classes, children, parent):
     """Recursive helper function for getclasstree()."""
     results = []
-    classes.sort(key=lambda c: (c.__module__, c.__name__))
+    classes.sort(key=attrgetter('__module__', '__name__'))
     for c in classes:
         results.append((c, c.__bases__))
         if c in children: