diff --git a/Lib/functools.py b/Lib/functools.py
index 03de69a..e92a2fc 100644
--- a/Lib/functools.py
+++ b/Lib/functools.py
@@ -96,6 +96,7 @@
 def cmp_to_key(mycmp):
     """Convert a cmp= function into a key= function"""
     class K(object):
+        __slots__ = ['obj']
         def __init__(self, obj, *args):
             self.obj = obj
         def __lt__(self, other):