Index: sessions.py
===================================================================
--- sessions.py	(revision 1766)
+++ sessions.py	(working copy)
@@ -9,7 +9,10 @@
 class SessionWrapper(object):
     def __init__(self, session_key):
         self.session_key = session_key
-        self.modified = False
+        self.modified = False
+
+    def __contains__(self, key):
+        return key in self._session
 
     def __getitem__(self, key):
         return self._session[key]