The usual
diff --git a/Lib/dos-8x3/userdict.py b/Lib/dos-8x3/userdict.py
index 3c48415..9b6e73b 100755
--- a/Lib/dos-8x3/userdict.py
+++ b/Lib/dos-8x3/userdict.py
@@ -34,3 +34,7 @@
                 self.data[k] = v
     def get(self, key, failobj=None):
         return self.data.get(key, failobj)
+    def setdefault(self, key, failobj=None):
+        if not self.data.has_key(key):
+            self.data[key] = failobj
+        return self.data[key]