Explicitly raise an exception in __cmp__ -- this clarifies that cmp()

is not supported on sets.  (Unfortunately, sorting a list of sets may
still return random results because it uses < exclusively, but for
sets that inly implements a partial ordering.  Oh well.)

Lib/sets.py

@@ -102,6 +102,11 @@ class BaseSet(object):
         """
         return self._data.iterkeys()
 
+    # Three-way comparison is not supported
+
+    def __cmp__(self, other):
+        raise TypeError, "can't compare sets using cmp()"
+
     # Equality comparisons using the underlying dicts
 
     def __eq__(self, other):