Thought of a bug, wrote a test, discovered it was a bug, fixed it.

src/fix.py

@@ -127,6 +127,8 @@ def all(p, seq):
     return True
 
 def rsplit(s, sep=None, maxsplit=-1):
+    if sep is not None:
+        sep = sep[::-1]
     L = s[::-1].split(sep, maxsplit)
     L.reverse()
     return [s[::-1] for s in L]

test/test_fix.py

@@ -94,6 +94,7 @@ class FunctionsTest(unittest.TestCase):
                          ['foo bar', 'baz']) 
         self.assertEqual(rsplit('foo        bar baz', maxsplit=1),
                          ['foo        bar', 'baz'])
+        self.assertEqual(rsplit('foobarbaz', 'bar'), ['foo', 'baz'])
 
 
 # vim:set shiftwidth=4 tabstop=8 expandtab textwidth=78: