diff -r c87e00a6258d Lib/urllib/request.py
--- a/Lib/urllib/request.py	Thu Sep 25 19:48:15 2014 +1000
+++ b/Lib/urllib/request.py	Thu Sep 25 15:25:38 2014 +0200
@@ -1052,7 +1052,8 @@
             H = lambda x: hashlib.md5(x.encode("ascii")).hexdigest()
         elif algorithm == 'SHA':
             H = lambda x: hashlib.sha1(x.encode("ascii")).hexdigest()
-        # XXX MD5-sess
+        else: # MD5-sess
+            raise NotImplementedError("Unsupported Digest Authentication Algorithm '%s'" % algorithm)
         KD = lambda s, d: H("%s:%s" % (s, d))
         return H, KD