The new default protocol is two.

rhettinger · rhettinger · commit ba99c5887286 · 2008-02-10T20:41:56.000Z
diff --git a/Lib/shelve.py b/Lib/shelve.py
@@ -75,7 +75,7 @@ def __init__(self, dict, protocol=None, writeback=False,
                  keyencoding="utf-8"):
         self.dict = dict
         if protocol is None:
-            protocol = 0
+            protocol = 2
         self._protocol = protocol
         self.writeback = writeback
         self.cache = {}
