diff -Naur old/shelve.py new/shelve.py
--- old/shelve.py	2011-03-22 21:48:37.000000000 +0000
+++ new/shelve.py	2011-03-22 21:52:48.000000000 +0000
@@ -90,6 +90,8 @@
         self.writeback = writeback
         self.cache = {}
         self.keyencoding = keyencoding
+        self._BytesIO = BytesIO
+        self._Pickler = Pickler
 
     def __iter__(self):
         for k in self.dict.keys():
@@ -119,8 +121,8 @@
     def __setitem__(self, key, value):
         if self.writeback:
             self.cache[key] = value
-        f = BytesIO()
-        p = Pickler(f, self._protocol)
+        f = self._BytesIO()
+        p = self._Pickler(f, self._protocol)
         p.dump(value)
         self.dict[key.encode(self.keyencoding)] = f.getvalue()