def __reduce__(self):
return (type(self), (self.default_factory,), None, None, self.iteritems())<EOL>
__reduce__ must return a 5-tuple as follows:

   - factory function
   - tuple of args for the factory function
   - additional state (here None)
   - sequence iterator (here None)
   - dictionary iterator (yielding successive (key, value) pairs

   This API is used by pickle.py and copy.py.