new_contents: from callback_function import CallbackFunction
  
  class ReplaceCallback(CallbackFunction):
      
      def __init__(self, old_key, new_key, datatype=unicode):
          CallbackFunction.__init__(self, old_key, new_key, datatype)
  
  
      def __call__(self, _, val):
          if val:
              return (self._new_key, self._datatype(val))
          else:
              return (None, None)
  
subject: Create replace_callback.py