new_contents: 
  class BaseWrapper(object):
  
      def set_up():
          pass
  
      def get_key(key, default=None):
          raise NotImplementedError()
  
      def get_key_list(keys, default=None):
          return [ get_key(key, default) for key in keys ]
  
      def set_key_list(key_value_tuples):
          return [ set_key(key, value) for key, value in key_value_tuples ] 
  
      def set_key(key, value):
          raise NotImplementedError()
  
subject: Add a base class for the backends