code: def get_key(self, key_type, key_id):
          
          return "{0}:{1}{2}{3}".format(key_type, self._hash_start, key_id,
                                        self._hash_stop)
docs: get_key constructs a key given a key type and a key id.
          Keyword arguments:
          key_type -- the type of key (e.g.: 'friend_request')
          key_id -- the key id (e.g.: '12345')
  
          returns a string representing the key
          (e.g.: 'friend_request:{12345}')