code: def __get_data_size(self):
          
          response = self.__response
          if response == const.CMD_PREPARE_DATA:
              size = unpack('I', self.__data[:4])[0]
              return size
          else:
              return 0
docs: Checks a returned packet to see if it returned CMD_PREPARE_DATA,
          indicating that data packets are to be sent
  
          Returns the amount of bytes that are going to be sent