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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def __get_data_size(self):
 """ 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 """

        response = self.__response
        if response == const.CMD_PREPARE_DATA:
            size = unpack('I', self.__data[:4])[0]
            return size
        else:
            return 0