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def luhn(self):
        all_sum = 0
        self.__card_number = str(self.__card_number)
        self.__card_number = list(self.__card_number)
        for element in range(len(self.__card_number)):
            if element % 2 == 0:
                self.__card_number[element] = int(self.__card_number[element]) * 2
                if self.__card_number[element] > 9:
                    number = self.__card_number[element]
                    self.__card_number[element] = number // 100 + number // 10 % 10 + number % 10  # sum digits of number
            self.__card_number[element] = int(self.__card_number[element])
            all_sum += self.__card_number[element]
        checksum = 0
        while checksum < 10:
            if all_sum % 10 == 0:
                checksum = str(checksum)
                break
            else:
                all_sum += 1
                checksum += 1
        self.i = str(self.i)
        self.i = list(self.i)
        self.i.append(checksum)
        self.__card_number = self.i
 
        self.__card_number = "".join(self.__card_number)