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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 custom_code(self, mask: str = '@###', char: str = '@', digit: str = '#') -> str: """Generate custom code using ascii uppercase and random integers. :param mask: Mask of code. :param char: Placeholder for characters. :param digit: Placeholder for digits. :return: Custom code. """

        char_code = ord(char)
        digit_code = ord(digit)
        code = bytearray(len(mask))

        def random_int(a: int, b: int) -> int:
            b = b - a
            return int(self.random() * b) + a

        _mask = mask.encode()
        for i, p in enumerate(_mask):
            if p == char_code:
                a = random_int(65, 91)  # A-Z
            elif p == digit_code:
                a = random_int(48, 58)  # 0-9
            else:
                a = p
            code[i] = a
        return code.decode()