如何实现类方法__mul__和__div__以使用长算法？

Question

首先，我知道Python从一开始就可以处理大量数字。但是我需要自己上课与他们一起工作。我意识到添加和 sub ，并且不完全了解如何实现 mul 和 div ？

如何实现类方法 mul 和 div 以使用长算法？

这是我的代码：

class Calc:


    def __init__(self, num):
        self.num = num
        self._data_len = len(num)


    def __len__(self):
        return self._data_len


    def __getitem__(self, index):
        return self.num[index]


    def __str__(self):
        return self.num


    def __add__(self, num2):
        self.num = self.num[::-1]
        num2 = num2[::-1]
        shift = 0
        result = ''
        maxlen = max(len(self.num), len(num2))
        for i in range(maxlen):
            if i < len(self.num):
                a1 = int(self.num[i])
            else:
                a1 = 0
            if i < len(num2):
                b1 = int(num2[i])
            else:
                b1 = 0
            r = a1 + b1 + shift
            shift = r // 10
            r = r % 10
            result += str(r)

        if shift > 0:
            result += str(shift)

        return result[::-1]


    def __sub__(self, num2):
        if len(self.num) & len(num2) < 10:
            return int(self.num) - int(num2)
        else:
        char = ''
        shift = 0
        result = ''
        if len(self.num) < len(num2):
            self.num, num2 = num2, self.num
            char = '-'
            self.num = self.num[::-1]
            num2 = num2[::-1]
            maxlen = max(len(self.num), len(num2))
            for i in range(maxlen):
                if i < len(self.num):
                    a1 = int(self.num[i])
                else:
                    a1 = 0
                if i < len(num2):
                    b1 = int(num2[i])
                else:
                    b1 = 0
                r = a1 - b1 - shift
                if r >= 0:
                    result += str(r)
                    shift = 0
                else:
                    r += 10
                    shift = 1
                    result += str(r)

        if char == '-':
            result += char
        return result[::-1]

示例：

a = '15321321321392130912301293120931'
b = '130382138921839219838129398123891938189'

a1 = Calc(a)
a2 = Calc(b)

a3 = a1 + a2
>>>>130382154243160541230260310425185059120    


a = '15321321321392130912301293120931'
b = '130382138921839219838129398123891938189'

a1 = Calc(a)
a2 = Calc(b)

a3 = a1 - a2
>>>>-130382123600517898445998485822598817258

a = '15321321321392130912301293120931'
b = '130382138921839219838129398123891938189'

a1 = Calc(a)
a2 = Calc(b)

a3 = a1 * a2
>>>> mul??

有什么建议吗？