没有得到预期的输出 - python 类

Question

This is the output I am getting for the code given below: -4,-4

The expected output: 8,-4

class arthemetic():
    def __init__(self,a):
        self.a=a
        self.b=a

    def __add__(self, other):
         self.a=self.b
         self.a+=other.a
         return self
    def __sub__(self, other):
        self.a = self.b
        self.a-=other.a
        return self
    def __str__(self):
        return str(self.a)


if __name__ =='__main__':
    a =arthemetic(2)
    b=arthemetic(6)
    print(*map(str,[a+b,a-b]))

Answer 1

You shouldn't mutate the object within your __add__ and __sub__ methods but return new objects instead. By not doing that, the operations become non-deterministic. Both a + b and a - b return the a object after mutating it. That means that doing a - b actually changes the result from the previous a + b . Since both reference the same object, you get the same result in both cases.

_{-- from poke 's comment}