我在添加方法时遇到麻烦

Question

my code:

class Test(object):
    def __init__(self, number):
        self.number = number
    def __str__(self):
        return "This number is {}".format(self.number)
    def __add__(self, second):
        self.number = self.number + second.number
        return self
c1 = Test(1)
c2 = Test(2)
c2 = c1 + c2
print(c2)
print(c1)

Output:

This number is 3
This number is 3

my trouble is that it changes the c1 and c2, when I try to add them. How do I keep c1 the same?

Answer 1

Return a new object:

class Test(object):
    def __init__(self, number):
        self.number = number

    def __str__(self):
        return "This number is {}".format(self.number)

    def __add__(self, second):
        return Test(self.number + second.number)

Note that when you rebind c2 like this:

c2 = c1 + c2

its original value is lost. With your code, both c1 and c2 will point to the first Test object ( c1 + c2 returns self from c1 's __add__ ).

With my code, the original second object will be deleted and a new object will be bound to c2 .

Answer 2

Your problem lies in the fact that you modify self in your __add__ method. You're not supposed to do that (though you can, since Python considers all people involved to be "consenting adults"... and sometimes, it would be the correct solution. Though I can't think of a scenario where it would be)

And self in your __add__ method gets bound to c1, so you modify c1; and then you assign it to c2.

Try adding print(c1 is c2) at the end. It will print True .

Pavel's __add__ behaves as an __add__ method is supposed to.