覆盖 __add__ 方法后的 TypeError [英] TypeError after overriding the __add__ method

问题描述

我想了解 __add__ 是如何工作的:

I am trying to understand how __add__ works:

class MyNum:
    def __init__(self,num):
        self.num=num
    def __add__(self,other):
        return MyNum(self.num+other.num)
    def __str__(self):
        return str(self.num)

如果我把它们放在一个列表中

If I put them in a list

d=[MyNum(i) for i in range(10)]

这有效

t=MyNum(0)
for n in d:
    t=t+n
print t

但这不会:

print sum(d)
TypeError: unsupported operand type(s) for +: 'int' and 'instance'

我做错了什么?我怎样才能让 sum() 起作用?

What am I doing wrong? How can I get the sum() to work?

更新

我的问题是如何在支持 __add__ 的对象列表上使用 sum，需要尽可能保持通用.

My problem is how to use the sum on a list of objects that support the __add__, need to keep it as generic as possible.

推荐答案

您还需要定义 __radd__ 以使其工作.

You need to define __radd__ as well to get this to work.

__radd__ 是反向添加.当 Python 尝试计算 x + y 时，它首先尝试调用 x.__add__(y).如果失败，则返回到 y.__radd__(x).

__radd__ is reverse add. When Python tries to evaluate x + y it first attempts to call x.__add__(y). If this fails then it falls back to y.__radd__(x).

这允许您仅通过触摸一个类来覆盖加法.例如，考虑 Python 必须如何计算 0 + x.尝试调用 0.__add__(x) 但 int 对您的类一无所知.您不能很好地更改 int 中的 __add__ 方法，因此需要 __radd__.我想这是依赖倒置的一种形式.

This allows you to override addition by only touching one class. Consider for example how Python would have to evaluate 0 + x. A call to 0.__add__(x) is attempted but int knows nothing about your class. You can't very well change the __add__ method in int, hence the need for __radd__. I suppose it is a form of dependency inversion.

正如 Steven 指出的那样，sum 就地操作，但从 0 开始.所以第一个加法是唯一需要使用 __radd__ 的加法.作为一个很好的练习，您可以检查是否是这种情况！

As Steven pointed out, sum operates in place, but starts from 0. So the very first addition is the only one that would need to use __radd__. As a nice exercise you could check that this was the case!

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