如何正确地对类实例执行算术？例如。求和House类实例列表的n_people属性

Question

如何正确地对类实例执行算术？在下面的示例中，类表示House并包含表示该房屋中人数的n_people属性。

目标是对House类实例执行求和并返回房屋中的总人数。当House类实例加在一起时，这似乎有效：

class House:

    def __init__(self, n_people):
        self.n_people = n_people

    def __add__(self, n_people):
        return n_people + self.n_people

House(3) + House(4)

在汇总House类实例数组时失败：

houses = [House(i) for i in range(10)]
sum(houses)

产生的错误是：

TypeError                                 Traceback (most recent call last)
<ipython-input-66-197f26932b89> in <module>
      1 houses = [House(i) for i in range(10)]
----> 2 sum(houses)

TypeError: unsupported operand type(s) for +: 'int' and 'House'

在Python中对类实例执行算术的正确方法是什么？

Answer 1

你面临的问题是sum从左到右工作，所以在前两个House实例相加后，你的代码试图执行int + House，只定义了House + int。

要解决这个问题，您需要定义__radd__，在这种情况下调用它：

class House:

    def __init__(self, n_people):
        self.n_people = n_people

    def __add__(self, n_people):
        return n_people + self.n_people

    def __radd__(self, n_people):
        return n_people + self.n_people

House(3) + House(4)
houses = [House(i) for i in range(10)]
sum(houses)

输出：

45