python - 为列表设置 __setattr__ 时出错

Question

我正在实现一个类的所有预定义函数，当我实现我的__setattr__() 来更新列表时我卡住了

class testop:
    def __init__(self):
        self.li=[x*2 for x in list(range(5))]
    def __getitem__(self,index):
        return self.li[index]
    def __setattr__(self,attr,value):
        self.__dict__[attr]+=value #self.__dict__[attr].append(value)

if __name__=='__main__':
    testop().li=3

错误:

line 10, in <module>
    testop().li=30
  File "C:/Users/Desktop/testop.py", line 3, in __init__
    self.li=[x*2 for x in list(range(5))]
  File "C:/Users/Desktop/testop.py", line 7, in __setattr__
    self.__dict__[attr]+=value
KeyError: 'li'

对于 list 以外的普通变量，它确实工作正常，谁能说出我缺少的是什么？

Thanks for the answers and i am getting it clear why self.__dict__[attr] fail to find the key, Hmmmmmm but the normal variables it does work fine even when not adding it before hand to the dict?

Answer 1

问题是当 __init__ 被调用时，li 还不存在。

基本上，发生的事情是作为 __init__ 的一部分，您调用 __setattr__ 来设置 li。但是，li 还不存在，所以你得到一个键错误。

您需要做的是在尝试添加之前检查该属性是否存在。

class testop:
    def __init__(self):
        self.li = [x * 2 for x in list(range(5))]

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

    def __setattr__(self, attr, value):
        if attr in self.__dict__:
            self.__dict__[attr] += value
        else:
            pass  # Either create it or throw an exception, whatever you want to do.


if __name__ == '__main__':
    testop().li = 3