列为类属性 [英] List as class attribute

问题描述

我不完全了解，以下示例中的类属性整数或列表的区别是什么:

I do not fully understand, what the difference of class attribute integer or list in the following example is:

class MyClass(object):

    a = [1]
    b = 1

    def __init__(self):
        print(self.a, id(MyClass.a))
        print(self.b, id(MyClass.b))

    def modify(self):
        self.a.append(2)
        print("modfied list attribute:", MyClass.a, id(MyClass.a))
        print("modfied listattribute:", self.a, id(self.a))

        self.b += 1
        print("modfied int attribute:", MyClass.b, id(MyClass.b))
        print("modfied int attribute:", self.b, id(self.b))

x = MyClass()
x.modify()
y = MyClass()

在此示例的输出中，可以看到，如果我在一个实例(x)中更改了列表，则MyClass中的实际属性也会被更改.但是，在一个实例中更改该整数后，该类的整数将保持不变.我认为，这与列表的功能有关，即，如果附加了某些值，则ID仍会保留.

In the output of this example one can see, that if I alter the list in one instance (x), the actual atribute in MyClass is altered. The integer however when changed in one instance remains for the class the same. I assume, that this has to do with features of a list, i.e. that the id remains if some value is appended.

有人可以简要地给我一个简短的解释，这种行为背后的主要原因是什么?

Can someone give me a short explanation in a nutshell, what the main reason behind this behaviour is?

推荐答案

int 值是不可变的.您没有修改现有的 int ，而是将 self.b 替换为不同 int 值.

int values are immutable. You didn't not modify an existing int, but replaced self.b with a different int value.

list 值是可变的，而 list.append 修改调用它的实例. self.a 就地修改，而不用新列表替换现有列表.

list values are mutable, and list.append modifies the instance that invokes it. self.a is modified in-place without replacing the existing list with a new one.

基本上，如果不知道 + = 的值类型，就无法预测其行为.在 b = 0;b + = 1 ， b 指的是 int 的新实例.但是在 b = [];中b + = [2,3] ，则扩展了现有列表(就像您调用了 b.extend([2,3])一样).

Basically, you cannot predict the behavior of += without knowing the type of the value using it. In b = 0; b += 1, b refers to a new instance of int. But in b = []; b += [2,3], the existing list is extended (as if you had called b.extend([2,3])).

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