为什么一个值改变了，而第二个值却没有改变? [英] why did one value change but the second value did not?

问题描述

a = [ 1, 2 ]
b = a
a.append(3)
print(b) # shows [ 1 2 3 ] which means b changed

c = 4
d = c
c = 8
print(d) # shows 4 which means d did not change

为什么b改变了，而d没有改变? .append更改了存储在b中的值，但是c = 8并没有更改d中的值

Why did b change, but d did not? the .append changed the value stored in b, but c = 8 did not change the value in d

推荐答案

这完全取决于您分配的对象是mutable还是immutable.

It's all about if the object you assign is mutable or immutable.

简单地说-可变对象创建后可以更改，不可变对象不能更改.

Simply put - mutable objects can be changed after they are created, immutable objects can't.

考虑到您有一个分配给对象的变量a，当您将新变量指向a变量时，有两种可能性:

Considering you have a variable a that is assigned to an object, when you point a new variable to the a variable, there are two possibilities:

1. 如果对象是mutable->，则只需使用新变量指向同一对象
2. 如果对象是immutable->，则将新对象分配给新变量.

1. If an object is mutable -> you will just point to the same object with your new variable
2. If an object is immutable -> you will assign a new object to your new variable.

您的第一种情况:

• 首先创建一个列表a，它是一个可变对象
• 然后使用第二个变量b指向同一列表对象.
• 更改值时，只需更改两个变量都指向的可变对象即可.

• First you create a list a, which is a mutable object
• Then you point with a second variable b to the same list object.
• When you change value, you just change a mutable object, to which both variables are pointing.

第二种情况:

• 首先，您将变量c分配给不可变的int=4对象.
• 然后将其分配给第二个变量d.
• 接下来，您将为变量c分配一个新的不可变的int=8对象.

• First you assign a variable c to an immutable int=4 object.
• Then you assign it to the second variable d.
• And what happens next, is that you assign a new immutable int=8 object to the variable c.

有很多关于对象是可变对象的文章，例如:

There is plenty of articles about what does it mean that an object is mutable, for example: https://medium.com/@meghamohan/mutable-and-immutable-side-of-python-c2145cf72747

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