请教一个 Python 中 for in 列表修改值的问题

[jonathanchoo]

代码如下：

case1: 
lists = []

v1 = {"name": "aa", "age": 18}
lists.append(v1)

for v in lists:
    v["name"] = "bb"

print(lists) //[{'name': 'bb', 'age': 18}]


case2:
list2 = []

v2 = "a"
list2.append(v2)

for v in list2:
    v = "b"

print(list2) //a

按我的理解，for in 出来的 value 不应该是第二种情况，属于值拷贝吗，为什么在 case1 中，可以直接修改 list 中的值啊，是因为 list 中的字典类型是引用类型吗？

[crclz]

是的。其他语言会得出同样的结论。

[qgb]

str 是不可变对象，直接赋值 v 没有修改 list2，只是把 v 换了一个对象。你在 case1 中直接赋值 v={} 也是一样的

[laike9m]

你不能以 C/C++ 的思维模型来理解 Python 的变量
可以参考： http://foobarnbaz.com/2012/07/08/understanding-python-variables/

[hxysnail]

这个问题需要在 Python 对象模型中找答案，Python 对象都是通过指针引用的。Python 中的变量，可以理解成一个与变量名绑定在一起的对象指针。

case1 中，临时变量 v 引用列表保存的 dict 对象，然后对 dict 对象进行修改，也就是对列表中的对象进行修改。case2 中，临时变量一开始引用列表中保存的 string 对象，但后来修改临时变量将它指向一个新对象，列表引用的对象没有任何变化。

关于 Python 对象模型，可以参考这个系列： https://fasionchan.com/python-source/object-model/overview/

[ClericPy]

https://www.google.com/search?q=python+mutable+and+immutable

[jonathanchoo]

@hxysnail
@laike9m
获益匪浅，非常感谢！

[johnsona]

变量就是一个标签
两个例子分别指向一个你的列表 和你新创建的字符串

[IgniteWhite]

补充 Python 官方教程（ The Python Tutorial, by Guido van Rossum 本人）里的选段，作为参考（我自己正好在读）：

https://docs.python.org/3/tutorial/classes.html#a-word-about-names-and-objects

9.1. A Word About Names and Objects
Objects have individuality, and multiple names (in multiple scopes) can be bound to the same object. This is known as aliasing in other languages. This is usually not appreciated on a first glance at Python, and can be safely ignored when dealing with immutable basic types (numbers, strings, tuples). However, aliasing has a possibly surprising effect on the semantics of Python code involving mutable objects such as lists, dictionaries, and most other types. This is usually used to the benefit of the program, since aliases behave like pointers in some respects. For example, passing an object is cheap since only a pointer is passed by the implementation; and if a function modifies an object passed as an argument, the caller will see the change — this eliminates the need for two different argument passing mechanisms as in Pascal.

以及
https://docs.python.org/3/tutorial/classes.html#class-and-instance-variables

As discussed in A Word About Names and Objects, shared data can have possibly surprising effects with involving mutable objects such as lists and dictionaries.

[James369]

你举的两个例子，正好很好的说明了 Python 中可变对象和不可变对象的区别。

[madpecker009]

请不要尝试在循环中修改列表的值。其他语言同理

[duole]

可变对象 和 不可变对象的原因。度娘查一下 python 不可变对象都有哪些吧