了解清单清单 [英] Understanding lists of lists

问题描述

可能重复:
Python列表列表中的意外功能
Python列表混乱

Possible Duplicate:
Unexpected feature in a Python list of lists
Python list confusion

考虑以下代码:

a = [[]] * 3
a[1].append("foo")

我希望a的值变为:

[[], ["foo"], []]

相反，a的每个元素都会更新:

instead, every element of a is updated:

[["foo"], ["foo"], ["foo"]]

有人可以解释为什么列表中的每个元素而不是仅定义的元素(a[1])被更新了吗?我的逻辑有根本上的错误吗?

Can someone please explain why did each element of the list got updated instead of just the defined element (a[1])? Is there something fundamentally wrong in my logic?

对于运行我的Python2.7的价值而言

For what it's worth I'm running Python2.7

推荐答案

Python中的列表是对象，这意味着当您将列表分配给另一个变量时，更改其中任何一个都会更改实际列表，例如

Lists in Python are objects, this means when you assign a list to another variable, changing either will change the actual list, for example

a = []
b = a
b.append("hi")
print a
// a is now ["hi"]

完成任务时

a = [[]] * 3

这跟说的一样

inner_list = []
outer_list = [inner_list, inner_list, inner_list]

换句话说，所有内部列表都是 same 对象.正如您所想，没有不同的对象.

In other words, all the inner lists are the same object. Not different objects as you think.

要获得想要的效果，您应该这样做:

To get the effect you want, you should do:

outer_list = []
for i in range(0, 3):
    outer_list.append([])

哪个会创建3个内部列表对象，然后将其放入外部对象.

Which creates a 3 inner lists objects and puts the into the outer object.

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