Python清单-正确/错误的前两个元素? [英] Python list - True/False first two elements?

问题描述

发布此问题后，经历了一些试验和错误，我观察到以下现象:

Following some trial and error after posting this question, I observe the following phenomena:

>>> [1,2][True]
2
>>>>[1,2][False]
1
>>>>[1,2,3][True]
2

如果我添加第三个或后续元素，则无效.

If I add a third or subsequent element, it has no effect.

有人可以指出我对这些观察结果的解释吗?我想这是一些与任何Python列表中的前两个元素有关的常规属性吗?

Can someone point me to the an explanation of these observations? I presume this is some general property relating to the first two elements in any Python list?

谢谢

推荐答案

由于 [1,2,3] [True] 有两组[] 的解释方式不同.

What's happening here is a little confusing, since [1,2,3][True] has two sets of []s that are being interpreted in different ways.

如果我们将代码分成几行，那么发生的事情会更加清楚.

What's going on is a little more clear if we split the code over a few lines.

第一组 [] 构造一个列表对象.让我们为该对象分配名称 a :

The first set of []s construct a list object. Let's assign that object the name a:

>>> [1,2,3]
[1, 2, 3]
>>> a = [1,2,3]
>>>

第二组 [] 指定该列表内的索引.您通常会看到这样的代码:

The second set of [] specify an index inside that list. You'd usually see code like this:

>>> a[0]
1
>>> a[1]
2
>>>

但是直接使用列表对象而不用给它起一个名字同样有效:

But it's just as valid to use the list object directly, without ever giving it a name:

>>> [1,2,3][0]
1
>>> [1,2,3][1]
2

最后， True 和 False 可用作索引的事实是因为它们被视为整数.来自数据模型文档:

Lastly, the fact that True and False are useable as indexes is because they're treated as integers. From the data model docs:

共有三种类型的整数:

There are three types of integers:

普通整数....

长整数.....

布尔值

这些代表真值False和True.他们俩代表值False和True的对象是唯一的布尔值对象.布尔类型是纯整数的子类型，布尔类型几乎在所有值中，值的行为分别类似于值0和1上下文，例外情况是当转换为字符串时，分别返回字符串"False"或"True".

These represent the truth values False and True. The two objects representing the values False and True are the only Boolean objects. The Boolean type is a subtype of plain integers, and Boolean values behave like the values 0 and 1, respectively, in almost all contexts, the exception being that when converted to a string, the strings "False" or "True" are returned, respectively.

因此， [1,2,3] [True] 等效于 [1,2,3] [1]

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