Python中有合法使用list [True]，list [False]吗? [英] Is there any legitimate use of list[True], list[False] in Python?

问题描述

由于True和False是int的实例，因此以下内容在Python中有效:

Since True and False are instances of int, the following is valid in Python:

>>> l = [0, 1, 2]
>>> l[False]
0
>>> l[True]
1

我理解为什么会这样.但是，我发现这种行为有点出乎意料，并且可能导致难以调试的错误.它肯定咬了我几次.

I understand why this happens. However, I find this behaviour a bit unexpected and can lead to hard-to-debug bugs. It has certainly bitten me a couple of times.

有人能想到合法使用True或False的索引列表吗?

Can anyone think of a legit use of indexing lists with True or False?

推荐答案

在过去，有人使用这种行为来产生穷人的

In the past, some people have used this behaviour to produce a poor-man's conditional expression:

['foo', 'bar'][eggs > 5]  # produces 'bar' when eggs is 6 or higher, 'foo' otherwise

但是，使用正确的条件表达式添加到Python 2.5的语言中后，由于您指出的原因，人们对此一无所获:依靠布尔值作为整数的子类对于维护人员来说太神奇"了，不易理解.

However, with a proper conditional expression having been added to the language in Python 2.5, this is very much frowned upon, for the reasons you state: relying on booleans being a subclass of integers is too 'magical' and unreadable for a maintainer.

因此，除非您正在进行代码搜寻(故意生成非常紧凑和晦涩的代码)，否则请使用

So, unless you are code-golfing (deliberately producing very compact and obscure code), use

'bar' if eggs > 5 else 'foo'

取而代之的是

，它的另一个优点是，它们选择的两个表达式都是 lazily 求值的；如果eggs > 5为false，则永远不会执行if之前的表达式.

instead, which has the added advantage that the two expressions this selects between are lazily evaluated; if eggs > 5 is false, the expression before the if is never executed.

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