为什么索引超出范围的子字符串切片会起作用? [英] Why does substring slicing with index out of range work?

问题描述

为什么 'example'[999:9999] 不会导致错误?既然 'example'[9] 这样做了，那么它背后的动机是什么?

根据这种行为，我可以假设 'example'[3] 本质上/内部与 'example'[3:4] 不同，即使虽然两者都产生相同的 'm' 字符串.

解决方案

你说得对！'example'[3:4] 和 'example'[3] 根本不同，在序列边界之外切片(至少对于内置函数)不会不会导致错误.

一开始可能会让人感到惊讶，但仔细想想却是有道理的.索引返回单个项目，但切片返回项目的子序列.所以当你试图索引一个不存在的值时，没有什么可以返回的.但是当你在边界之外对一个序列进行切片时，你仍然可以返回一个空序列.

这里令人困惑的部分原因是字符串的行为与列表略有不同.看看当你对一个列表做同样的事情时会发生什么:

<预><代码>>>>[0, 1, 2, 3, 4, 5][3]3>>>[0, 1, 2, 3, 4, 5][3:4][3]

这里的区别很明显.在字符串的情况下，结果似乎是相同的，因为在 Python 中，没有字符串之外的单个字符这样的东西.单个字符只是一个 1 个字符的字符串.

(有关在序列范围之外进行切片的确切语义，请参阅 mgilson 的回答.)

Why doesn't 'example'[999:9999] result in error? Since 'example'[9] does, what is the motivation behind it?

From this behavior I can assume that 'example'[3] is, essentially/internally, not the same as 'example'[3:4], even though both result in the same 'm' string.

解决方案

You're correct! 'example'[3:4] and 'example'[3] are fundamentally different, and slicing outside the bounds of a sequence (at least for built-ins) doesn't cause an error.

It might be surprising at first, but it makes sense when you think about it. Indexing returns a single item, but slicing returns a subsequence of items. So when you try to index a nonexistent value, there's nothing to return. But when you slice a sequence outside of bounds, you can still return an empty sequence.

Part of what's confusing here is that strings behave a little differently from lists. Look what happens when you do the same thing to a list:

>>> [0, 1, 2, 3, 4, 5][3]
3
>>> [0, 1, 2, 3, 4, 5][3:4]
[3]

Here the difference is obvious. In the case of strings, the results appear to be identical because in Python, there's no such thing as an individual character outside of a string. A single character is just a 1-character string.

(For the exact semantics of slicing outside the range of a sequence, see mgilson's answer.)

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