1. 在字符串中倒转字符或单词(的顺序)

1.1. 问题 Problem

You want to reverse the characters or words in a string.

你想要在一个字符串中倒转字符或单词(的顺序).

1.2. 解决 Solution

Strings are immutable, so we need to make a copy. A list is the right intermediate data structure, since it has a reverse method that does just what we want and works in place:

字符串是不可变的,所以我们需要产生一个拷贝.一个列表正好可用作数据结构媒介,因为它有一个reverse方法,恰好适合做我们想要做的工作:

   1 revchars = list(astring)       # string -> list of chars
   2 revchars.reverse(  )           # reverse the list in place
   3 revchars = ''.join(revchars)   # list of strings -> string

To flip words, we just work with a list of words instead of a list of characters:

对于翻转单词,我只是用一个单词列表取代上面的字符列表:

   1 revwords = astring.split(  )       # string -> list of words
   2 revwords.reverse(  )               # reverse the list in place
   3 revwords = ' '.join(revwords)      # list of strings -> string

Note that we use a '　' (space) joiner for the list of words, but a ' ' (empty string) joiner for the list of characters.

注意,我们使用一个'　'(1个空格符)的join来连接单词列表,而用 ' '(空字符串)连接字符列表.

If you need to reverse by words while preserving untouched the intermediate whitespace, regular-expression splitting can be useful:

如果你需要在倒转词序时原样保留中间的空白字符,规则表达式会很有用的:

   1 import re
   2 revwords = re.split(r'(\s+)', astring)     # separators too since '(...)'
   3 revwords.reverse(  )                       # reverse the list in place
   4 revwords = ''.join(revwords)               # list of strings -> string

Note that the joiner becomes the empty string again in this case, because the whitespace separators are kept in the revwords list by using re.split with a regular expression that includes a parenthesized group.

注意,这里再次使用了空字符串作为连接符,因为通过使用re.split并用一个包含括号分组的规则表达式作参数时,字串中的空白分隔符保留在了revwords列表中.

1.3. 讨论 Discussion

The snippets in this recipe are fast, readable, and Pythonic. However, some people have an inexplicable fetish for one-liners. If you are one of those people, you need an auxiliary function (you can stick it in your built-ins from sitecustomize.py) like this:

这个配方中的代码片断是快速的,可读的且Pythonic.可是,有些人总是说不清地迷信单行语句.如果你是这些人中的一个,你需要一个这样的附助函数(你可以从sitecustomize.py中把它们贴到你的"内置工具"(built-ins)中)

   1 def reverse(alist):
   2     temp = alist[:]
   3     temp.reverse(  )
   4     return temp

or maybe this, which is messier and slower:

或也许是这样,这是杂乱且缓慢的:

   1 def reverse_alternative(alist):
   2     return [alist[i-1] for i in range(len(alist), 0, -1)]

This is, indeed, in-lineable, but not worth it in my opinion.

这确实是单行的,但在我看来它没有价值.

Anyway, armed with such an almost-built-in, you can now do brave new one-liners, such as:

无论如何,用如此这样的"内置工具"(almost-built-in),你现在能勇敢的写一个新的单行语句,如:

   1 revchars = ''.join(reverse(list(astring)))
   2 revwords = ' '.join(reverse(astring.split(  )))

In the end, Python does not twist your arm to make you choose the obviously right approach: Python gives you the right tools, but it's up to you to use them.

最终,python不会扭着你的胳膊让你去选择正确的途径:python给了你正确的工具,但使用它们是你的事.

1.4. 参考 See Also

The Library Reference section on sequence types; Perl Cookbook Recipe 1.6.