Python Cookbook

Reversing a String by Words or Characters

Credit: Alex Martelli

Problem

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

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:

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

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

revwords = astring.split(  )       # string -> list of words
revwords.reverse(  )               # reverse the list in place
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.

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

import re
revwords = re.split(r'(\s+)', astring)     # separators too since '(...)'
revwords.reverse(  )                       # reverse the list in place
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.

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:

def reverse(alist):
    temp = alist[:]
    temp.reverse(  )
    return temp

or maybe this, which is messier and slower:

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

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

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

revchars = ''.join(reverse(list(astring)))
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.

See Also

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