Please explain Python sequence reversal

Question

Not sure where I picked this up, but it stuck and I use it all the time.

Can someone explain how this string reversal works? I use it to test for palindromic strings without converting it to a mutable type first.

>>> word = "magic"
>>> magic = word[::-1]
>>> magic
'cigam'

I would put my best guess, but I don't want to walk in with any preconceptions about the internals behind this useful trick.

It's good you changed from "sort" to "reversal", but it's also not "in-place". The variable `string` remains the same. — David Robinson, Jan 08 '12 at 20:25
@DavidRobinson Thank you for pointing that out. It seems my preconceptions were holding me back more so than I realized. — yurisich, Jan 08 '12 at 20:32
you should use something like "_string" instead of "string" as your variable name, as string is a module in python. — Rusty Rob, Jan 08 '12 at 20:34
_"The variable string remains the same."_ So, it's a invariable variable....... — eyquem, Jan 08 '12 at 23:06
possible duplicate of [reverse a string in Python](http://stackoverflow.com/questions/766141/reverse-a-string-in-python) — yurisich, Jan 09 '12 at 00:05

score 7 · Accepted Answer · answered Jan 08 '12 at 20:18

7

The slice notation goes like this:

my_list[start:end:step]

So, when you do [::-1], it means:

start: nothing (default)
end: nothing (default)
step: -1 (descendent order)

So, you're going from the end of the list (default) to the first element (default), decreasing the index by one (-1).

So, as many answers said, there is no sorting nor in-place swapping, just slice notation.

answered Jan 08 '12 at 20:18

juliomalegria

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I did some reading...an equivalent statement would be `word[None:None:-1]`...this helped me look at my example from a couple of new angles. Thanks! – yurisich Jan 08 '12 at 20:46

score 4 · Answer 2 · edited Jan 11 '12 at 16:13

4

You can have a look here - it is an extended slice.

edited Jan 11 '12 at 16:13

Ethan Furman

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answered Jan 08 '12 at 20:16

fkerber

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Why not? It is an official documentation that explains it better than I can (apart from copying the explanation). – fkerber Jan 08 '12 at 20:20
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Upvoted! If something is explained well in the documentation, Sharing a link just avoids rework. – Krishna Bharadwaj Jan 08 '12 at 20:22
@KrishnaBharadwaj I disagree. Here's the quote: *Negative values also work to make a copy of the same list in reverse order*. I already understand this...I would like to know more. An example goes a long way toward helping me understand the process python goes through to produce this answer. – yurisich Jan 08 '12 at 20:25
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@Droogans: Then ask for an example. Links are perfectly acceptable answers on SO. – Ethan Furman Jan 08 '12 at 21:54
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@EthanFurman Link-only answers are generally frowned upon, see [this meta post](http://meta.stackexchange.com/questions/8231/are-answers-that-just-contain-links-elsewhere-really-good-answers). – Dave Newton Jan 11 '12 at 14:38

score 3 · Answer 3 · answered Jan 08 '12 at 20:15

3

"What's New in Python 2.3", section 15, "Extended Slices".

answered Jan 08 '12 at 20:15

Ignacio Vazquez-Abrams

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@Droogans: Would you rather I say "That's how it's worked since 2.3."? Because that's all it is. – Ignacio Vazquez-Abrams Jan 08 '12 at 20:20
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If you were posting a comment...yes. – yurisich Jan 08 '12 at 20:22

John Machin · Answer 4 · 2012-01-08T22:33:24.533

This "trick" is just a particular instance of applying a slice operation to a sequence. You can use it to produce a reversed copy of a list or a tuple as well. Another "trick" from the same family: [:] is often used to produce a (shallow) copy of a list.

"What's new in Python 2.3" is an unexpected entry point into the maze. Let's start at a more obvious(?) place, the current 2.X documentation for sequence objects.

In the table of sequence operations, you'll see a row with Operation = s[i:j:k], Result = "slice of s from i to j with step k", and Notes = "(3)(5)".

Note 3 says "If i or j is negative, the index is relative to the end of the string: len(s) + i or len(s) + j is substituted. But note that -0 is still 0."

Note 5 says "The slice of s from i to j with step k is defined as the sequence of items with index x = i + n*k such that 0 <= n < (j-i)/k. In other words, the indices are i, i+k, i+2*k, i+3*k and so on, stopping when j is reached (but never including j). If i or j is greater than len(s), use len(s). If i or j are omitted or None, they become “end” values (which end depends on the sign of k). Note, k cannot be zero. If k is None, it is treated like 1."

We have k == -1, so the indices used are i, i-1, i-2, i-3 and so on, stopping when j is reached (but never including j). To obtain the observed effect, the "end" value used for i must be len(s)-1, and the "end" value used for j must be -1. Thus the indices used are last, last-1, ..., 2, 1.

Another entry point is to consider how we might produce such a result for any sequence if [::-1] didn't exist in the language:

def reverse_traversal_of_sequence(s):
    for x in range(len(s) - 1, -1, -1):
        do_something_with(s[x])

Please explain Python sequence reversal

4 Answers4