Python negative step slice, how, without None, to expresse the equivalent to [::-1]

Question

I've read with interest Understanding slicing and What does 'result[::-1]' mean?

>>> a = 'Python'
# a[start:stop:step]  with start and stop inverted if stem < 1
>>> a[::-1]
'nohtyP'

I got that:

>>> a[::-1] == a[None:None:-1]
True

But I still asking to myself the following question:

Why I cant create with some default start stop values the equivalent of a[::-1] ?

>>> a[len(a):0:-1]
'nohty'
>>> a[len(a)+1:0:-1]
'nohty'
>>> a[len(a)+1:-1:-1]
''

Can I catch the P of Python with explicit indices ?

Does https://stackoverflow.com/questions/12521798/what-are-the-default-slice-indices-really answer your question? — Thierry Lathuille, Aug 14 '22 at 12:18
Does this answer your question? [What are the default slice indices \*really\*?](https://stackoverflow.com/questions/12521798/what-are-the-default-slice-indices-really) — mkrieger1, Aug 14 '22 at 12:21
[This answer](https://stackoverflow.com/a/12521981), unfortunately the best parts are in the comments. — mkrieger1, Aug 14 '22 at 12:39

score 3 · Accepted Answer · answered Aug 14 '22 at 12:37

3

You can reverse the string with explicit indices if you use negative indices

print(a[-1:-len(a)-1:-1]) # nohtyP

answered Aug 14 '22 at 12:37

Guy

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I like `'Python'[~0:~6:-1]` – Kelly Bundy Aug 14 '22 at 12:39
@KellyBundy nice, but I think it will be confusing for many people, – Guy Aug 14 '22 at 12:41
Meh, ever since I realized that `~` gives backwards indexing, it has been quite natural :-) – Kelly Bundy Aug 14 '22 at 12:44
Fun fact, just the other day I saw that some guy named Guido once thought [there is no number](https://mail.python.org/pipermail/python-ideas/2013-October/023733.html) that achieves this. – Kelly Bundy Aug 14 '22 at 12:49

score -2 · Answer 2 · answered Aug 14 '22 at 12:28

I think not because the stop value in the slice is not included in the slice.

You are looking for a stop value that includes P, in other words includes index 0. However that is not possible due to how python uses negative numbers in slices.

a[-1:0:-1] -> 'nohty'

We would expect to get P if we go down by one from 0, use -1, but -1 is interpreted as last index, hence the string will be empty.

a[-1:-1:-1] -> ''

TL;DR: nope, not possible

Python negative step slice, how, without None, to expresse the equivalent to [::-1]

2 Answers2