Re: Slices by length

On 2025-09-06 13:47, Rob Cliffe via Python-list wrote:
> I quite often find myself writing expressions of the form
>       someString[x : x+n]
> where n is often an int and x may be an int, a variable, or a (possibly
> complicated) expression.
> It would be more natural to be able to specify the slice not by its
> startpoint and ENDPOINT,
> but by its startpoint and LENGTH, particularly when x is a long
> expression and has to be repeated or pre-stored in a variable.
> I have worked with languages that had this facility, and I miss it.
> The possible syntaxes are restricted by the need to avoid ambiguity with
> existing valid code.
> It could be for example
>       someString[x ! n]
> (of course, x and n could be arbitrary expressions).
> One point to be decided would be whether negative n would produce
>       (a) an empty string, so that s[x ! n] was always the same as s[x :
> x+n]
>       (b) a slice ending at x, e.g. "01234567"[4!-3] would equal "123"
> (or less plausibly "321").
> I don't have a strong opinion on this; there may be good reasons for
> preferring one to another.
> Does anybody think this is a good idea?
>
I'm not happy about the syntax.
How about `someString[x +: n]`?

As for a negaitve length, it should probably return an empty string 
given that `someString[x : x+n]` returns an empty string if `n` is negative.

The syntax would leave open the possibility of adding `someString[x -: 
n]` in the future for option b.

On a related note, a quick search revealed that the D programming 
language lets you write `someString[x .. $]` where `$` in that context 
is the length. That lets you write `someString[x .. $ - n]`. In Python 
that could be `someString[x : $ - n]`, equivalent to `someString[x : 
len(someString) - n]`, even if `n` is 0.
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