Re: Slices by length

On 07/10/2025 20:37, Thomas Passin wrote:
> On 10/7/2025 2:49 PM, Rob Cliffe via Python-list wrote:
>>
>>
>> On 06/09/2025 17:21, MRAB wrote:
>>> 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]`?
>> Yes thank you, I think that's a great improvement, and of course it's 
>> currently not legal syntax.
>> Best wishes
>> Rob Cliffe
>>
>>>
>>> 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.
>
> Since there are at least two options, (a) and (b), the proposed syntax 
> is not obvious and intuitive. So it would add to the programmer's 
> cognitive load for no or little benefit.  Let's not advocate for 
> adding to cognitive load.
>
Thanks for your answer, but I don't really agree.
It may not be obvious now that we're all used to it, but there are also 
two possible meanings for "abcd"[1:-1].  Should it be "bc" (as it is), 
or an empty string?
As for cognitive load:
     (a) Someone may think of an even better syntax which makes it more 
intiuitive.
           Maybe s[x->n]   ?  I'd welcome more suggestions.
     (b) When the slice start is a conplicated expression and the slice 
length is very simple (like a literal int), this proposal would remove 
the need to either repeat the expression or first put it in a temporary 
variable, which may well improve readability.

In fairness, it is already possible to write
      s[x:][:n]
which gives the "right" answer if n is non-negative, and is quite readable,
although this is slower than s[x:x+2], at least in tests that I did 
(and, I would hazard a guess, always).
Best wishes
Rob Cliffe
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