Re: Slices by length

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.

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