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.
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