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Specify start and length, beside start and end, in slices

Antoon Pardon apardon at forel.vub.ac.be
Mon May 24 10:53:56 EDT 2004 
Op 2004-05-21, Terry Reedy schreef <tjreedy at udel.edu>:
>
> "Noam Raphael" <noamr at correctme.users.sourcephorge.net> wrote in message
> news:c8l3s3$27o$1 at news.iucc.ac.il...
>
>> Many times I find myself asking for a slice of a specific length, and
>> writing something like l[12345:12345+10].
>> This happens both in interactive use and when writing Python programs,
>> where I have to write an expression twice (or use a temporary variable).
>
> With an expression, I'd go for the temp var.
>
>> Wouldn't it be nice if the Python grammar had supported this frequent
>> use?
>
> I take this as 'directly support' versus the current indirect support via
> start+len.
> My answer: superficially (in isolation) yes, but overall, in the context of
> Python's somewhat minimalistic grammar/syntax, no.  Two ways to slice might
> easily be seen as one too many.  In addition, the rationale for this, your
> favorite little addition, would admit perhaps 50 others like it.
>
>> My idea is that the expression above might be expressed as l[12345:>10].
>
> Sorry, this strike me as ugly, too much like and easily confused with
> l[12345:-10], and too much looking like a syntax error.
>
> Given that some other languages slice with (start,len) arguments (but not
> then, that I remember or know of, also with a start,stop option), I am
> *sure* that Guido thought carefully about the issue.  A plus with his
> choice is ability to offset (index) from the end *without* calling the len
> function.

Well I hate his choice. It is inconsistent with the fact that generally
l[a:b] produces the empty list when a > b.

It is only inconsistent with the Zen of python which says there should
be only way to do something.

-- 
Antoon Pardon
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