[Python-Dev] RE: iterable slices

[Damien Morton]

I hadnt thought of that, but its kind of elegant.

for i in (a:b:c:d:e):  

Would probably be an error. Trying to make an iterator out of anything
but a 2 or 3 element slice should fail, unless you supply your own
iterator factory.

Omitted slice elements being None is a nice idea, and would work with
the RDF notation

(subj::obj) -> slice(subj, None, obj)

It gets a little hairy when you start doing things like this, however

(::) -> slice(None, None, None)

And what then of the 1-element slice? (if its necessary at all)

(1:) -> slice(1, None) or slice(1)??

> -----Original Message-----
> From: Alex Martelli [mailto:aleax@aleax.it] 
> Sent: Thursday, 2 May 2002 02:39
> To: Damien Morton; python-dev@python.org
> Subject: iterable slices
> 
> 
> On Thursday 02 May 2002 08:04, Damien Morton wrote:
> 	...
> > Could the notation   a:b:c:d:... be generalised, with 
> slices becoming a
> > kind of tuple. For backwards compatability, the first three 
> elements 
> > of the tuple could be accessed using the start, stop, step 
> attributes.
> 	...
> > {1, 2, 3} -> dict([slice(1), slice(2), slice(3)]}
> >
> > A slice on its own would be writen:
> >
> > (1:2:3)  -> slice(1,2,3)
> >
> > A 1-element slice might be written similarily to a 1-element tuple:
> > (1:) -> slice(1)
> 
> And presumably None could be omitted, e.g. (:1) as slice(None,1).
> 
> Related but somewhat orthogonal to this idea -- I'm starting 
> to think that slices should be iterable, with 
> iter(slice(a,b,c)) yielding exactly the same numbers as 
> iter(range(a,b,c)) or iter(xrange(a,b,c)).  If any such 
> abbreviated notation existed for slices, then "for i in 
> (:6):" might work.  Risks: perhaps error-prone ("for i in 
> {:6}:", "for i in (6:):", etc, might be likely typos yielding 
> unexpected behavior); no idea of what behavior would be 
> expected of "for i in (a:b:c:d:e):".
> 
> 
> Alex
>