[Python-Dev] PEP Proposal: Revised slice objects & lists use slice objects as indexes

Mon Mar 10 01:35:09 CET 2008

On Sun, Mar 9, 2008 at 7:21 PM, Forrest Voight <voights at gmail.com> wrote:
> This would simplify the handling of list slices.
>> Slice objects that are produced in a list index area would be different,
> and optionally the syntax for slices in list indexes would be expanded
> to work everywhere. Instead of being containers for the start, end,
> and step numbers, they would be generators, similar to xranges.

I am not sure what you are trying to propose here. The slice object
isn't special, it's just a regular built-in type.
 >>> slice(1,4)
 slice(1, 4, None)
 >>> [1,2,3,4,5,6][slice(1,4)]
 [2, 3, 4]
I don't see how introducing new syntax would simplify indexing.
> Lists would accept these slice objects as indexes, and would also
> accept any other list or generator.
>
Why lists should accept a list or a generator as index? What is the
use case you have in mind?
> Optionally, the 1:2 syntax would create a slice object outside of list
> index areas.

Again, I don't see how this could be useful...
>> >>> list(1:5)
> [1, 2, 3, 4]
>> >>> list(1:5:2)
> [1, 3]
>
list(range(1,5,2))?
> >>> range(30)[1:5 + 15:17]
> [1, 2, 3, 4, 15, 16]
>
This is confusing, IMHO, and doesn't provide any advantage over:
 >>> s = list(range(30))
 >>> s[1:5] + s[15:17]
If you really needed it, you could define a custom class with a fancy
__getitem__
 class A:
 def __getitem__(self, x):
 return x
 >>> A()[1:3,2:5]
 (slice(1, 3, None), slice(2, 5, None))
P.S. You should consider using the python-ideas
(http://mail.python.org/mailman/listinfo/python-ideas) mailing list,
instead of python-dev for posting suggestions.
Cheers,
-- Alexandre