# [Python-Dev] extended slicing for lists

Michael Hudson mwh21@cam.ac.uk
28 Jul 2000 21:58:11 +0100

```Michael Hudson <mwh21@cam.ac.uk> writes:

> Guido van Rossum <guido@beopen.com> writes:
>
> > > Good idea.  Are lists and tuples going to support seq[a:b:c] anytime
> > > soon?
> >
> > It's on my wish list, but can't be done for 2.0.
>
> This would involve the list type implementing the mapping API wouldn't
> it?  This would seem to be a little on the silly side.  Never mind;
> here's a quick-hack patch that "seems to work" for lists:
[snip]

I've now put this (or a slightly less buggy version) onto sourceforge.

Guido said (after I bothered him in private email) that he'd expect
that:

>>> l = range(10)
>>> l[2:10:2] = [0]*5
>>> l
[0, 1, 0, 3, 0, 5, 0, 7, 0, 9]
>>>

And also:

>>> l = range(10)
>>> l[4:-1:-1] = range(5)
>>> l
[4, 3, 2, 1, 0, 5, 6, 7, 8, 9]
>>>

(and presumably del l[a:b:c])

Now I'd expect *that* as well, but I was wondering what should happen
if the lengths of the lists don't match up ... just bail?  That would
be easiest, but it doesn't quite gel well with current behaviour for
simple slices.  Presuming an error, what about these (assuming d is a
list of the wrong length):

l[a:b:-1] = d
l[a:b:1] = d
l[a:b:] = d

I think these should probably be errors too, but I'd welcome any
opinions people have.

Cheers,
M.

--
A witty saying proves nothing.                           -- Voltaire

```