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

Mon Mar 10 11:57:30 CET 2008

> 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.

 The idea is to have slice objects be generators. You could make a
 slice like 1:10:2 , and that would make a slice object which could be
 used as a list index. The list would return a list with the
 corresponding item for every index in the generator. Then, lists could
 transparently be used as list indexes, or you could supply your own
 generator instead of a slice object.
 > I don't see how introducing new syntax would simplify indexing.
 It would move the slice logic from the list to the slice object. Now
 the slice object is just a container, but with this it would be
 useful.
 > Why should lists accept a list or a generator as an index? What is the
> use case you have in mind?

 For example, a multiple selection box's selection would be changed
 into its values by supplying the chosen indexes into a list of values.
 > > 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))?
 It would only be useful as shorthand for xrange, but its addition
 capabilities would be useful.
 > > >>> 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]
 >
 I don't think it's confusing. Also, an advantage would be if the slice
 object were being automatically generated, this would simplify the
 code.
 > 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
 >