"Raymond Hettinger" <python@rcn.com> writes:
> In Py2.3, __getitem__ conveniently supports slices for
> builtin sequences: 'abcde'.__getitem__(slice(2,4))
>
> For user defined classes to emulate this behavior, they need
> to test the index argument to see whether it is a slice and then
> loop over the slice indices like this:
>
> class SquaresToTen:
> """Acts like a list of squares but
> computes only when needed"""
>
> def __len__(self):
> return 11
>
> def __getitem__(self, index):
> if isinstance(index, slice):
> return [x**2 for x in range(index.start, index.stop, index.step)]
You can spell that
if isinstance(index, slice):
return [x**2 for x in range(*index.indices(11))]
and, as a bonus, it'll work more often <wink> (consider
"SquaresToTen()[7::-2]", for example).
> This could be simplified somewhat by making slices iterable so that
> the __getitem__ definition looks more like this:
>
> def __getitem__(self, index):
> if isinstance(index, slice):
> return [x**2 for x in index]
> else:
> return index**2
However to make omitted slice places work, you need to pass in the
length of the sequence, so I don't think this can fly.
Cheers,
M.
--
We've had a lot of problems going from glibc 2.0 to glibc 2.1.
People claim binary compatibility. Except for functions they
don't like. -- Peter Van Eynde, comp.lang.lisp
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