[Python-ideas] More useful slices

[Greg Ewing]

Skip Montanaro wrote:
> optimizers like PyPy which aim to be precisely compatible with CPython
> semantics must look up "range" every time it occurs and decide if it's
> the real builtin range function, in which case it can emit/execute
> machine code which is something like the C for loop:
> 
>     for (i=start; i<stop; i+=step) {
>         do something interesting with i
>     }

I'd rather have a more flexible way of ierating over integer
ranges that's not biased towards closed-start open-end
intervals.

While that's convenient for indexing sequences,
if that's all you want to do you're better off iterating
over the sequence directly, maybe using enumerate and/or
zip. If you really need to iterate over ints, you
probably need them for some other purpose, in which
case you're likely to want any combination of
open/closed start/end.

For Pyrex I came up with a syntax that allows specifying
any combination equally easily and clearly:

    for 0 <= i < 10:
       # closed start, open end

    for 0 <= i <= 10:
       # both ends closed

    for 0 < i < 10:
       # both ends open

    for 10 >= i >= 0:
       # both ends closed, going backwards

etc.

I think something like this would be a much better use
of new syntax than just syntactic sugar for something
that's not used very often.

-- 
Greg