[Python-ideas] Where did we go wrong with negative stride?

[Ron Adam]

On 10/28/2013 08:00 AM, Nick Coghlan wrote:
> In this vein, I started wondering if it might be worth trying to come up
> with a syntax to control whether the ends of a slice were open or closed.
>
> Since mismatched paren types would be too confusing, perhaps abusing some
> binary operators as Chris suggested could help:
>
> "[<i:" closed start of slice (default)
> "[i<:" open start of slice
> ":>j]" open end of slice (default)
> ":j>]" closed end of slice
> ":>j:k]" open end of slice with step
> ":j>:k]" closed end of slice with step
>
> Default slice: "[<0:-1>:1]"
> Reversed slice: "[<-1:0>:-1]"
>
> This makes it possible to cleanly include the final element as a closed
> range, rather than needing to add or subtract 1 (and avoids the zero trap
> when indexing from the end).

I think a reverse index object could be easier to understand.  For now it 
could be just a subclass of int.  Then 0 and rx(0) would be distinguishable 
from each other.  (-i and rx(i) would be too.)

     seq[0:rx(0)]        Default slice.
     seq[0:rx(0):-1]     Reversed slice.  (compare to above)

     seq[rx(5): rx(0)]   The last 5 items.


A syntax could be added later.  (Insert preferred syntax below.)

     seq[\5:\0]           The last 5 items



How about this example, which would probably use names instead of the
integers in real code.

     >>> "abcdefg"[3:10]       # 10 is past the end.  (works fine)
     'defg'

Sliding the range 5 to the left...

     >>> "abcdefg"[-2:5]       # -2 is before the beginning?  (Nope)
     ''                        # The wrap around gotcha!

The same situation happens when indexing from the right side [-i:-j], and 
sliding the range to the right.  Once j >= 0, it breaks.


It would be nice if these worked the same on both ends.  A reverse index 
object could fix both of these cases.

Cheers,
    Ron