negative stride list slices

[Shalabh Chaturvedi]

Shalabh Chaturvedi wrote:

> I'd like to add:
> 
>   +---+---+---+---+---+
>   | H | e | l | p | A |
>   +---+---+---+---+---+
>   0   1   2   3   4   5
>  -5  -4  -3  -2  -1
>   |                   |
>  start               end   (defaults for +ve step)
>  end                 start (defaults for -ve step)
> 
> (Is this correct?)

Apparently not.

 >>> "HelpA"[-1:-2:-1]
'A'

I would expect 'p'. Also,

 >>> "HelpA"[:-1:-1]
''

And I definitely expected 'A'. Now I find it confusing too.

Does anyone else find it intuitive to expect that S[b:a:-1] be exactly 
reverse of S[a:b:1]?

This is only true for S[::1] and S[::-1]. In other cases, for -ve step, 
the -ve indices seem to indicate a shifted position. I'm using Python 2.3.3.

--
Shalabh