What does the syntax [::-1] really mean?

[Robert Kern]

Casey wrote:
> On Oct 4, 4:32 pm, Duncan Booth <duncan.bo... at invalid.invalid> wrote:
>> Duncan Booth <duncan.bo... at invalid.invalid> wrote:
>>> the invariant that you are looking for is that for all non-negative a, b:
>>>    x[a:b:1] reversed is x[-len(x)+b-1:-len(x)+a-1:-1]
>> I should of course have said "all a, b in the range 0 <= a <= len(x) and 0
>> <= b <= len(x)".
> 
> Thanks, again! I figured it out from Fred's and your initial posts.
> IM(ns)HO, this is non-intuitive.
> 
> I would expect that the reverse of x[i,j,k] would be x[j,i,-k]; eg;
> x[0:len(x):1] would be x[len(x):0:-1].  This representation is little
> more than syntactic sugar for x[y for y in range(i,j,k)], with a -1
> adjustment to i and j if k is < 0 (due to the way range operates) and
> with appropriate boundary checking on i and j.  The default values for
> i and j would become (0,len(x)), flopped for k < 0.
> 
> There may be a good (or not-so-good) reason this representation
> wouldn't have worked or would have caused problems.  Or maybe it is
> just personal preference.

It would be inconsistent for the reasons that Neil gave.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
 that is made terrible by our own mad attempt to interpret it as though it had
 an underlying truth."
  -- Umberto Eco