Slice objects with negative increment

[Paul Hughett]

Alex Martelli <aleax at aleax.it> wrote:
: Paul Hughett wrote:
:         ...
:> Hmm.  Is the value of r[-1:-1:-1] equal to array([12, 11, 10, 9, 8, 7,
:> 6, 5, 4, 3, 2, 1, 0]) or to an empty array?  It could plausibly be

: Any slice x[a:a] is empty.  Why ever should x[a:a:-1] be any different
: from x[a:a:1]?!  I suggest you download and install Numeric so you
: can easily check on such doubts.

: Slicing rules are simple.  [details omitted]

Perhaps, but they give some results that I find anomalous.  Consider this
mini-problem:  Given i and j such that j > i >= 0, write a snippet of
code to construct a slice object that selects elements j-1, j-2, ..., i.
If I understand the interpretation of slices correctly, i = 0 must
be treated as a special case and the code must look something like

   if i == 0 :
      slc = slice(j, None, -1)
   else :
      slc = slice(j, i, -1)

: Even having just one convention might be a problem if the convention
: was bad -- complicated, irregular, inconsistent.  I think Python's
: arrangement here is quite good.

I'm less convinced of that, in light of the example just given.
Having to treat element zero as a special case could come as a nasty
surprise to some programmer.


Paul Hughett