[Python-Dev] Negative index in insert()

Tim Peters tim.one@home.com
Sun, 26 Aug 2001 02:14:15 -0400

[Eric S. Raymond]
> Given
> >>> a = [1, 2, 3, 4]
> because
> >>> a[-2]
> -3
> I expected that a.insert(-2, 0) would yield [1, 2, 0, 3, 4].  It was a
> rude shock to discover that
> >>> a
> [0, 1, 2, 3, 4]
> In fact I think this may be the nastiest surprise Python has
> handed me since I started using it.

I never noticed it, but agree it's strange.  Did you open a feature request
on SourceForge?  Note that array objects (created by the array module) share
this behavior.

> The reference manual says "same as s[i:i] = [x]  if i >= 0" which
> of course doesn't cover the i < 0 case.

Which formally means its behavior is undefined.

> David Beasley's reference says "Inserts x at index i" which sounds like
> the behavior I was expecting but didn't get.

Ya, so file a bug report with David <wink>.

> Is this a deliberate design choice, an oversight, or a plain bug?  If
> it's a choice, it's damn poorly documented -- this deserves at least a
> footnote in the list methods table.  If it's an oversight or bug, I
> volunteer to fix it.

Only Guido *may* be able to say for sure.  Plausible:  the behavior for i<0
was deliberate at the start, and when Python *grew* negative indices (it
didn't always accept them), fear of incompatibility prevented changing
.insert behavior (in the other cases where negative indices "grew a
meaning", the old behavior was to raise an exception; but .insert accepted
them silently).

Digging thru CVS, the current behavior for i<0 has always been that way
(since listobject.c rev 1.1); and the "if i >= 0" qualifier was added to
libtypes.tex a bit over 6 years ago, with checkin comment

    correct description of list.insert()

Hard call!  I agree it should change, but if it breaks code the "but it was
formally undefined" comeback doesn't really warm peoples' hearts.

first-one-to-suggest-a-future-stmt-dies-ly y'rs  - tim