On Thu, Nov 26, 2020 at 01:27:46PM +1300, Greg Ewing wrote:
> On 26/11/20 12:41 pm, Steven D'Aprano wrote:
> > a = "abcdef"
> > a[-2] # returns a result
>
> Yes, but did you *intend* that result, or did the -2
> result from a calculation that should have returned a
> positive index but went wrong? Python has no way to
> tell.
Ah, now the penny drops! You are suggesting that the negative index
might be a miscalculation. Okay, fair enough.
Be happy that if the index is *sufficiently* negative, it will still
raise an out-of-bounds IndexError.
But is there something fundamentally harder about verifying your
calculation in the half-open interval `-n <= index < n` versus the
half-open interval `0 <= index n`? I don't think there is.
Personally, I have miscalculated indices and got the wrong *postive*,
but still within bounds, value far more than an unwanted negative value.
E.g. I expected to calculate an index of 3, but got 7 instead. That's
much harder to test against than an unwanted negative:
assert index >= 0
and the problem is solved.
In any case, as you correctly point out, we're not going to dump
negative indices, so this discussion is rather hypothetical.
--
Steve
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