[Python-ideas] Where did we go wrong with negative stride?

[Guido van Rossum]

On Sun, Oct 27, 2013 at 10:40 AM, MRAB <python at mrabarnett.plus.com> wrote:

> On 27/10/2013 17:04, Guido van Rossum wrote:
>
>> In the comments of
>> http://python-history.**blogspot.com/2013/10/why-**
>> python-uses-0-based-indexing.**html<http://python-history.blogspot.com/2013/10/why-python-uses-0-based-indexing.html>
>> there were some complaints about the interpretation of the bounds for
>> negative strides, and I have to admin it feels wrong. Where did we go
>> wrong? For example,
>>
>> "abcde"[::-1] == "edcba"
>>
>> as you'd expect, but there is no number you can put as the second bound
>> to get the same result:
>>
>> "abcde"[:1:-1] == "edc"
>> "abcde"[:0:-1] == "edcb"
>>
>> but
>>
>> "abcde":-1:-1] == ""
>>
>> I'm guessing it all comes from the semantics I assigned to negative
>> stride for range() long ago, unthinkingly combined with the rules for
>> negative indices.
>>
>>  For a positive stride, omitting the second bound is equivalent to
> length + 1:
>
> >>> "abcde"[:6:1]
> 'abcde'
>

Actually, it is equivalent to length; "abcde"[:5:1] == "abcde" too.


> For a negative stride, omitting the second bound is equivalent to
> -(length + 1):
>
> >>> "abcde"[:-6:-1]
> 'edcba'
>

Hm, so the idea is that with a negative stride you you should use negative
indices. Then at least you get a somewhat useful invariant:

if -len(a)-1 <= j <= i <= -1:
    len(a[i:j:-1]) == i-j

which at least somewhat resembles the invariant for positive indexes and
stride:

if 0 <= i <= j <= len(a):
    len(a[i:j:1]) == j-i

For negative indices and stride, we now also get back this nice theorem
about adjacent slices:

if -len(a)-1 <= i <= -1:
    a[:i:-1] + a[i::-1] == a[::-1]

Using negative indices also restores the observation that a[i:j:k] produces
exactly the items corresponding to the values produced by range(i, j, k).

Still, the invariant for negative stride looks less attractive, and the
need to use negative indices confuses the matter. Also we end up with -1
corresponding to the position at one end and -len(a)-1 corresponding to the
position at the other end. The -1 offset feels really wrong here.

I wonder if it would have been simpler if we had defined a[i:j:-1] as the
reverse of a[i:j]?

What are real use cases for negative strides?

-- 
--Guido van Rossum (python.org/~guido)
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