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

[Tim Peters]

[Tim\
>> All strides start the same way, by (conceptually) selecting s[i:j]
>> first.  Then the stride is applied to that contiguous slice, starting
>> with the first element and taking every abs(k) element thereafter.
>> Finally, if k is negative, that sequence is reversed.  k=1, k=-1, k=2,
>> k=-2, ..., all the same.

[Terry]
> I think this is wrong. Conceptually reverse first, if indicated by negative
> stride, then select: see my previous post for my rationale, and below.

I already replied to your previous post, and agreed with you :-)

> ... I think that [::k] should continue to work
> as it does. I believe Guido only suggested deprecating negative strides with
> non-default endpoints, which implies negative strides *with* default
> endpoints should continue as are. We should not break more than necessary.

Take "yes" for an answer ;-)

>> ...
>> Did you notice that Guido titled this thread "Where did we go wrong
>> with negative stride?".;-)

> I did,

And did you notice that I posed that question to someone else? ;-)

> and I explained exactly where I thing we went wrong, which was to
> make the interpretation of i and j depend on the sign of k. Undoing this
> does not mandate B instead of A.

Agreed.