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

[ron adam]

On Wed, 30 Oct 2013 20:22:22 +1000, Nick Coghlan 
<ncoghlan at gmail.com> wrote:
> Regardless, my main point is this: slices are just objects. The 
syntax:


>    s[i:j:k]


> is just syntactic sugar for:


>   s[slice(i, j, k)]


> That means that until people have fully explored exactly the 
semantics
> they want in terms of the existing object model, just as I did for
> rslice(), then there are *zero* grounds to be discussing syntax
> changes that provide those new semantics.

(Sending from gmane on limited tablet)

I think its sugar for a function that takes *args and depending on 
its contents makes a slice, or multiple slices, plus what ever is 
left over.  In the case of a simple index its just whats left over. 

Slice syntax is simple on purpose so that they can pass through more 
than just ints.  It the responsibility of the object that uses them 
to make sense of the slices. 

So it may possible to pass a callable index modifier through too.

    def reversed(obj, slice_obj):
        ...
        return reversed_slice_obj

    a[i:j:k, reversed]


If it's also passed the object to be sliced,  self from __getitem__ 
method, it could set end values and maybe do other things like 
control weather exceptions are raised or not.  Its kind of like 
decorating a slice.   For example it could do an infanite wrap around 
slice by normalizing the indices of the slice object.

Ican't test it on this tablet unfornately.   

Cheers,  Ron Adam.