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

[Paul Moore]

On 28 October 2013 22:41, Guido van Rossum <guido at python.org> wrote:
> I'm not sure I like new syntax. We'd still have to find a way to represent
> this with slice() and also with range().

It's a shame there isn't an indexing syntax where you can supply an
iterator that produces the set of indexes you want and returns the
subsequence - then we could experiment with alternative semantics in
user code.

So, for example (silly example, because I don't have the time right
now to define an indexing function that matches any of the proposed
solutions):

    >>> def PrimeSlice():
    >>>    yield 2
    >>>    yield 3
    >>>    yield 5
    >>>    yield 7

    >>> 'abcdefgh'[[PrimeSlice()]]
    'bceg'

But of course, to make this user-definable needs new syntax in the
first place :-(

Paul