[Numpy-discussion] NEP 21: Simplified and explicit advanced indexing

Sebastian Berg sebastian at sipsolutions.net
Tue Jun 26 06:48:11 EDT 2018

On Tue, 2018-06-26 at 02:27 -0700, Robert Kern wrote:
> On Tue, Jun 26, 2018 at 1:36 AM Sebastian Berg <sebastian at sipsolution
> s.net> wrote:
> > On Tue, 2018-06-26 at 01:21 -0700, Robert Kern wrote:
> > > On Tue, Jun 26, 2018 at 12:58 AM Sebastian Berg
> > > <sebastian at sipsolutions.net> wrote:
> > 
> > <snip>
> > 
> > > > 
> > > > Yes, that is true, but I doubt you will find a lot of code path
> > > > that
> > > > need the current indexing as opposed to vindex here,
> > > 
> > > That's probably true! But I think it's besides the point. I'd
> > wager
> > > that most code paths that will use .vindex would work perfectly
> > well
> > > with current indexing, too. Most of the time, people aren't
> > getting
> > > into the hairy corners of advanced indexing.
> > > 
> > 
> > Right, the proposal was to have DeprecationWarnings when they
> > differ,
> > now I also thought DeprecationWarnings on two advanced indexes in
> > general is good, because it is good for new users.
> > I have to agree with your argument that most of the confused should
> > be
> > running into broadcast errors (if they expect oindex vs. fancy). So
> > I
> > see this as a point that we likely should just limit ourselves at
> > least
> > for now to the cases for example with sudden transposing going on.
> > 
> > However, I would like to point out that the reason for the more
> > broad
> > warnings is that it could allow warping normal indexing at some
> > point.
> > 
> I don't really understand this. You would discourage the "normal"
> syntax in favor of these more specific named syntaxes, so you can
> introduce different behavior for the "normal" syntax and encourage
> everyone to use it again? Just add more named syntaxes if you want
> new behavior! That's the beauty of the design underlying this NEP.
> > Also it decreases traps with array-likes that behave differently.
> If we were to take this seriously, then no one should use a bare []
> ever.
> I'll go on record as saying that array-likes should respond to `a[rr,
> cc]`, as in Juan's example, with the current behavior. And if they
> don't, they don't deserve to be operated on by skimage functions.
> If I'm reading the NEP correctly, the main thrust of the issue with
> array-likes is that it is difficult for some of them to implement the
> full spectrum of indexing possibilities. This NEP does not actually
> make it *easier* for those array-likes to implement every
> possibility. It just offers some APIs that more naturally express
> common use cases which can sometimes be implemented more naturally
> than if expressed in the current indexing. For instance, you can
> achieve the same effect as orthogonal indexing with the current
> implementation, but you have to manipulate the indices before you
> pass them over to __getitem__(), losing information along the way
> that could be used to make a more efficient lookup in some array-
> likes.
> The NEP design is essentially more of a way to give these array-likes 
> standard places to raise NotImplementedError than it is to help them
> get rid of all of their NotImplementedErrors. More specifically, if
> these array-likes can't implement `a[rr, cc]`, they're not going to
> implement `a.vindex[rr, cc]`, either.
> I think most of the problems that caused these libraries to make
> different choices in their __getitem__() implementation are due to
> the fact that these expressive APIs didn't exist, so they had to
> shoehorn them into __getitem__(); orthogonal indexing was too useful
> and efficient not to implement! I think that once we have .oindex and
> .vindex out there, they will be able to clean up their __getitem__()s
> to consistently support whatever of the current behavior that they
> can and raise NotImplementedError where they can't.

Right, it helps mostly to be clear about what an object can and cannot
do. So h5py or whatever could error out for plain indexing and only
support `.oindex`, and we have all options cleanly available.

And yes, I agree that in itself is a big step forward.

The thing is there are also very strong opinions that the fancy
indexing behaviour is so confusing that it would ideally not be the
default since it breaks comparing analogy slice objects.

So, personally, I would argue that if we were to start over from
scratch, fancy indexing (multiple indexes), would not be the default
plain indexing behaviour.
Now, maybe the pain of a few warnings is too high, but if we wish to
move, no matter how slowly, in such regard, we will have to swallow it
The suggestion was to make that as easy as possible with adding an
attribute indefinitely.
Otherwise, even a possible numpy replacement might have difficulties to
chose a different default for indexing for years to come...

Practically, I guess some warnings might have to wait a longer while,
just because it could be almost impossible to avoid them in code
working with different numpy versions.

- Sebastian

> -- 
> Robert Kern
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> NumPy-Discussion at python.org
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