[Numpy-discussion] Request for comments on PEP 637 - Support for indexing with keyword arguments

[Hameer Abbasi]

Hello,

For consistency with the current API, I’d suggest an empty tuple.

arr[] # arr.__setitem__(())
arr[1] # arr.__setitem__(1) (could be the only exception)
arr[1, 2] # arr.__setitem__((1, 2)) # NOT a.__setitem__(1, 2), note the missing parantheses
arr[*a, **kw] # arr.__setitem__(a, **kw), perhaps a future extension

If we do
a[] # arr.__setitem__()
a[1] # arr.__setitem__(1)
a[1, 2] # arr.__setitem__((1, 2))
a[1, 2, x=3] # arr.__setitem__((1, 2), x=3)
a[*a, **kw] # arr.__setitem__(a, **kw)

But honestly, if I’m being candid, the form I like most is:

arr[*a, **kw] # arr.__setitem__(a, kw)
arr[*a] # arr.__setitem__(a)
arr[**kw] # arr.__setitem__((), kw)

This is driven by my love for generality and hatred of special cases. So I’m +1 for empty tuple or the third form.

Best regards,
Hameer Abbasi

--
Sent from Canary (https://canarymail.io)

> On Friday, Sep 25, 2020 at 4:22 PM, Eric Wieser <wieser.eric+numpy at gmail.com (mailto:wieser.eric+numpy at gmail.com)> wrote:
> I agree with Stephan's suggestion of having no default value for positional indices, and letting the user supply it.
>
> It seems I replied badly to the mail on the python-ideas list, and my response ended up as a separate thread, at https://mail.python.org/archives/list/python-ideas@python.org/thread/ZQ4CHT37YUHYEFYFBQ423QNZMFCRULAW/.
>
> Eric
> On Thu, 24 Sep 2020 at 22:22, Aaron Meurer <asmeurer at gmail.com (mailto:asmeurer at gmail.com)> wrote:
> > This PEP also opens the possibility of allowing a[] with nothing in the getitem. Has that been considered?
> >
> > Aaron Meurer
> > On Thu, Sep 24, 2020 at 1:48 PM Sebastian Berg <sebastian at sipsolutions.net (mailto:sebastian at sipsolutions.net)> wrote:
> > > On Wed, 2020-09-23 at 21:41 -0700, Stephan Hoyer wrote:
> > > > On Wed, Sep 23, 2020 at 2:22 PM Stefano Borini <
> > > > stefano.borini at gmail.com (mailto:stefano.borini at gmail.com)>
> > > > wrote:
> > >
> > > <snip>
> > >
> > > > >
> > > > Hi Stefano -- thanks for pushing this proposal forward! I am sure
> > > > that
> > > > support for keyword indexing will be very welcome in the scientific
> > > > Python
> > > > ecosystem.
> > > >
> > > > I have not been following the full discussion on PEP 637, but I
> > > > recall
> > > > seeing another suggestion earlier for what this could be resolved
> > > > into:
> > > >
> > > > type(obj).__getitem__(obj, x=23)
> > > >
> > > > I.e., not passing a positional argument at all.
> > > >
> > > > The author of a class that supports keyword indexing could check this
> > > > sort
> > > > of case with a positional only argument with a default value, e.g.,
> > > >
> > > > def __getitem__(self, key=MY_SENTINEL, /, **kwargs):
> > > >
> > > > where MY_SENTINEL could be any desired sentinel value, including
> > > > either
> > > > None or (). Is there a reason for rejecting this option? It seems
> > > > like a
> > > > nice explicit alternative to prespecifing the choice of sentinel
> > > > value.
> > > >
> > > > I guess the concern might be that this would not suffice for
> > > > __setitem__?
> > > >
> > > >
> > > > >
> > > > > You can see a detailed discussion in the PEP at L913
> > >
> > > <snip>
> > >
> > > > I guess the case this would disallow is distinguishing between
> > > > obj[None,
> > > > x=23] and obj[x=23]?
> > > >
> > > > Yes, this could be a little awkward potentially. The tuple would
> > > > definitely
> > > > be more natural for NumPy users, given the that first step of
> > > > __getitem__/__setitem__ methods in the broader NumPy ecosystem is
> > > > typically
> > > > packing non-tuple keys into a tuple, e.g.,
> > > >
> > > > def __getitem__(self, key):
> > > > if not isinstance(key, tuple):
> > > > key = (key,)
> > > > ...
> > > >
> > > > That said:
> > > > - NumPy itself is unlikely to support keyword indexing anytime soon.
> > > > - New packages could encourage using explicit aliases like
> > > > "np.newaxis"
> > > > instead of "None", which in general is a best practice already.
> > > > - The combined use of keyword indexing *and* insertion of new axes at
> > > > the
> > > > same time strikes me as something that would be unusual in practice.
> > > > From
> > > > what I've seen, it is most useful to either use entirely unnamed or
> > > > entirely named axes. In the later case, I might write something like
> > > > obj[x=None] to indicate inserting a new dimension with the name "x".
> > > >
> > >
> > > Just to briefly second these points and the general support thanks for
> > > the hard work! I do also wonder about the `key=custom_default`
> > > solution, or whether there may be other option to address this.
> > >
> > > For NumPy, I do hope we can play with the idea of adding support if
> > > this PEP lands. But agree that labeled axes in NumPy is unlikely to be
> > > on the immediate horizon, and I am not sure how feasible it is.
> > >
> > > My main in the discussion on python-ideas was `arr[]` doing something
> > > unexpected, but it was long decided that it will remain a SyntaxError.
> > >
> > > For the question at hand, it seems to me that mixing labeled and
> > > unlabeled indexing would be an error for array-like objects.
> > > In that case, the worst we get seems a quirk where `arr[None, x=4]` is
> > > not an error, when it should be an error.
> > > That does not really strike me as a blocker.
> > >
> > > I am not a fan of such quirks. But some trade-off seems unavoidable
> > > considering the backward compatibility constraints and differences
> > > between array-likes and typing use of `__getitem__`.
> > >
> > > - Sebastian
> > >
> > >
> > > > I think we could definitely live with it either way. I would lean
> > > > towards
> > > > using an empty tuple, but I agree that it feels a little uglier than
> > > > using
> > > > None (though perhaps not surprising, given the already ugly special
> > > > cases
> > > > for tuples in the indexing protocol).
> > > >
> > > > Best,
> > > > Stephan
> > > > _______________________________________________
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