On Sat, Aug 15, 2020 at 8:27 PM Guido van Rossum <gu...@python.org> wrote:
> On Sat, Aug 15, 2020 at 8:02 PM Todd <toddr...@gmail.com> wrote: > >> On Sat, Aug 15, 2020 at 7:26 PM Stefano Borini <stefano.bor...@gmail.com> >> wrote: >> >>> > QUESTION >>> > Suppose we have >>> > >>> d[x=1, y=2] = 42 >>> > >>> d[x=1, y=2] >>> > 42 >>> > where d is an instance of a suitable class X that has no special >>> knowledge of keywords. >>> >>> Initially, when I wrote the pep, the idea was that there was no >>> distinction of kwargs and normal args. Basically the idea was that >>> currently the only "metainfo" associated to every argument is purely >>> positional (e.g. the meaning of position 1 is implicit). But index 1 >>> can have a specific semantic meaning (e.g. it could be a day). >>> So in practice they would be one and the same, just that you add >>> non-positional semantic meaning to indexes, and you can refer to them >>> either through the position, or this additional semantic meaning. >>> >>> In other words, if you claim that the first index is day, and the >>> second index is detector, somehow, there is no difference between >>> these >>> >>> d[3, 4] >>> d[day=3, detector=4] >>> d[detector=4, day=3] >>> >>> In fact, my initial feeling would be that you can use either one or >>> the other. You should not be able to mix and match. >>> >>> the pep went through various revisions, and we came to a possible >>> proposal, but it's not set in stone. >>> >> >> This would definitely not be sufficient for xarray, which I see as being >> one of the main users of this syntax. The whole point is to be able to >> specify an arbitrary subset labeled dimensions. >> > > Are you saying that for xarray it is important to distinguish between > `d[day=3, detector=4]` and `d[detector=4, day=3]`? If we just passed the > keyword args to `__getitem__` as an extra `**kwds` argument (which > preserves order, since Python 3.6 at least), that should work, right? If > not, can you clarify? > An extra **kwds would be quite sufficient for xarray. We don't need to distinguish between `d[day=3, detector=4]` and `d[day=4, detector=3]`, at least not any differently from normal Python keyword arguments. What might not suffice is a required one-to-one mapping between positional and keyword arguments. Xarray has a notion of a "Dataset" that contains multiple arrays, some of which may have different dimensions or dimensions in a different order. For this reason, on Dataset we allow keyword-based indexing, but not positional-based indexing. This is basically the same as the use-cases for keyword-only parameters. One question that comes up: should d[**kwargs] be valid syntax? d[*args] currently is not, but that's OK since d[tuple(args)] is identical. On the other hand, we probably do need d[**kwargs] since there's no way to dynamically unpack keyword arguments (short of directly calling __getitem__). And perhaps for symmetry this suggests d[*args] should be valid, too, defined as equivalent to d[tuple(args)]. > > -- > --Guido van Rossum (python.org/~guido) > *Pronouns: he/him **(why is my pronoun here?)* > <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/> > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/TGNFCU24YK36BOLKULDB36JRW73VTJGC/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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