On Wed, Sep 2, 2020 at 4:23 AM Steven D'Aprano <st...@pearwood.info> wrote:
> On Wed, Sep 02, 2020 at 12:55:43PM +1200, Greg Ewing wrote: > > On 2/09/20 3:44 am, Steven D'Aprano wrote: > > >(9) Keyword-only subscripts are permitted: > > > > > > obj[spam=1, eggs=2] > > > # calls type(obj).__getitem__(spam=1, eggs=2) > > > > > > del obj[spam=1, eggs=2] > > > # calls type(obj).__delitem__(spam=1, eggs=2) > > > > An alternative is to pass an empty tuple as the index when there > > are no positional args. That would make the following less of a > > special case: > > I think that will interfere with people who need to choose their own > default for the index. > > Unifying the behaviour of the getter and the setter only makes sense if > you have both; for immutable objects (hence no setter) that need only > keyword subscripts, why bother declaring an index you're never going to > use when you actually want this signature? > > __getitem__(self, *, keyword=default) > > Having to declare a positional index parameter I don't want, simply for > consistency with a setitem I'm not even using, might be a tad annoying. > The whole backwards compatibility thing is more than a bit annoying, isn't it? So maybe accepting that you'll always get a key, and it may be `()`, isn't the worst part. OTOH, maybe we can cook up a scheme where we have two API families, `__getitem__/__setitem__` and `__getindex__/__setindex__`. The signature of `__setindex__` has the value first (after `self`). The rules could be something like - If keywords are present, always use `__getindex__/__setindex__` (fail if they aren't present); if multiple positional indices are present, these become multiple arguments. - Otherwise, if only `__getindex__/__setindex__` are present, call those, and lose the distinction between `d[1]` and `d[1,]`; `d[1, 2]` becomes two positional arguments. - Otherwise, if only `__getitem__/__setitem__` are present, call those in a 100% backwards compatible way. - Otherwise, if both are present, call `__getitem__/__setitem__` if no outer-level comma is present (so for `d[1]` as well as for `d[(1, 2)]`), and call `__getindex__/__setindex__` if an outer-level comma is present (so for `d[1,]` as well as for `d[1, 2]`). This can be done relatively cleanly with changes at both the bytecode level and the C API level. I'm not sure I like this better than the "pure `__getitem__/__setitem__` scheme, because the API duplication is troublesome, -- --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/>
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