On Fri, Sep 25, 2020 at 11:26 PM Ricky Teachey <ri...@teachey.org> wrote:
> On Sat, Sep 26, 2020 at 1:51 AM Guido van Rossum <gu...@python.org> wrote: > >> But this requires introspection. >> > > I'm sorry Guido I'm staring at your message and reread mine several times > and I don't understand where introspection is required. It seems like all > the parser has to do is what it has always done-- pass the arguments to the > item dunders and let the chips fall where they may (other than perhaps > adjusting the TypeError message that could result). But you don't have to > explain to me, I am certain you know what you're talking about. > > Anyway: I don't have an opinion on which of the offered possibilities is > the best SENTINEL. > > But I would also suggest considering adding the Ellipses object, ..., to > the list of possibilities. It's a unique value that sort of stands out > (though it is Truthy rather than Falsey like None and the empty tuple). > I meant that introspection would be needed to determine whether a default index was given or not. On second thought, I made a mistake, and it's worse -- it's just impossible to do it right from every POV. IIUC you're proposing that the user add a default to the `index` value, like this: ``` def __setitem__(self, index=42, value=None, **kwargs): ... ``` But in order to be correct we'd have to make `index` and `value` (and `self`) positional-only arguments (PEP 570), otherwise you'd get problems with `a[43, value=1] = 2`, so the def should really be ``` def __setitem__(self, index=42, value=None, /, **kwargs): ... ``` But then the interpreter must pass `index` and `value` by position. How would it pass `value` without passing `index`? By keyword -- but we've just determined it must be a positional-only arg. Ergo, we've reached a contradiction, and there's no solution. -- --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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