On Sat, Aug 29, 2020 at 7:30 PM Steven D'Aprano <st...@pearwood.info> wrote:
> I don't think that anyone wants adding a keyword to a single-valued > subscript to change it to a tuple. At least, I really hope that nobody > wants this! > > So given the current behaviour: > > obj[1] # calls __getitem__(1) > obj[1,] # calls __getitem__((1,)) > > I expect that the first will be the most common. If we add a keyword to > the subscript: > > obj[1, a=3] > > I would expect that the it turns into `__getitem__(1, a=3)` which is > almost surely what the reader and coder expects. It would be quite weird > for the subscript 1 to turn into a tuple just because I add a keyword. > > That does leave the second case a little trickier to add a keyword to, > it would require a pair of parens to disambiguate it from above: > > obj[(1,), a=3] > > but I think that's likely to be obvious to the developer who is adding > in the keyword where previously no keyword existed. > That's a fair ruling. In general, when keywords are present, the rule that you can always omit an outermost pair of parentheses is no longer true. That is, d[(...)] and d[...] are always equivalent regardless what "..." stands for, as long as (...) is a valid expression (which it isn't if there are slices involved). Example: ``` d[1] ~~~ d[(1)] d[1,] ~~~ d[(1,)] d[1, 2] ~~~ d[(1, 2)] ``` But there is absolutely no such rule if keywords are present. FYI, Jonathan's post (once I "got" it) led me to a new way of reasoning about the various proposals (__keyfn__, __subscript__ and what I will keep calling "Steven's proposal") based on what the compiler and interpreter need to do to support this corner case. My tentative conclusion is that Steven's proposal is superior. But I have been reviewing my reasoning and pseudo-code a few times and I'm still not happy with it, so posting it will have to wait. -- --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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