On Mon, Aug 17, 2020 at 10:04 AM Jean Abou Samra <j...@abou-samra.fr> wrote:
[Jean] > Finally, was as-based syntax considered as an alternative to walrus >> patterns, that is, PATTERN as NAME instead of NAME := PATTERN, as we have >> in the try statement? Given the extensive discussion, I guess it might have >> been rejected, so it could deserve a place under "Rejected ideas" -- this >> holds for all the above too, which I'm sorry about if it's dumb. >> > [Guido] > I don't recall it was discussed. A very early draft (that wasn't > published) used `as` instead of `case`, and that draft used `:=` for this > purpose. It's always made sense to use that, so I've never considered `as`. > Do you think there are situations where `as` has a clear advantage over > `:=`? > > [Jean] > Let's see: it probably avoids some uglinesses that occur with the use of = > in class patterns. > > case Expr(value=(add := BinOp(op=Add()))): > vs. > case Expr(value=(BinOp(op=Add()) as add)) > > There is also > case Line(start := Point(x, y), end): > which might be confused with > case Line(start=Point(x, y), end): > > However, the main reason why I was asking is that 'as' parallels well with > the current context that is closest to pattern matching, namely the try > statement. The walrus parallels with assignment expressions, so this is in > fact a story of wether patterns should look more like expressions or the > LHS of an assignment, as I see it. > > The walrus highlights the structure that you're trying to fit in: "This is > an Expr, having as a value an addition, which looks like ...". By contrast, > having the name after the pattern makes you think like the interpreter: > "Try to fit this into BinOp(op=Add()), and if that succeds, put it in > 'add'." Both are defendable. I think the latter would be a little easier to > teach, so I'd encourage considering it. > Okay, I opened an issue in the PEP team's internal tracker about this: https://github.com/gvanrossum/patma/issues/140 -- --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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