On Mon, 9 May 2022 at 01:07, MRAB <pyt...@mrabarnett.plus.com> wrote:
>
> On 2022-05-08 13:44, Chris Angelico wrote:
> > On Sun, 8 May 2022 at 22:09, Valentin Berlier <berlie...@gmail.com> wrote:
> >>
> >> A while ago there was a discussion about allowing "match" patterns for the
> >> walrus operator. This would cover iterable unpacking as you suggested
> >> along with all the patterns allowed in match statements.
> >>
> >> if [x, y, z] := re.match(r"...").groups():
> >> print(x, y, z)
> >>
> >> The walrus expression would evaluate to None if the pattern on the left
> >> can't be matched.
> >>
> >
> > What if it does match, though? That's a little less clear. I like the
> > idea, but I'm not entirely sure what this should do, for instance:
> >
> > print( {"x": x} := some_dict )
> >
> > Clearly it should assign x to some_dict["x"] if possible, but then
> > what should get printed out? A small dict with one key/value pair? The
> > original dict? There would need to be a guarantee that the return
> > value is truthy, otherwise it'll create bizarre situations where you
> > try to match and it looks as if the match failed.
> >
> Perhaps the solution would be to add another operator, "?=", that would
> return True if the binding succeeded and False if it failed.
>
It doesn't actually even need to be a different operator. There is
currently no semantic meaning for "pattern := expr" save in the strict
case where the pattern is a simple name, so semantics can be granted
according to what's most useful.
ChrisA
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