On Tue, Dec 31, 2019 at 8:23 PM Andrew Barnert via Python-ideas <
[email protected]> wrote:
> > K = 42
> > x, K, z = vec
> Yes. I’m surveying the way other languages deal with this to try to figure
> out what might fit best for Python.
> Some languages use special syntax to mark either values or targets:
>
> let x, K, let z = vec
> x, @K, z = vec
>
> But the simplest solution is to nothing: you have to stick it in an
> expression that isn’t a valid target, or
> It’s a target. And I think that might actually work. If the pattern
> matching library includes this (or you write it yourself):
>
What is and isn't a valid target. For example: 'map[key]', 'seq[pos]',
'obj.attr'.
Generally a dictionary with key should be a target. Or likewise a sequence
with position. But they might be invalid too at runtime. So maybe it's
fine for parser to think they are a target, and just blow up with
IndexError or the like, as now.
But what about that attribute? Maybe it's an assignment target. Or maybe
it's a read-only property that contains a desirable value to match. Or
maybe it's not read only, but a plain attribute that already exists with a
desirable value to match.
The bigger problem in my mind is that what I'd like to match isn't
generally a specific value. It's more like:
type[int], type[int], type[float] = vec
Or:
type[int], y, z = vec
... I don't like my "proposed" syntax, but it kinda-sort vaguely resembles
stuff in PEP 484 and typing module, so using that as placeholder.
Or I even want to match:
type[int], y < 42, z = vec
So your syntax gets me maybe 20% of what I'd want to do if we had pattern
matching. And it has edge cases that of course I could learn, but would
not be intuitive.
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