If you don't want the overlapping with the existing syntax angle brackets
can be used:
match eggs:
case Spam<a=1, b=2>: ...
On Sat, Nov 21, 2020 at 7:31 PM David Mertz <me...@gnosis.cx> wrote:
> On Sat, Nov 21, 2020 at 12:23 PM Steven D'Aprano <st...@pearwood.info>
> wrote:
>
>> Clearly Spam(a=1, b=2) does not necessarily result in an instance with
>> attributes a and b. But the pattern `Spam(a=1, b=2)` is intended to be
>> equivalent to (roughly):
>>
>> if (instance(obj, Spam)
>> and getattr(obj, a) == 1
>> and getattr(obj, b) == 2)
>>
>> it doesn't imply that obj was *literally* created by a call to
>> the constructor `Spam(a=1, b=2)`, or even that this call would be
>> possible.
>>
>
> I think this explanation makes me not worry about the fact that `Spam(a=1,
> b=2)` in a pattern looks a lot like a constructor. Like some other
> commenters, I was vaguely bothered that the identical spelling might have
> these different meanings in different contexts. But I think a match case
> just clearly enough IS a different context that using slightly different
> intuitions is no real conceptual stretch for remembering or teaching it.
>
> As a strawman, we could use different syntax for "match the thing of class
> Spam that has attributes with these values:
>
> match eggs:
> case Spam[a=1, b=2]: ...
>
> Or:
>
> match eggs:
> case Spam{a=1, b=2}: ...
>
> Well, the square brackets COULD mean something different if PEP 637 is
> adopted. But even supposing the curly braces could be fit into the
> grammar. Yes, it sort of suggests the connection between dictionaries and
> Spam.__dict__. But it still reads as "this is something special that I
> have to think about a little differently."
>
> Even where there are capture variables, I think I'd be completely
> comfortable thinking about the different context for:
>
> match eggs:
> case Spam(a=x, b=2): ...
>
> --
> The dead increasingly dominate and strangle both the living and the
> not-yet born. Vampiric capital and undead corporate persons abuse
> the lives and control the thoughts of homo faber. Ideas, once born,
> become abortifacients against new conceptions.
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--
Thanks,
Andrew Svetlov
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