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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