[Python-Dev] Third and hopefully final post: PEP 557, Data Classes

[Brett Cannon]

On Thu, 30 Nov 2017 at 05:00 Eric V. Smith <eric at trueblade.com> wrote:

> On 11/30/2017 6:59 AM, Antoine Pitrou wrote:
> >
> > Or, simply, is_dataclass_instance(), which is even longer, but far more
> > readable thanks to explicit word boundaries :-)
>
> That actually doesn't bother me. I think this API will be used rarely,
> if ever. Or more realistically, it should be used rarely: what actually
> happens will no doubt surprise me.
>
> So I'm okay with is_dataclass_instance() and is_dataclass_class().
>
> But then I'm also okay with dropping the API entirely. nametuple has
> lived for years without it, although Raymond's advice there is that if
> you really want to know, look for _fields. See
> https://bugs.python.org/issue7796#msg99869 and the following discussion.
>

My question was going to be whether this is even necessary. :) Perhaps we
just drop it for now and add it in if we find there's a public need for it?

-Brett


>
> Eric.
>
> >
> > On Thu, 30 Nov 2017 10:16:58 +0100
> > Antoine Pitrou <solipsis at pitrou.net> wrote:
> >> isdataclass() testing for instance-ship does sound like a bug magnet to
> >> me.
> >>
> >> If isdataclassinstance() is too long (that's understandable), how about
> >> isdatainstance()?
> >>
> >> Regards
> >>
> >> Antoine.
> >>
> >>
> >> On Wed, 29 Nov 2017 17:02:21 -0800
> >> Guido van Rossum <guido at python.org> wrote:
> >>>   On Wed, Nov 29, 2017 at 3:51 PM, Carl Meyer <carl at oddbird.net>
> wrote:
> >>>
> >>>> On 11/29/2017 03:26 PM, Eric V. Smith wrote:
> >>>>> I've posted a new version of PEP 557, it should soon be available at
> >>>>> https://www.python.org/dev/peps/pep-0557/.
> >>>>>
> >>>>> The only significant changes since the last version are:
> >>>>>
> >>>>> - changing the "compare" parameter to be "order", since that more
> >>>>> accurately reflects what it does.
> >>>>> - Having the combination of "eq=False" and "order=True" raise an
> >>>>> exception instead of silently changing eq to True.
> >>>>>
> >>>>> There were no other issues raised with the previous version of the
> PEP.
> >>>>
> >>>> Not quite; I also raised the issue of isdataclass(ADataClass)
> returning
> >>>> False. I still think that's likely to be a cause of bug reports if
> left
> >>>> as-is.
> >>>>
> >>>
> >>> I tried to look up the discussion but didn't find much except that you
> >>> flagged this as an issue. To repeat, your concern is that isdataclass()
> >>> applies to *instances*, not classes, which is how Eric has designed
> it, but
> >>> you worry that either through the name or just because people don't
> read
> >>> the docs it will be confusing. What do you suppose we do? I think
> making it
> >>> work for classes as well as for instances would cause another category
> of
> >>> bugs (confusion between cases where a class is needed vs. an instance
> >>> abound in other situations -- we don't want to add to that). Maybe it
> >>> should raise TypeError when passed a class (unless its metaclass is a
> >>> dataclass)? Maybe it should be renamed to isdataclassinstance()?
> That's a
> >>> mouthful, but I don't know how common the need to call this is, and
> people
> >>> who call it a lot can define their own shorter alias.
> >>>
> >>
> >>
> >>
> >
> >
> >
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