[Python-Dev] Issues with PEP 526 Variable Notation at the class level

Tin Tvrtković tinchester at gmail.com
Sun Dec 10 14:17:25 EST 2017


I'm one of the attrs contributors, and the person who initially wrote the
slots functionality there.

We've given up on returning a new class always since this can conflict with
certain metaclasses (have you noticed you can't make a slots attrs class
inheriting from Generic[T]?) and with PEP 487. I think with PEP 487 it's
becoming especially evident class creation is not necessarily an idempotent

I'm currently brainstorming alternative APIs for slots. The best solution
would be for Python to actually offer a way to add slotness to a class
after it's been defined, and Guido has expressed approval (

Personally I really like slot classes, both for their memory
characteristics and the fact the attributes need to be enumerated
beforehand and typos get turned into errors, so I'd welcome any development
on this front. :)

Date: Sat, 9 Dec 2017 08:52:15 -0500
> From: "Eric V. Smith" <eric at trueblade.com>
> To: Nathaniel Smith <njs at pobox.com>
> Cc: Python Dev <python-dev at python.org>
> Subject: Re: [Python-Dev] Issues with PEP 526 Variable Notation at the
>         class level
> Message-ID: <f76fa8aa-36f6-9ac7-5789-68e3c0c61e0b at trueblade.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
> On 12/8/2017 9:14 PM, Nathaniel Smith wrote:
> > On Dec 7, 2017 12:49, "Eric V. Smith" <eric at trueblade.com
> > <mailto:eric at trueblade.com>> wrote:
> >
> >     The reason I didn't include it (as @dataclass(slots=True)) is
> >     because it has to return a new class, and the rest of the dataclass
> >     features just modifies the given class in place. I wanted to
> >     maintain that conceptual simplicity. But this might be a reason to
> >     abandon that. For what it's worth, attrs does have an
> >     @attr.s(slots=True) that returns a new class with __slots__ set.
> >
> >
> > They actually switched to always returning a new class, regardless of
> > whether slots is set:
> >
> > https://github.com/python-attrs/attrs/pull/260
> In the end, it looks like that PR ended up just refactoring things, and
> the decision to always return a new class was deferred. I still haven't
> finished evaluating exactly what the refactoring does, though.
> Eric.
> > You'd have to ask Hynek to get the full rationale, but I believe it was
> > both for consistency with slot classes, and for consistency with regular
> > class definition. For example, type.__new__ actually does different
> > things depending on whether it sees an __eq__ method, so adding a method
> > after the fact led to weird bugs with hashing. That class of bug goes
> > away if you always set up the autogenerated methods and then call
> > type.__new__.
> They have a bunch of test cases that I'll have to review, too.
> Eric.
> -
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