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Hello,

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 operation.

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 (https://github.com/ericvsmith/dataclasses/issues/60#issuecomment-348719029).

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@trueblade.com>
To: Nathaniel Smith <njs@pobox.com>
Cc: Python Dev <python-dev@python.org>
Subject: Re: \[Python-Dev\] Issues with PEP 526 Variable Notation at the
class level
Message-ID: <f76fa8aa-36f6-9ac7-5789-68e3c0c61e0b@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@trueblade.com
\> eric@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.
\-