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

Antoine Pitrou solipsis at pitrou.net
Sun Dec 10 14:47:45 EST 2017 
Hi,

On Sun, 10 Dec 2017 19:17:25 +0000
Tin Tvrtković <tinchester at gmail.com> wrote:
> 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.

Hmm... I understand you may be restricted by backwards compatibility
here.  But dataclasses don't have that issue, so we could decide we're
incompatible with certain dataclasses from day 1.

> 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).

The problem is less the API but the implementation.  As Guido pointed
out, this means the instance layout may now change after class
definition.  One possibility would be to allow changing the layout
before the first instance (or even subclass) is instantiated, after
which it would raise an error.

Regards

Antoine.
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