[Python-Dev] PEP487: Simpler customization of class creation

Sylvain Corlay sylvain.corlay at gmail.com
Thu Jul 21 02:43:56 EDT 2016

In any case I find this PEP great. If we can implement a library like
traitlets only using these new hooks without a custom metaclass, it will be
a big improvement.

My only concern was that calling the hook __set_name__ was misleading while
it could not set the name at all and do pretty much anything else.


On Thu, Jul 21, 2016 at 4:53 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 21 July 2016 at 03:40, Sylvain Corlay <sylvain.corlay at gmail.com> wrote:
> > My point is that in any real-world implementation of traits, __set_name__
> > will do a lot more than setting the name, which makes the name
> misleading.
> I suspect the point of disagreement on that front may be in how we
> view the names of the existing __get__, __set__ and __delete__ methods
> in the descriptor protocols - all 3 of those are in the form of event
> notifications to the descriptor to say "someone is getting the
> attribute", "someone is setting the attribute" and "someone is
> deleting the attribute". What the descriptor does in response to those
> notifications is up to the descriptor, with it being *conventional*
> that they be at least plausibly associated with the "obj.attr",
> "obj.attr = value" and "del obj.attr" operations (with folks voting by
> usage as to whether or not they consider a particular API's side
> effects in response to those notifications reasonable).
> The new notification is merely "someone is setting the name of the
> attribute", with that taking place when the contents of a class
> namespace are converted into class attributes.
> However, phrasing it that way suggest that it's possible we *did* miss
> something in the PEP: we haven't specified whether or not __set_name__
> should be called when someone does someone does "cls.attr = descr".
> Given the name, I think we *should* call it in that case, and then the
> semantics during class creation are approximately what would happen if
> we actually built up the class attributes as:
>     for attr, value in cls_ns.items():
>         setattr(cls, attr, value)
> Regards,
> Nick.
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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