[Python-Dev] PEP 487: Simpler customization of class creation

Guido van Rossum gvanrossum at gmail.com
Sun Jul 3 21:02:22 EDT 2016

OK, I see this point now. Still looking for time to review the rest of your

--Guido (mobile)
On Jul 3, 2016 3:29 PM, "Martin Teichmann" <lkb.teichmann at gmail.com> wrote:

> Hi Guido,
> sorry I missed your post...
> >> One of the big issues that makes library authors reluctant to use
> >> metaclasses
> >> (even when they would be appropriate) is the risk of metaclass
> conflicts.
> >
> > Really? I've written and reviewed a lot of metaclasses and this has never
> > worried me. The problem is limited to multiple inheritance, right? I
> worry a
> > lot about MI being imposed on classes that weren't written with MI in
> mind,
> > but I've never particularly worried about the special case of
> metaclasses.
> Yes, the problem only arises with MI. Unfortunately, that's not
> uncommon: if you want to implement an ABC with a class from a
> framework which uses metaclasses, you have a metaclass conflict. So
> then you start making MyFrameworkABCMeta-classes.
> The worst is if you already have a framework with users out there. No
> way you add  a metaclass to your class, however convenient it would
> be. Because you never now if some user out there had gotten the idea
> to implement an ABC with it. Sure, you could let your metaclass
> inherit from ABCMeta, but is this really how it should be done?
> (This has already been mentioned by others over at python-ideas:
> https://mail.python.org/pipermail/python-ideas/2016-February/038506.html)
> Greetings
> Martin
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