[Python-ideas] A (meta)class algebra (original) (raw)

Petr Viktorin encukou at gmail.com
Fri Feb 13 14:02:49 CET 2015

On Fri, Feb 13, 2015 at 12:59 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

On 13 February 2015 at 20:55, Petr Viktorin <encukou at gmail.com> wrote:

On Fri, Feb 13, 2015 at 5:43 AM, Martin Teichmann

This becomes even worse if you want to add a metaclass to a class. Then you have to change every class that inherits from it. Now you say that code with multiple metaclasses is a bad thing in general, but I don't agree. There are very simple examples where it makes a lot of sense. Say, you want to write a class with traits that implements an abstract base class. Wouldn't it be cool if you could write

This is the one thing keeping metaclasses from being truly general – using them limits inheritance. The problem is that metaclasses are too powerful. FWIW, that's why http://legacy.python.org/dev/peps/pep-0422/ exists - it aims to standardise Zope's notion of "class initialisers" so you can have definition time behaviour that gets inherited by subclasses (unlike explicit class decorators), without running the risk of introducing metaclass conflicts, and without making the metaclass system even more complicated. Then the cases that can be safely combined with other metaclasses would merely stop using a custom metaclass on 3.5+, and instead rely entirely on having an appropriate base class. The last round of reviews (back before 3.4 was released) raised a bunch of interesting questions/challenges, and then somebody (cough) decided it would be a good idea to go spend more time helping with PyPI ecosystem wrangling instead :) Regards, Nick.

Oh! I missed that PEP.

I think this is a much better way to go. It defines a simple thing, rather than providing a way for a powerful thing to say "I'm simple!" :)

Martin, would this proposal work for your needs?

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