[Python-Dev] Possible rough edges in Python 3 metaclasses (was Re: Language reference updated for metaclasses) (original) (raw)

Nick Coghlan ncoghlan at gmail.com
Wed Jun 6 11:31:25 CEST 2012

On Wed, Jun 6, 2012 at 5:09 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:

On Wed, Jun 6, 2012 at 1:28 AM, PJ Eby <pje at telecommunity.com> wrote:

To be clear, what I specifically proposed (as I mentioned in an earlier thread) was simply to patch buildclass in order to restore the missing metaclass hook.  (Which, incidentally, would make ALL code using metaclass cross-version compatible between 2.x and 3.x: a potentially valuable thing in and of itself!)

As for metaclasses being hard to compose, PEP 422 is definitely a step in the right direction.  (Automatic metaclass combining is about the only thing that would improve it any further.) Just as a warning, I doubt I'll be able to persuade enough people that this is a feature worth including in the short time left before 3.3 feature freeze. It may end up being necessary to publish metaclass and explicit decorator based variants (with their known limitations), with a view to gaining support for inclusion in 3.4.

Upgrading this warning to a fact: there's no way this topic can be given the consideration it deserves in the space of the next three weeks. I'll be changing the title of 422, spend more time discussing the problem (rather than leaping to a conclusion) and retargeting the PEP at 3.4.

If you do decide to play around with monkeypatching build_class, please make clear to any users that it's a temporary fix until something more robust and less implementation dependent can be devised for 3.4.

Cheers, Nick.

-- Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

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