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

[Nick Coghlan]

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 __build_class__ 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