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

[Nick Coghlan]

On Tue, Jun 5, 2012 at 10:10 AM, PJ Eby <pje at telecommunity.com> wrote:
> On Mon, Jun 4, 2012 at 7:18 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>>
>> On Tue, Jun 5, 2012 at 8:58 AM, PJ Eby <pje at telecommunity.com> wrote:
>> > On Mon, Jun 4, 2012 at 6:15 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> >>
>> >> It's actually the pre-decoration class, since the cell is initialised
>> >> before the class is passed to the first decorator. I agree it's a
>> >> little
>> >> weird, but I did try to describe it accurately in the new docs.
>> >
>> > I see that now; it might be helpful to explicitly call that out.
>> >
>> > This is adding to my list of Python 3 metaclass gripes, though.  In
>> > Python
>> > 2, I have in-the-body-of-a-class decorators implemented using
>> > metaclasses,
>> > that will no longer work because of PEP 3115...
>>
>> I'm not quite following this one - do you mean they won't support
>> __prepare__, won't play nicely with other metaclasses that implement
>> __prepare__, or something else?
>
>
> I mean that class-level __metaclass__ is no longer supported as of PEP 3115,
> so I can't use that as a way to non-invasively obtain the enclosing class at
> class creation time.
>
> (Unfortunately, I didn't realize until relatively recently that it wasn't
> supported any more; the PEP itself doesn't say the functionality will be
> removed.  Otherwise, I'd have lobbied sooner for a better migration path.)

As in the "def __metaclass__(name, bases, ns): return type(name,
bases, ns)" functionality?

You can still pass an ordinary callable as the metaclass parameter and
it will behave the same as the old class level __metaclass__
definition.

I personally wouldn't be averse to bringing back the old spelling for
the case where __prepare__ isn't needed - you're right that it's a
convenient way to do a custom callable that gets inherited by
subclasses.

Cheers,
Nick.

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