[Python-3000] PEP for Metaclasses in Python 3000

[Guido van Rossum]

On 3/14/07, Jack Diederich <jackdied at jackdied.com> wrote:
> I've snipped all the bits I understand and/or agree with.

I'm only quoting what I think needs clarification and/or hasn't
already been addressed. Note you're responding to an old thread; Talin
posted a new version of the PEP and I checked it in:
http://www.python.org/dev/peps/pep-3115/

> I don't understand the keyword args other than 'metaclass.'
> If class D inherits from class C does it also get passed the 'Implements'
> keyword?

No, it's up to the metaclass to define what happens. It has enough
info to tell which interfaces C implements, so if it wants to it can
easily make D implement them too without being explicitly told so.

> I like the more explicit
>
>   class C(metaclass=Implements(I1, I2)):
>     ...

You can do it either way. The PEP however won't force people to do it
this way if they like the other way.

> Agreed, I currently use metaclasses that pull behavior flags from the
> class body and it is less than pretty.  In my experience when you have
> those behavior flags in the class they change for every subclass too.
> If they didn't they would be defined in the original metaclass. I think
> of metaclasses as 'apply always' and class decorators as 'apply once.'

But that's really between the metaclass and the decorator; the
language doesn't constrain you. (I feel I'm repeating myself; we're so
deep into meta-land that we can only provide mechanism, not policy on
how to use it.)

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)