[Python-3000] PEP for Metaclasses in Python 3000
Guido van Rossum
guido at python.org
Wed Mar 14 19:50:25 CET 2007
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/)
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