[Python-3000] Metaclasses in Python 3000: Draft 2

[Guido van Rossum]

On 3/15/07, Talin <talin at acm.org> wrote:
> Guido van Rossum wrote:
> > While you're at it, could you also add a pointer to this patch as a
> > sample implementation?
> >
> > http://python.org/sf/1681101
>
> That is way cool.

It is now way cooler -- it *works*! The build_class function is
implemented in Python. To try this out, go to the SF patch. I'm
soliciting help at this point!!

> One other thing that I think might be good to have (at some point after
> we get a working implementation) is some sort of HOWTO document (apart
> from the PEP) that lays out a set of guidelines or advice on creating
> metaclasses. Perhaps a Python cookbook recipe or something.
>
> The reason I say this is because one of the things that I noticed is
> that writing a metaclass that handles all corner cases properly can be
> quite tricky. As an example, if you're doing anything with ordered
> fields, you need to make sure that the base class's ordered fields are
> properly incorporated into the calculation.
>
> Exploiting the new metaclass mechanism fully requires a deeper knowledge
> of the way function bodies are evaluated and how the type() constructor
> works. The fact that there was some confusion as to what methods of the
> dict interface needed to be supported is evidence of this.
>
> The PEP gives you a mechanism for declaring and using metaclasses, and
> invites you to play with the class machinery at a deeper level, but it
> doesn't tell much about the internal mechanism and structure of classes
> themselves, or what is/isn't valid in terms of tweaking them. Thus, for
> example, if I want a class whose instances don't have a dictionary, but
> instead store their members in a flat array (a la __slots__), I would
> need to know what parts of the system will choke, expecting a dict to be
> there when there isn't one, and how to get around that.

Yes, this would be a great thing to add to the documentation somewhere.

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