[Python-3000] metaclass syntax [was: Discussions with no PEPs]

[Guido van Rossum]

Those look like use cases for metaclasses, but I don't see how they
require setting a custom dict *while the class suite is being
executed*. The metaclass can create a new dict from its dict argument
and use the new dict to construct the class.

On 3/9/07, Jim Jewett <jimjjewett at gmail.com> wrote:
> On 3/8/07, Talin <talin at acm.org> wrote:
>
> > First, there is a purely cosmetic argument about how metaclasses ought
> > to be specified syntactically, which I won't go into in any detail. Most
> > of the proposals centered around what I will call 'Class Definition
> > Keyword Arguments' (CDKA), in other words keyword arguments that are
> > passed in along with the list of base classes.
>
> You need to in the PEP though, particularly since class decorators are
> now available.  (These remove the need for some of the existing
> metaclass usage.)
>
> > ... a means to
> > supply a custom, dictionary-like object that would be used to collect
> > the class member definitions as the class was being evaluated.
>
> ...
>
> > Now, it was pointed out that the only use cases for a custom dictionary
> > that anyone could think of were all about preserving the ordering of
> > declarations.
>
> Not quite true.
>
> (1)  immutable class dictionaries.  These are typical for extension
> classes, but a real pain for python classes.
>
> (2)  almost-immutable -- with callbacks when values are
> added/changed/deleted.  PJE just pointed out that doing this on even
> just the __bases__ attribute could make generic functions safer.
>
> (3)  actually-private variables
>
> -jJ
>


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