[Python-3000] metaclass syntax [was: Discussions with no PEPs]
Guido van Rossum
guido at python.org
Fri Mar 9 16:06:56 CET 2007
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
--Guido van Rossum (home page: http://www.python.org/~guido/)
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