[Python-Dev] Possible rough edges in Python 3 metaclasses (was Re: Language reference updated for metaclasses)

[Michael Foord]

On 5 Jun 2012, at 08:53, Nick Coghlan wrote:

> [snip...]
> 
> Now, one minor annoyance with current class decorators is that they're
> *not* inherited. This is sometimes what you want, but sometimes you
> would prefer to automatically decorate all subclasses as well.
> Currently, that means writing a custom metaclass to automatically
> apply the decorators. This has all the problems you have noted with
> composability.
> 
> It seems then, that a potentially clean solution may be found by
> adding a *dynamic* class decoration hook. As a quick sketch of such a
> scheme, add the following step to the class creation process (between
> the current class creation process, but before the execution of
> lexical decorators):
> 
>    for mro_cls in cls.mro():
>        decorators = mro_cls.__dict__.get("__decorators__", ())
>        for deco in reversed(decorators):
>            cls = deco(cls)
> 
> Would such a dynamic class decoration hook meet your use case? Such a
> hook has use cases (specifically involving decorator inheritance) that
> *don't* require the use of sys._getframes(), so is far more likely to
> achieve the necessary level of consensus.
> 
> As a specific example, the unittest module could use it to provide
> test parameterisation without needing a custom metaclass.


Heh, you're effectively restoring the old Python 2 metaclass machinery with a slightly-less-esoteric mechanism. That aside I really like it. 

Michael


--
http://www.voidspace.org.uk/


May you do good and not evil
May you find forgiveness for yourself and forgive others
May you share freely, never taking more than you give.
-- the sqlite blessing 
http://www.sqlite.org/different.html