[Python-Dev] PEP 487 vs 422 (dynamic class decoration) (original) (raw)

Eric Snow ericsnowcurrently at gmail.com
Mon Apr 6 18:12:07 CEST 2015

On Fri, Apr 3, 2015 at 6:44 AM, Martin Teichmann <lkb.teichmann at gmail.com> wrote:

Nick, I couldn't agree more with you, yet I think PJ actually brought up a very interesting point. Post-processing is a very common thing these days, and has been re-written so many times that I think it is about time that something like it should be in the standard library.

Here's another approach that would help. Support a mechanism for inheriting class decorators. Classes would have an attribute like subclass_decorators that would hold a tuple of all the inherited decorators. They would be applied, in order, in build_class before any explicit decorators are.

One way to accomplish this is with a meta-decorator, e.g. "classutil.inherited_decorator". You would decorate a class decorator with it:

def inherited_decorator(deco): def new_deco(cls): cls = deco(cls) try: inherited = cls.subclass_decorators except AttributeError: cls.subclass_decorators = (deco,) else: cls.subclass_decorators = inherited + (deco,) return cls

@inherited_decorator def register(cls): registry.add(cls) return cls

@register class X: ...

The downside to the meta-decorator is that is isn't apparent when the class decorator is used that it will be inherited. It could also be used directly to make it more apparent:

def register(cls): registry.add(cls) return cls

@inherited(register) class X: ...

However, that doesn't read well. Syntax would be better, but is a harder sell and a little grittier:

def register(cls): registry.add(cls) return cls

@@register class X:

-eric

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