class -> class_decorator(ya-mixin)

class ValidationError(object):
    pass

class Foo(object):
    def clean(self):
        print "hey"

        if self.cleaned_data["x"] < self.cleaned_data["y"]:
            raise ValidationError

class Bar(object):
    def clean(self):
        print "hoo"

        if self.cleaned_data["x"] < self.cleaned_data["z"]:
            raise ValidationError


class FooBar(object):
    def clean(self):
        pass #???

def deco_from_class(source):
    get_real = lambda m : m.__func__ if hasattr(m, "__func__") else m.im_func

    def deco(cls):
        if not hasattr(cls, "clean"):
            cls.clean = get_real(source.clean)
        else:
            def wrapped(original):
                def _wrapped(self):
                    val = original(self)
                    get_real(source.clean)(self)
                    return val
                return _wrapped
            cls.clean = wrapped(cls.clean)
        return cls
    return deco
    

@deco_from_class(Bar)    
@deco_from_class(Foo)
class FooBar(object):
    cleaned_data = dict(x=10, y=5, z=1)

FooBar().clean()

# foo,  bar