Class Decorators, Inheritance, super(), and maximum recursion

Question

I\'m trying to figure out how to use decorators on subclasses that use super(). Since my class decorator creates another subclass a decorated class seems to pre

Answer 1

As you might already be aware, the problem arises from the fact that the name SubClassAgain in SubClassAgain.print_class is scoped to the current module's global namespace. SubClassAgain thus refers to the class _DecoratedClass rather than the class that gets decorated. One way of getting at the decorated class is to follow a convention that class decorators have a property referring to the decorated class.

def class_decorator(cls):
    class _DecoratedClass(cls):
        original=cls
        def __init__(self):
            print '_DecoratedClass.__init__'
            return super(_DecoratedClass, self).__init__()
    return _DecoratedClass

@class_decorator
class SubClassAgain(BaseClass):
    original
    def print_class(self):
        super(self.__class__.original, self).print_class()

Another is to use the __bases__ property to get the decorated class.

@class_decorator
class SubClassAgain(BaseClass):
    def print_class(self):
        super(self.__class__.__bases__[0], self).print_class()

Of course, with multiple decorators, either of these become unwieldy. The latter also doesn't work with subclasses of the decorated class. You can combine decorators and mixins, writing a decorator that adds a mixin into a class. This won't help you override methods.

def class_decorator(cls):
    class _DecoratedClass(object):
        def foo(self):
            return 'foo'
    cls.__bases__ += (_DecoratedClass, )
    return cls

Lastly, you can work directly with the class attributes to set methods.

def class_decorator(cls):
    old_init = getattr(cls, '__init__')
    def __init__(self, *args, **kwargs):
        print 'decorated __init__'
        old_init(self, *args, **kwargs)
    setattr(cls, '__init__', __init__)
    return cls

This is probably the best option for your example, though the mixin-based decorator also has its uses.