Question

I’m trying to decorate a class with another class. I also want to inherit from the decorated class, but I get some errors. Here’s my code:

class Decorator:
    def __init__(self, decorated):
        pass

@Decorator
class Foo:
    pass

class Goo(Foo):
    pass

The error I get when I try to subclass from Foo is this:

Traceback (most recent call last):
   File “test.py”, line 9, in
      class Goo(Foo):
TypeError: __init__() takes exactly 2 positional arguments (4 given)

By adding another init function to Decorator…

def __init__(self, *args):
    for arg in args:
        print(arg)

… I get the following output:

<class ‘__main__.Foo’>
Goo
(<__main__.Decorator object at 0x010073B0>,)
{‘__module__’: ‘__main__’}

What are those parameters and how should I be using them inside Decorator?

Answer 1

I’ll try to answer the “what are those parameters” question. This code:

@Decorator
class Foo:
    pass

is equivalent to:

class Foo:
    pass
Foo = Decorator(Foo)

This means that Foo ends up being an instance of the Decorator class instead of being a class.

When you try to use this instance as a base of a class (Goo), Python will have to determine a metaclass that will be used to create the new class. In this case it will use Foo.__class__ which equals to Decorator. Then it will call the metaclass with (name, bases, dict) arguments and expect it to return a new class.

This is how you end up with these arguments in Decorator.__init__.

More about this can be found here:
http://www.python.org/download/releases/2.2.3/descrintro/#metaclasses
(particularly the “When a class statement is executed…” part)