How to decorate a class?

Question

In Python 2.5, is there a way to create a decorator that decorates a class? Specifically, I want to use a decorator to add a member to a class and change the constructor to take a value for that member.

Looking for something like the following (which has a syntax error on 'class Foo:':

def getId(self): return self.__id

class addID(original_class):
    def __init__(self, id, *args, **kws):
        self.__id = id
        self.getId = getId
        original_class.__init__(self, *args, **kws)

@addID
class Foo:
    def __init__(self, value1):
        self.value1 = value1

if __name__ == '__main__':
    foo1 = Foo(5,1)
    print foo1.value1, foo1.getId()
    foo2 = Foo(15,2)
    print foo2.value1, foo2.getId()

I guess what I'm really after is a way to do something like a C# interface in Python. I need to switch my paradigm I suppose.

Answer 1

Apart from the question whether class decorators are the right solution to your problem:

In Python 2.6 and higher, there are class decorators with the @-syntax, so you can write:

@addID
class Foo:
    pass

In older versions, you can do it another way:

class Foo:
    pass

Foo = addID(Foo)

Note however that this works the same as for function decorators, and that the decorator should return the new (or modified original) class, which is not what you're doing in the example. The addID decorator would look like this:

def addID(original_class):
    orig_init = original_class.__init__
    # Make copy of original __init__, so we can call it without recursion

    def __init__(self, id, *args, **kws):
        self.__id = id
        self.getId = getId
        orig_init(self, *args, **kws) # Call the original __init__

    original_class.__init__ = __init__ # Set the class' __init__ to the new one
    return original_class

You could then use the appropriate syntax for your Python version as described above.

But I agree with others that inheritance is better suited if you want to override __init__.