Question

What is a simple example of decorating a class by defining the decorator as a class?

I’m trying to achieve what has been implemented in Python 2.6 using PEP 3129 except using classes not functions as Bruce Eckel explains here.

The following works:

class Decorator(object):
    def __init__(self, arg):
        self.arg = arg

    def __call__(self, cls):
        def wrappedClass(*args):
            return cls(*args)
        return type("TestClass", (cls,), dict(newMethod=self.newMethod, classattr=self.arg))

    def newMethod(self, value):
        return value * 2

@Decorator("decorated class")
class TestClass(object):
    def __init__(self):
        self.name = "TestClass"
        print "init %s"%self.name

    def TestMethodInTestClass(self):
        print "test method in test class"

    def newMethod(self, value):
        return value * 3

Except, in the above, wrappedClass is not a class, but a function manipulated to return a class type. I would like to write the same callable as follows:

def __call__(self, cls):
        class wrappedClass(cls):
            def __init__(self):
                ... some code here ...
        return wrappedClass

How would this be done?

I’m not entirely sure what goes into “””… some code here …”””

Answer 1

If you want to overwrite new_method(), just do it:

class Decorator(object):
    def __init__(self, arg):
        self.arg = arg
    def __call__(self, cls):
        class Wrapped(cls):
            classattr = self.arg
            def new_method(self, value):
                return value * 2
        return Wrapped

@Decorator("decorated class")
class TestClass(object):
    def new_method(self, value):
        return value * 3

If you don’t want to alter __init__(), you don’t need to overwrite it.