Decorate a class in Python by defining the decorator as a class

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.

After this, the class NormalClass becomes a ClassWrapper instance:

def decorator(decor_arg):

    class ClassWrapper:
        def __init__(self, cls):
            self.other_class = cls

        def __call__(self,*cls_ars):
            other = self.other_class(*cls_ars)
            other.field += decor_arg 
            return other

    return ClassWrapper

@decorator(" is now decorated.")
class NormalClass:
    def __init__(self, name):
        self.field = name

    def __repr__(self):
        return str(self.field)

Test:

if __name__ == "__main__":

    A = NormalClass('A');
    B = NormalClass('B');

    print A
    print B
    print NormalClass.__class__

Output:

A is now decorated. <br>
B is now decorated. <br>
\__main__.classWrapper