The class arguments

If a class D is supposed to be a decorator, then D will be used like so:

@D
def fn(*args, **kwargs):
    ...

That will be translated to

def fn(*args, **kwargs):
    ...

fn = D(fn)

This shows two things:

1. the function is the argument that the class D must accept when it is instantiated; and

2. the function that is decorated is replaced by the instance of the class.

To comply with the first point, D must be implemented like so:

class D:
    def __init__(self, fn):
        self.fn = fn

If you define D as seen above, you can already use it as a decorator:

@D
def fn():
    pass

The code runs and you can see that fn is now an instance of D:

print(fn)  # <__main__.D object at 0x102b31a90>