[Python] Using python decorators associated with an instance level object

I am trying to create a decorator which enables me to register callback functions to an instance of ClassB. I would like to use this decorator in other classes within my program (eg. ClassA) but want to instantiate ClassB as an instance variable of ClassA instead of instantiating it as a module level variable. When instantiating at module level everything works fine, however when instantiating as an instance variable of ClassA it fails.

class ClassB():

def __init__(self):
self.callbacks = []

def register_callback(self):
def wrapper(fn):
self.callbacks.append(fn)
return fn
return wrapper


Instantiating at module level works well:

import ClassB

class_b = ClassB()

class ClassA():

def __init__(self):
pass

@class_b.register_callback()
def do_something(self):
# do stuff



Instantiating as an instance variable does not work:

import ClassB

class ClassA():

def __init__(self):
self.class_b = SomeClassB()

@self.class_b.register_callback()
def do_something(self):
# do stuff



What is the correct syntax/ way to achieve what I am trying to do?

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