Using python decorators associated with an instance level object - Stack Overflow

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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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