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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_callback(self, func):
 """ Registers a call back function """

        if func is None: return
        func_list = to_list(func)

        if not hasattr(self, 'callback_list'):
            self.callback_list = func_list
        else:
            self.callback_list.extend(func_list)