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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 callbacks(cls, eventType=None):
 """ Returns a list of callback methods that can be invoked whenever an event is processed. :return: {subclass of <Event>: <list>, ..} """

        key = '_{0}__callbacks'.format(cls.__name__)
        try:
            callbacks = getattr(cls, key)
        except AttributeError:
            callbacks = {}
            setattr(cls, key, callbacks)

        return callbacks.get(eventType, []) if eventType is not None else callbacks