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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 set_callback(self, method, extra=None):
 """ Sets a callback method, called once the result has been computed or in case of exception. The callback method must have the following signature: ``callback(result, exception, extra)``. :param method: The method to call back in the end of the execution :param extra: Extra parameter to be given to the callback method """

        self.__callback = method
        self.__extra = extra
        if self._done_event.is_set():
            # The execution has already finished
            self.__notify()