def _run_on_completion(self, func, *args, **kwargs):
        """Function to call when request has finished, after having been _set(). The first argument passed to func will
        be the request itself. Additional parameters are NOT validated. If the request is already finished, the given
        function will be run immediately (in same thread).
        """
        if self._complete_func is not None:
            raise ValueError('Completion function already set for %s: %s' % (self.id_, self._complete_func))

        if not self.__event.is_set():
            self._complete_func = partial(func, self, *args, **kwargs)
        else:
            self.__run_completion_func(partial(func, self, *args, **kwargs), self.id_)
Function to call when request has finished, after having been _set(). The first argument passed to func will
        be the request itself. Additional parameters are NOT validated. If the request is already finished, the given
        function will be run immediately (in same thread).