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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 start_external_service(self, service_name, conf=None):
 """ Start external service service_name with configuration conf. :param service_name: Name of service to start :param conf: :return: nothing """

        if service_name in self._external_services:
            ser = self._external_services[service_name]
            service = ser(service_name, conf=conf, bench=self.bench)
            try:
                service.start()
            except PluginException:
                self.logger.exception("Starting service %s caused an exception!", service_name)
                raise PluginException("Failed to start external service {}".format(service_name))
            self._started_services.append(service)
            setattr(self.bench, service_name, service)
        else:
            self.logger.warning("Service %s not found. Check your plugins.", service_name)