def empty_server_interceptor_decorator(target_server, interceptor):
    return target_server

class Tracer:
    def __init__(self, tracer=None,
            interceptor=None,
            server_decorator=empty_server_interceptor_decorator):
        self.tracer = tracer
        self.interceptor = interceptor
        self.server_decorator=server_decorator

    def decorate(self, server):
        return self.server_decorator(server, self.interceptor)

    def close(self):
        self.tracer and self.tracer.close()