This decorator is similar to coroutine,except it does not return a
Future and the callback argument is not treated specially.

正如gen.coroutine文档所说

From the caller’s perspective,@gen.coroutine is similar to the
combination of @return_future and @gen.engine.

gen.engine基本上是一个比coroutine更老的,更简化的版本.如果您正在编写新代码,则应遵循文档的建议并始终使用tornado.gen.coroutine.

如果你查看两个函数的代码(删除了文档),这是非常明显的.

发动机：

def engine(func):
    @functools.wraps(func)
    def wrapper(*args,**kwargs):
        runner = None

        def handle_exception(typ,value,tb):
            if runner is not None:
                return runner.handle_exception(typ,tb)
            return False
        with ExceptionStackContext(handle_exception) as deactivate:
            try:
                result = func(*args,**kwargs)
            except (Return,StopIteration) as e:
                result = getattr(e,'value',None)
            else:
                if isinstance(result,types.GeneratorType):
                    def final_callback(value):
                        if value is not None:
                            raise ReturnValueIgnoredError(
                                "@gen.engine functions cannot return values: "
                                "%r" % (value,))
                        assert value is None
                        deactivate()
                    runner = Runner(result,final_callback)
                    runner.run()
                    return
            if result is not None:
                raise ReturnValueIgnoredError(
                    "@gen.engine functions cannot return values: %r" %
                    (result,))
            deactivate()
            # no yield,so we're done
    return wrapper

协程：

def coroutine(func):
    @functools.wraps(func)
    def wrapper(*args,**kwargs):
        runner = None
        future = TracebackFuture()

        if 'callback' in kwargs:
            callback = kwargs.pop('callback')
            IOLoop.current().add_future(
                future,lambda future: callback(future.result()))

        def handle_exception(typ,tb):
            try:
                if runner is not None and runner.handle_exception(typ,tb):
                    return True
            except Exception:
                typ,tb = sys.exc_info()
            future.set_exc_info((typ,tb))
            return True
        with ExceptionStackContext(handle_exception) as deactivate:
            try:
                result = func(*args,None)
            except Exception:
                deactivate()
                future.set_exc_info(sys.exc_info())
                return future
            else:
                if isinstance(result,types.GeneratorType):
                    def final_callback(value):
                        deactivate()
                        future.set_result(value)
                    runner = Runner(result,final_callback)
                    runner.run()
                    return future
            deactivate()
            future.set_result(result)
        return future
    return wrapper

乍一看,这些都可能很难理解.但是,显然代码非常相似,除了@ gen.coroutine对回调kwarg有一些特殊的处理,它构建/返回一个Future. @ gen.engine有一些代码,如果你试图从它返回一些东西,而不是把它放在Future中,它会特意抛出一个错误.