如何在dynamodb中使用boto3扫描滤波器

我正试着用boto3用dynamodb测试样品过滤器。我的简单例子-写的几乎与 documented .scan :

In [24]: list(User.scan())
Out[24]: [users_info-dev<giovanni>, users_info-dev<nicola>, users_info-dev<ping>]

尝试筛选密钥时:

User.scan(FilterExpression=Attr('user_id').eq('giovanni'))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
...

ValueError: Attribute FilterExpression specified for filter does not exist.

from boto3.dynamodb.conditions import Key, Attr

错误是什么,执行此扫描的正确方法是什么?