无法使用 boto3 从 DynamoDB StringSet 中删除项目

Question

I have a table that contains an attribute of type StringSet. The attributes name is allDevices . I am trying to remove a device id from the StringSet. However, when I run the below code, it does not remove the string from the StringSet. It also does not throw any error. When attempting to remove deviceID fakeID1 response is :

 {'Attributes': {'allDevices': {'fakeID1', 'fakeID2'}}, 'ResponseMetadata': {'RequestId': '<removed>', 'HTTPStatusCode': 200, 'HTTPHeaders': {'server': 'Server', 'date': 'Fri, 22 Feb 2019 09:04:52 GMT', 'content-type': 'application/x-amz-json-1.0', 'content-length': '93', 'connection': 'keep-alive', 'x-amzn-requestid': '<removed>', 'x-amz-crc32': '<removed>'}, 'RetryAttempts': 0}}

How do I get DynamoDB to remove a string from the StringSet?

Python Code:

 dynamodb = boto3.resource('dynamodb', region_name='us-east-1') table = dynamodb.Table('sniffer-users') try: response = table.update_item( Key={ 'username': user, }, UpdateExpression="DELETE allDevices :s", ExpressionAttributeValues={ ':s': set(deviceID), }, ReturnValues="UPDATED_NEW" ) print(response) except ClientError as e: print(e.response['Error']['Message']) return -9

Things I have tried:

• Removing set() from around deviceID
• Change DELETE to REMOVE
• Banging my head against a wall, begging & pleading
• Explicitly setting fakeID1 to make it read ':s': set('fakeID1'),
• Using the syntax ':s': {'SS': [oldDeviceID]},
• Quadruple checking that allDevices is the same in code as on DDB
• Adding something to allDevices - this works flawlessly
• Changing runtime from Python 3.7 to Python 2.7

Answer 1

You just need to make a minor modification to your ExpressionAttributeValues. To convert a single string into a set, you must first make it a list.

ExpressionAttributeValues={
    ':s': set([deviceID]),
}

Notice the difference below between putting your variable inside [] to make it list and not doing so.

>>> deviceID = "WB529"
>>> set(deviceID)
set(['9', '2', 'B', '5', 'W'])
>>> set([deviceID])
set(['WB529'])

This delete operation will indicate it is successful and return "updated" attributes even if no changes have been made. I have some really similar code; it is impossible to tell from the response if you removed an element from the stringset or if it was never there to begin with.

I haven't tried it, but I believe that adding a ConditionExpression will give you that feedback (the operation will either succeed or will throw an exception).