Dynamodb transact_write_items throws a exception validating ConditionExpression

[LiamCato]

Describe the bug

When using an object supplied from from boto3.dynamodb.conditions import Attr rather than string expression transact_write_items raises an exception:

Unknown parameter in input: "ExpressionAttributeNames", must be one of: TransactItems, ReturnConsumedCapacity, ReturnItemCollectionMetrics, ClientRequestToken

When a ConditionExpression is supplied for a put.

Expected Behavior

I expect the transact_write_items call to function the same where the string representation of a ConditionExpression is supplied or the python object is supplied.

Current Behavior

The string expression works as expected, whereas when using the Attr object, the inject_condition_expressions function unpacks the api_params into:

{'TransactItems': [{'Put': {'TableName': 'my-table', 'Item': {'PK': {'S': 'Test'}, 'SK': {'S': 'Test'}, 'data': {'S': 'Test'}}, 'ConditionExpression': 'attribute_not_exists(#n0)'}}], 'ExpressionAttributeNames': {'#n0': 'SK'}, 'ClientRequestToken': 'c55280d1-da03-4d4f-9096-2cb6b9b2ff97'}

Note how the ExpressionAttributeNames is now outside the item and is validated as part of the top level members which causes the "Unknown parameter in input" exception.

Reproduction Steps

Example:

Works:

from boto3 import resource
dynamodb = resource("dynamodb")
table = dynamodb.Table("mytable")
body = [{'Put': {'TableName': 'my-table-name', 'Item': {'PK': 'Test', 'SK': 'Test', 'data': 'Test'}, 'ConditionExpression': 'attribute_not_exists(SK)' }}]
table.meta.client.transact_write_items(TransactItems=body)

Same action using the Attr object does not work:

from boto3 import resource
from boto3.dynamodb.conditions import Attr
dynamodb = resource("dynamodb")
table = dynamodb.Table("mytable")
body = [{'Put': {'TableName': 'my-table-name', 'Item': {'PK': 'Test', 'SK': 'Test', 'data': 'Test'}, 'ConditionExpression': Attr("SK").not_exists()}}]
table.meta.client.transact_write_items(TransactItems=body)

Possible Solution

No response

Additional Information/Context

No response

SDK version used

1.34.65

Environment details (OS name and version, etc.)

Ubuntu 22.04, python 3.11

[LiamCato]

I've created an example pull request that would fix this. I am not attached to it, it's just a suggestion and I'm happy to throw it away it the maintainers don't like it.

I do wonder however why all the expressions are not unpacked during the recursion like in my change, rather than putting just one of the transforms attributes there and then storing placeholders for later?

[tim-finnigan]

Thanks for reaching out and your patience here. I'm not fully clear on the issue here and think we need some more information.

Both code snippets you provided fail with the error: AttributeError: 'dynamodb.Table' object has no attribute 'transact_write_items'. Here is the documentation for that client method: https://boto3.amazonaws.com/v1/documentation/api/latest/reference/services/dynamodb/client/transact_write_items.html

Can you provide new code snippets highlighting the issue and what you're trying to do? Adding debug logs using boto3.set_stream_logger('') (with sensitive info redacted) may also give more insight here.

[LiamCato]

@tim-finnigan

Ooops, sorry about that, I foolishly used a resource instead of a client in the example. I've updated the example to use the client and tested the snippet to check it gives the error I've reported.

[LiamCato]

Hopefully my example change and test in the linked PR should give you insight into where in the code the issue is.

https://github.com/boto/boto3/pull/4054/files

[tim-finnigan]

Thanks for following up and your patience here. For readability I'll share your snippets again here but formatted and in Python markdown syntax:

1. (working)

from boto3 import resource

dynamodb = resource("dynamodb")
table = dynamodb.Table("mytable")

body = [
    {
        'Put': {
            'TableName': 'my-table-name',
            'Item': {
                'PK': 'Test',
                'SK': 'Test',
                'data': 'Test'
            },
            'ConditionExpression': 'attribute_not_exists(SK)'
        }
    }
]

table.meta.client.transact_write_items(TransactItems=body)

2. (not working)

from boto3 import resource
from boto3.dynamodb.conditions import Attr

dynamodb = resource("dynamodb")
table = dynamodb.Table("mytable")

body = [
    {
        'Put': {
            'TableName': 'my-table-name',
            'Item': {
                'PK': 'Test',
                'SK': 'Test',
                'data': 'Test'
            },
            'ConditionExpression': Attr("SK").not_exists()
        }
    }
]

table.meta.client.transact_write_items(TransactItems=body)

So the key difference to highlight is that 'ConditionExpression': 'attribute_not_exists(SK)' works but 'ConditionExpression': Attr("SK").not_exists() does not, because a string is expected for ConditionExpression, but you're using boto3.dynamodb.conditions.Attr.

It looks like #3259 overlaps with the request here, can you confirm? I think the changes in your PR would be breaking but maybe there is another approach that could work for converting that to a string.

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[LiamCato]

Hi Tim, yes I think that issue #3259 overlaps with this. If there was a string representation of the Attr object then this would likely not be an issue.

[tim-finnigan]

Thanks for following up and confirming. We may want to consolidate these issues and continue tracking in #3259, but I'll plan to bring these issues up for a future review with the team.