Bug: `multiValueQueryStringParameters` interferes with `queryStringParameters`

[aalvrz]

Expected Behaviour

When using both queryStringParameters and multiValueQueryStringParameters I expect all annotated query parameters to be set in my router handler.

Current Behaviour

If multiValueQueryStringParameters is used for one parameter and queryStringParameters for another, the latter will not be set in the router handler.

Code snippet

from typing import Annotated, Optional

from aws_lambda_powertools.event_handler.openapi.params import Query


@router.get("/foobar")
def foobar(
    ids: Annotated[Optional[list[str]], Query()] = None,
    status: Annotated[Optional[str], Query()] = None,
):
    assert ids  # This passes
    assert status is not None  # This fails


def test_issue():
    event = {
        "multiValueQueryStringParameters": {"ids": ["1", "2", "3"]},
        "queryStringParameters": {"status": "fizzbuzz"},
    }
    
    # Actually uses rest resolver
    response = handler(event, lambda_context)

Possible Solution

No response

Steps to Reproduce

Run test provided in code snippet.

Powertools for AWS Lambda (Python) version

3.21.0

AWS Lambda function runtime

3.13

Packaging format used

PyPi

Debugging logs

[aalvrz]

I don't understand if only one should be used and if so, which one?

[leandrodamascena]

Hey @aalvrz, first of all - I'm really sorry for the delay on this one.

I've investigated the root cause and here's what's happening is because in APIGatewayProxyEvent.resolved_query_string_parameters, we have:

return self.multi_value_query_string_parameters or super().resolved_query_string_parameters

When multiValueQueryStringParameters is present and non-empty (truthy), we return only that dict and completely ignore queryStringParameters. In your case, ids lives in multiValueQueryStringParameters and status lives in queryStringParameters - so status gets dropped.

The same pattern exists in ALBEvent.

Instead of an or that picks one or the other, we'll merge both: use multiValueQueryStringParameters as the base and fill in any missing keys from queryStringParameters. This way no parameter gets lost regardless of which field it appears in.

I'll include this fix in the next release. Thanks for the detailed reproduction - it made this straightforward to track down.

[aalvrz]

Thanks @leandrodamascena !