Bug: Open API Validation not validating response serialization when body is Falsy.

[amin-farjadi]

Expected Behaviour

When:

• resolver (of type APIGatewayResolver) has enable_validation=True and,
• route's function is annotated with a return type X (e.g. a Pydantic model) and,
• route's function return value is not of type X (and cannot be casted)

Expected:
raise a 422 (HTTPStatus.UNPROCESSABLE_ENTITY) as the result of error in response serialization

Current Behaviour

If function's return value is Falsy, the route is resolved to a 200 OK.

Code snippet

from typing import Literal

from aws_lambda_powertools.event_handler.api_gateway import APIGatewayRestResolver
from pydantic import BaseModel
import pytest

app = APIGatewayRestResolver(enable_validation=True)


class Todo(BaseModel):
    title: str
    status: Literal["Pending", "Done"]


@app.get("/none")
def return_none() -> Todo:
    return None


@app.get("/empty_list")
def return_empty_list() -> Todo:
    return list()


@app.get("/empty_dict")
def return_empty_dict() -> Todo:
    return dict()


@app.get("/empty_string")
def return_empty_string() -> Todo:
    return ""


def lambda_handler(event, context):
    return app.resolve(event, context)


# --- Tests below ---


@pytest.fixture()
def event_factory():
    def _factory(path: str):
        return {
            "httpMethod": "GET",
            "path": path,
        }

    yield _factory


@pytest.mark.parametrize(
    "path, body",
    [
        ("/empty_dict", "{}"),
        ("/empty_list", "[]"),
        ("/none", "null"),
        ("/empty_string", ""),
    ],
    ids=["empty_dict", "empty_list", "none", "empty_string"],
)
def test_unexpected_response_validation(path, body, event_factory):
    """Test to demonstrate cases where a non-OK response is expected from APIGatewayResolver."""
    event = event_factory(path)
    response = lambda_handler(event, None)
    assert response["statusCode"] == 200
    assert response["body"] == body


if __name__ == "__main__":
    pytest.main(["-v", __file__])

Possible Solution

Replace the falsy valuation of the response body in the _handle_response method of the OpenAPIValidationMiddleware class with a more robust class/type comparison.

Steps to Reproduce

• save snippet above to a file
• install the following dependencies
  • "aws-lambda-powertools>=3.4.1"
  • "pydantic>=2.10.5"
  • "pytest>=8.3.4"
• run the file

Powertools for AWS Lambda (Python) version

latest, 3.4.1

AWS Lambda function runtime

3.12

Packaging format used

PyPi

Debugging logs

[boring-cyborg]

Thanks for opening your first issue here! We'll come back to you as soon as we can.
In the meantime, check out the #python channel on our Powertools for AWS Lambda Discord: Invite link

[leandrodamascena]

Hey @amin-farjadi! Thanks for opening this, will check that.