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Bug: Open API Validation not validating response serialization when body is Falsy. #5887
Description
Expected Behaviour
When:
- resolver (of type APIGatewayResolver) has
enable_validation=Trueand, - 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
Activity
boring-cyborg commentedon Jan 20, 2025
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 commentedon Jan 21, 2025
Hey @amin-farjadi! Thanks for opening this, will check that.
anafalcao commentedon Jan 29, 2025
Hey @amin-farjadi , we were able to reproduce the bug, and we'll work on it. Thank you!
leandrodamascena commentedon Feb 17, 2025
Assigning to @anafalcao to see if it is possible to deliver in this current iteration.
7 remaining items
leandrodamascena commentedon Mar 7, 2025
Reopening this since it created a regression: #6216
amin-farjadi commentedon Feb 9, 2026
Hi @leandrodamascena, are you happy for me to implement a fix for this, taking into account the regression reported?
powertools-for-aws-oss-automation commentedon Mar 12, 2026
Warning
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github-actions commentedon Mar 20, 2026
This is now released under 3.26.0 version!