[MrNK2107]

Issue number: closes #8579

Summary

Changes

1. 404 hijacked by catch-all handlers. not_found_handler (sync and async) used lookup_exception_handler(NotFoundError), which walks the MRO up to Exception. A new _lookup_not_found_handler() walks the same MRO but stops before Exception/BaseException. Handlers for NotFoundError (@app.not_found) and ServiceError still format 404s as before. Only catch-all handlers are skipped, since no exception was actually raised.
2. body: null on ALB, and dict/tuple returns crashing. _call_exception_handler now passes the handler's return value through _to_response, the same adaptation a route's return value gets. As a result, ALBResolver._to_response turns body=None into "", and {...} / ({...}, status) are serialized as JSON.
   • The ALB response-size path already did this by hand after calling _call_exception_handler, so I removed that now-redundant line.

New tests:

• test_api_gateway.py: a catch-all handler no longer replaces the 404, a ServiceError handler still formats 404s, a None body on ALB becomes "", and dict and tuple returns are serialized.
• test_resolve_async.py: a catch-all handler no longer replaces the 404 on the async path.

All event_handler tests pass. Locally on Windows, test_openapi_encode_purepath and test_load_resolver_file_not_found also fail on an unmodified develop, because of path separators.

User experience

Before:

@app.exception_handler(Exception)
def generic(ex): return Response(500, "text/plain", "oops")
# GET /nope -> 500 "oops"

@app.exception_handler(ValueError)
def bad(ex): return {"error": str(ex)}, 400
# -> AttributeError: 'tuple' object has no attribute 'is_json'

After: GET /nope returns the default 404, and the tuple becomes a 400 JSON response. On ALBResolver, a handler returning Response(status_code=400, body=None) sends "body": "" instead of null.

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[boring-cyborg]

Thanks a lot for your first contribution! Please check out our contributing guidelines and don't hesitate to ask whatever you need.
In the meantime, check out the #python channel on our Powertools for AWS Lambda Discord: Invite link

[leandrodamascena]

Thanks @MrNK2107! I ran the suites and deployed this behind a real REST API: an unmatched route goes back to the default 404 even with a catch-all handler registered, a handler returning a tuple gives a proper 400 JSON instead of the AttributeError, and real exceptions still reach the catch-all. I also checked @app.not_found, a ServiceError handler, the [Exception, NotFoundError] list, the ALB None body, the Depends path and Bedrock, all behave. Nice touch dropping the now redundant _to_response in the ALB size path.

Approving now. This goes out in 3.38.0, so it will sit here a bit before I merge. No need to rebase or touch anything, I'll take care of it and you'll get the credit in the release notes. Thank you!

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.93%. Comparing base (9c6ca41) to head (5817f68).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
aws_lambda_powertools/event_handler/api_gateway.py	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8624      +/-   ##
===========================================
- Coverage    96.94%   96.93%   -0.01%     
===========================================
  Files          318      318              
  Lines        16051    16058       +7     
  Branches      1427     1430       +3     
===========================================
+ Hits         15560    15566       +6     
  Misses         349      349              
- Partials       142      143       +1     

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
1.6% Duplication on New Code

See analysis details on SonarQube Cloud

[boring-cyborg]

Awesome work, congrats on your first merged pull request and thank you for helping improve everyone's experience!

[powertools-for-aws-oss-automation]

Awesome work, congrats on your first merged pull request and thank you for helping improve everyone's experience!