Feature request: Automatically log exceptions using sys.eventhook

Use case

I find myself often writing a generic try/except statement in my handler to always log all exceptions. I’m not sure if this is a good pattern, maybe it already does this, so I might just be looking for guidance on current practices. (I didn’t see anything in the docs or other issues on this.)

from aws_lambda_powertools import Logger
from aws_lambda_powertools.utilities.typing import LambdaContext

logger = Logger()


@logger.inject_lambda_context
def handler(event: dict, context: LambdaContext) -> str:
    try:
        return "hello world"
    except Exception:
        logger.exception("Caught an unhandled error")
        raise

Solution/User Experience

What I’ve done in CLI utilities to capture those exceptions to the logger, is to override the sys.eventhook. This way, every exception is logged automatically. Would it be useful to do something similar for lambda functions?

def exception_hook(exc_type, exc_value, exc_traceback):
    """Log unhandled exceptions with hook for sys.excepthook."""
    log = logging.getLogger(service_name)
    log.exception(
        '',
        exc_info=(exc_type, exc_value, exc_traceback)
    )

sys.excepthook = exception_hook

Alternative solutions

No response

Acknowledgment

• This feature request meets Lambda Powertools Tenets
• Should this be considered in other Lambda Powertools languages? i.e. Java, TypeScript

[heitorlessa]

Great to hear @davideicardi !

As to your question, this is a hook for any exception that was either raised intentionally OR unforeseen. For the ServiceError case, Powertools will catch it and convert into a successful exit ({“statusCode”: 500, …}).

For the flexibility, we could still have a method where you provide your own function to handle any uncaught exception, like set_exception_hook. I initially thought of having a list of exceptions, “allow_list” if you will, but the premise that @lorengordon and Python docs states is primarily for uncaught exceptions – If you know what the exception is, it’s still business as usual since you can catch, log the exception with logger.exception, and decide whether to re-raise or handle gracefully.

For example:

Raised intentionally

from aws_lambda_powertools import Logger
import sys

logger = Logger(service="sample")


def exception_hook(exc_type, exc_value, exc_traceback):
    logger.exception("Unhandled exception", exc_info=(exc_type, exc_value, exc_traceback))


sys.excepthook = exception_hook


def main():
    raise ValueError("Oops!")  # this will crash the program thus calling `sys.excepthook`.


main()

Unexpected/unforeseen exception

from aws_lambda_powertools import Logger
import sys

logger = Logger(service="sample")


def exception_hook(exc_type, exc_value, exc_traceback):
    logger.exception("Unhandled exception", exc_info=(exc_type, exc_value, exc_traceback))


sys.excepthook = exception_hook


def main():
    a = {"key": "value"}
    a["non_existent_key"]  # KeyError will crash the program thus calling `sys.excepthook`.

main()

[heitorlessa]

Yes, we can figure out naming in a PR. Perhaps Logger(log_uncaught_exceptions=True), and in the docstring (and docs) we call out the the hook.

This makes it easier for other Powertools languages to reword exception to error and keep a similar UX 😉

I’m working on a few other issues now, but if you have the bandwidth to send a PR, we can include it in the next release!!