Best practice for handling errors in python

Question

try:
    msg_json = json.loads(message_string)
    if "task" in msg_json:
        job_type = msg_json["task"]
        return (job_type, msg_json)
    logger.error(
        "Could not parse message: must provide 'task' property",
        extra={"message_string": message_string},
    )
    return empty
except Exception:
    logger.exception(
        "Error parsing JSON message. Did you accidentally double-escape it?",
        extra={"message_string": message_string},
    )
    return empty

I have this code where i am trying to load some JSON formatted message string. After looking back at this piece of code i feel i maybe using try and catch in the wrong way and i was looking for suggestions as i am new to python and there may be cleaner approaches. There is no bug here but this is more for me to learn the "Cleaner" approach. As such i am open to all suggestions that explain the cleaner and more correct approach.

Avoid using generic except clauses like except Exception: because they can hide unexpected errors. Instead use except Exception as exc: and display or log the string value of exc so you have a clue as to what went wrong. — martineau
– martineau, Commented Jun 3, 2020 at 18:32
If you are just trying to catch a parsing error then you could put it around msg_json = json.loads(message_string) only. You should try to catch specific Exceptions instead of All Exceptions. — wwii
– wwii, Commented Jun 3, 2020 at 18:33

Samwise · Accepted Answer · 2020-06-03 18:29:37Z

1

You could handle both of your error cases in catch blocks, which makes your "happy path" code a bit cleaner and neatly groups all the error handling in one place:

try:
    msg_json = json.loads(message_string)
    return (msg_json["task"], msg_json)
except KeyError:
    logger.error(
        "Could not parse message: must provide 'task' property",
        extra={"message_string": message_string},
    )
    return empty
except Exception:
    logger.exception(
        "Error parsing JSON message. Did you accidentally double-escape it?",
        extra={"message_string": message_string},
    )
    return empty

answered Jun 3, 2020 at 18:29

Samwise

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3 Comments

Dinero Over a year ago

This is awesome and is there any best practice that suggests we should leave logic out of the "try" clause

Samwise Over a year ago

Any logic that you don't want to catch those exceptions from should not go in the try. Since you want to catch exceptions from both json.loads and the ['task'] lookup, and you want to always return an empty result rather than raising to the caller, it makes sense to have both of those in a try.

Samwise Over a year ago

There's kind of a larger best practice question about whether it's better to return an empty result than to raise, but that depends on the needs and expectations of the caller. In most cases I think it's better to raise, but that depends on there being any possibility that the caller can do something better than just ignoring it.

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