Python: Propagate an exception through a Try/Except Block with multiple Excepts

Question

Is there a way to propagate an exception in a try/except block from one except to the next?

I want to catch a specific error and then do a general error handling as well.

"raise" is letting the exception "bubble up" to an outside try/except, but not inside the try/except block that raised the error.

It should be ideally something like this:

import logging

def getList():
    try:
        newList = ["just", "some", "place", "holders"]
        # Maybe from something like: newList = untrustedGetList()

        # Faulty List now throws IndexError
        someitem = newList[100]

        return newList

    except IndexError:
        # For debugging purposes the content of newList should get logged.
        logging.error("IndexError occured with newList containing: \n%s",   str(newList))

    except:
        # General errors should be handled and include the IndexError as well!
        logging.error("A general error occured, substituting newList with backup")
        newList = ["We", "can", "work", "with", "this", "backup"]
        return newList

The problem I have is that when the IndexError gets catched with the first except, my general error handling in the second except block is not applied.

The only workaround I have for now is to include the general error handling code in the first block as well. Even if i wrap it in it's own functionblock it still seems less than elegant...

you could but your general error handling in a function and then call it both places, slightly more elegant — depperm
– depperm, Commented Dec 29, 2016 at 14:14
Impossible (at least in Python): execution cannot jump from one try/except or if/elif/else block to another one. This is like a rail junction: you go at full speed and go either left or right and are unable to move the train to another branch if you have already taken one. — ForceBru
– ForceBru, Commented Dec 29, 2016 at 14:16

Martijn Pieters · Accepted Answer · 2016-12-29 14:19:10Z

6

You have two options:

Don't catch IndexError with a dedicated except .. block. You can always test manually for the type of exception in the general block by catching BaseException and assigning the exception to a name (here e):

try:
    # ...
except BaseException as e:
    if isinstance(e, IndexError):
        logging.error("IndexError occured with newList containing: \n%s",   str(newList))

    logging.error("A general error occured, substituting newList with backup")
    newList = ["We", "can", "work", "with", "this", "backup"]
    return newList

Use nested try..except statements and re-raise:

try:
    try:
        # ...
    except IndexError:
        logging.error("IndexError occured with newList containing: \n%s",   str(newList))
        raise
except:
    logging.error("A general error occured, substituting newList with backup")
    newList = ["We", "can", "work", "with", "this", "backup"]
    return newList

answered Dec 29, 2016 at 14:19

Martijn Pieters

1.1m326 gold badges4.2k silver badges3.4k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Nimrod Over a year ago

So basically the answer to my question is NO. But at least whe have 3 different workarounds. Thx ;)

Collectives™ on Stack Overflow

Python: Propagate an exception through a Try/Except Block with multiple Excepts

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related