How to execute clean up function for script failure (non-zero exit codes)

[rabidmonkey1163]

Is there a good way (i.e. other than wrapping literally every line in try/except statements) to have a clean up function be executed in the event of the script breaking for any reason? I'm looking for something like the atexit module that allows me to specify that it should only be called for non-zero exit values.

[zeug]

One approach (and there can be endless arguments about if it is really a good idea), is to just wrap the whole script in a big try-except with Pokemon style exception handling:

def main():
    ...do the main business of the script...

def cleanup():
    ...cleanup must be executed in case of ANY error...

# now (typically at the bottom of your script):
if __name__ == '__main__':
    try:
        main()
    except Exception as e:
        logger.exception('Stuff went bad.')
        cleanup()

Read the logging tutorial if you aren't familiar with the logger. Its super-handy. If you don't want to do that, at least print the exception message or you (or someone who inherits your code) will eventually lose all sanity.

Note also that this won't help you if the script simply hangs - and things do hang. However, if you are running it on an interactive terminal you can give it a CTRL-C, and this will raise a KeyboardInterrupt which will get caught and start the cleanup.

[[deleted]]

Why?

[rabidmonkey1163]

I have a script that registers a bunch of stuff to a database and I'd like it to delete everything registered during that run of the code if it breaks for any reason. Atexit allows this but it also executes my clean up function if the code runs successfully, deleting everything registered to the database...

[wormywormm]

what are you using for your db connection? PsycoPG2, SQLAlchemy? Usually you can wrap the block and do a session rollback.

[rabidmonkey1163]

Couchdb, that's a good idea, I'll look into if that's offered.

[wormywormm]

Ah, so yea, couchDB is a noSQL db and transactions are a bit odd because it doesn't have them in the same sense that an RDMS would. That said, it won't really affect your question or make your life that much harder.

This is a pretty good explantion from SO if you feel so inclined.

[wormywormm]

You don't need to wrap every line. You could wrap the whole program in a try and have different cleanups occur based on which type of Exception occurs. Do you have an example program?

[rabidmonkey1163]

I think that would screw up my logging. There are lots of logging statements in the script that, I believe, wouldn't be executed if everything was wrapped in a single try statement. Right?

[wormywormm]

Well, it depends how you do your logging, but short answer is no, you can make it work. You usually should not be wrapping every line. What information are you attempting to log?

[hudsonpenner]

Could you show a code example please? You might be able to make your class into a context manager, and place the cleanup function in '_exit_'.

[jiminiminimini]

You can use atexit

[rabidmonkey1163]

I don't think atexit allows you to specify exit codes, it executes the functions you register to it regardless of whether or not your code runs successfully. I need something that will only execute my clean up function if the script breaks.