David Brownman

Errors Are Usually Bad! A Cautionary Tale

The worst part about programming is that the computer does exactly what you tell it to. Exactly.

Here at RelateIQ I work in Internal Tools, or building products for other employees. My most recent project has involved a pythong script moving data out of a database and into the product itself so a bunch of users could be slung through our new Marketo integration and contacted.

These jobs usually take a couple of hours to run, so we stick them in Docker containers and let Cronos take care of the rest. Because I wanted to make sure that the initial population run actually finished, I put an unusually aggressive try-except block in the code in the hope that it would gloss over any weird corner cases and legacy data. It looked something like this:

for user in users:
    try:
        uid = str(user['_id'])
        if uid in user_hash:
        # process and save the user object

        print "finished %s" % uid

    except:
        print "error %s" % uid

And this looked great. It was nigh unstoppable!

Before I put it in the container and pushed, I wanted to run it just to make sure everything was smooth. I fired it up and after some promising logging messages, I saw lots of successes.

=================
Begin IUL.py
=================
finished 0
finished 1
finished 2
finished 3
finished 4
finished 5
finished 5
...

And so forth. Great! Now I just need to kill the execution and run my build script and I'll be on my way. I hit Control+C and watched the interrupt get registered and ignored.

^C error 8
^C error 9
^C error 10
^C error 11
^C error 12
finished 13
finished 14
...

"That's interesting!", I thought with increasing trepidation. Ctrl+C was supposed to be my magic bullet, my ace in the hole for rescuing me from any terminal process that I had tricked myself into. Speaking to the rubber duck on my desk I explained how "It's weird, because python is clearly seeing the error happen because the message is changing accordingly, so the erro... oh."

Python was seeing the error, and it was doing exactly what I told it to. When a python script is ordered to stop, it registers a KeyboardInterrupt and acts accordingly. Unfortunately, that KeyboardInterrupt was being eaten up with anything else that could go wrong and was being ignored.

A process kill and a quick addendum fixed my dumb problem and the script ran as normal after that. It just goes to show-

Be careful what you wish for, because you just might get it!


Blog Index | | Random Post