I'd say, as a rule of thumb, whenever you are suspecting that something may fail badly enough to break the control flow, add any hint that you'd like to see yourself in runtime. Python may be a little bit cryptic without it at times. It's good enough that you'll get the failing line number and you always can peek into the source, but if you're suspecting a specific category of failures already, make yourself a favor and write that down. Specially, if the error may propagate from outside this script.