I recently wrote a blog post that involved exception handling, and gave short shrift to the part of exception handling I didn’t want to talk about in order to focus on the part I did want to talk about. For some readers, that clearly backfired.
My recent blog post about coercing Python objects to integers caught people’s attention in a way I hadn’t intended. The point I was trying to make was that an innocent-looking call like
int(an_object) calls the method
__int__(), and since that can be arbitrary code, it can raise arbitrary exceptions. Therefore, it’s insufficient to catch only the usual exceptions of ValueError and TypeError if you don’t know the type of
an_object in advance.
Here’s the code I suggested –
def int_or_else(value, else_value=None): """Given a value, returns the value as an int if possible. If not, returns else_value which defaults to None. """ try: return int(value) # I don't like catch-all excepts, but since objects can raise arbitrary # exceptions when executing __int__(), then any exception is # possible here, even if only TypeError and ValueError are # really likely. except Exception: return else_value
Several commenters objected to the fact that this code discards (and therefore silences/masks/hides) all exceptions. Here’s why I made that choice.
The Two Parts of Exception Handling
In Python, there’s two parts to consider about exception handling — what to catch, and what to do with the exception once you’ve caught it. My intention was to write only about the former.
The latter is an interesting topic, too. Once you’ve caught an exception, you might want to log it and then discard it, log it and then re-raise it, re-raise it as a different exception, silence it, let it pass up to the caller, modify its attributes and re-raise it, etc. There’s enough material for an entire blog post about different ways to react to an exception, and the pros and cons of each.
Someday I might write that post about different ways to react to trapped exceptions, and if I do, I’ll dedicate the entire post to the subject to give it the attention it deserves. That other blog post – that was not it. In fact, it was the opposite. I gave the topic of processing the trapped exception as little attention as possible so as not to detract attention from what I wanted to be the main topic (what exceptions need to be trapped).
My post was not advocacy of discarding exceptions, nor was it advocacy of not discarding exceptions. What’s the right choice? It depends. One situation where you might want to discard exceptions is in a blog post where you’re trying to keep the code as brief as possible for readability. Then again, you might regret that.
In the future, I’ll be clearer about what shortcuts I’m taking for brevity of presentation.
Agree? Disagree? I’d like to hear from you. I like it when people agree with me. Those who disagree can expand my horizons, and I like that too. In short, all civil comments are welcome. I feel I’ve spent enough time thinking about this topic for now, but that doesn’t make me right! Let me know what you think.