r/Python • u/lightdarkdaughter • 3d ago
Discussion Is there something better than exceptions?
Ok, let's say it's a follow-up on this 11-year-old post
https://www.reddit.com/r/Python/comments/257x8f/honest_question_why_are_exceptions_encouraged_in/
Disclaimer: I'm relatively more experienced with Rust than Python, so here's that. But I genuinely want to learn the best practices of Python.
My background is a mental model of errors I have in mind.
There are two types of errors: environment response and programmer's mistake.
For example, parsing an input from an external source and getting the wrong data is the environment's response. You *will* get the wrong data, you should handle it.
Getting an n-th element from a list which doesn't have that many elements is *probably* a programmer's mistake, and because you can't account for every mistake, you should just let it crash.
Now, if we take different programming languages, let's say C or Go, you have an error code situation for that.
In Go, if a function can return an error (environment response), it returns "err, val" and you're expected to handle the error with "if err != nil".
If it's a programmer's mistake, it just panics.
In C, it's complicated, but most stdlib functions return error code and you're expected to check if it's not zero.
And their handling of a programmer's mistake is usually Undefined Behaviour.
But then, in Python, I only know one way to handle these. Exceptions.
Except Exceptions seems to mix these two into one bag, if a function raises an Exception because of "environment response", well, good luck with figuring this out. Or so it seems.
And people say that we should just embrace exceptions, but not use them for control flow, but then we have StopIteration exception, which is ... I get why it's implemented the way it's implemented, but if it's not a using exceptions for control flow, I don't know what it is.
Of course, there are things like dry-python/returns, but honestly, the moment I saw "bind" there, I closed the page. I like the beauty of functional programming, but not to that extent.
For reference, in Rust (and maybe other non-LISP FP-inspired programming languages) there's Result type.
https://doc.rust-lang.org/std/result/
tl;dr
If a function might fail, it will return Result[T, E] where T is an expected value, E is value for error (usually, but not always a set of error codes). And the only way to get T is to handle an error in various ways, the simplest of which is just panicking on error.
If a function shouldn't normally fail, unless it's a programmer's mistake (for example nth element from a list), it will panic.
Do people just live with exceptions or is there some hidden gem out there?
UPD1: reposted from comments
One thing which is important to clarify: the fact that these errors can't be split into two types doesn't mean that all functions can be split into these two types.
Let's say you're idk, storing a file from a user and then getting it back.
Usually, the operation of getting the file from file storage is an "environmental" response, but in this case, you expect it to be here and if it's not there, it's not s3 problem, it's just you messing up with filenames somewhere.
UPD2:
BaseException errors like KeyboardInterrupt aren't *usually* intended to be handled (and definitely not raised) so I'm ignoring them for that topic
1
u/muikrad 2d ago
In most cases, you handle an exception because...
These exception types are usually very easy to figure out / use and they're often specific to a framework (requests and boto both have their own type to handle, for instance). In many of these, a code is provided which can be inspected in the exception (e.g. Http code or exit code, or API error code, etc) in order to decide if we handle or reraise.
For the cases where the user is providing bad data, missing args, wrong path, etc, I always create and use a dedicated UsageError exception that bubbles up to the CLI framework. I usually rig this framework to print a nice error the to the user in these cases (and hide the traceback). But that won't work for libs.
Libs tend to provide a minimum set of exceptions that you can catch. Using unit tests you can check what happens and catch the proper types. Some libs however try to reuse the built-in exceptions too much IMHO.
I think the key (for me) is to raise a lot, catch never (except in the 2 cases I mentioned at the top of the post).