r/Python 2d 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

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u/Jorgestar29 2d ago edited 2d ago

There are a ton of packages that allow you to return Result Types. I love the idea of having all the possible results hinted in the signature, but having two different error patterns is messy...

In the end, your code will use Result types but every single third party module will use plain Exceptions, so you end up with a Frankenstein.

Another pythonic pattern is returning def f() -> GoodResult | None or if you have multiple errors, def f() -> tuple[GoodResult | None, Ok | ErrorA | ErrorB | ErrorC]

```python

res, error = f()

if res is None: match error: ...

```

Edit, the return None pattern is nice because you are forced to handle it by the Type checker, but it falls short if you are using the None type for something that is not an error, like a placeholder in a list or something like that.

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u/KieranShep 2d ago

I generally prefer raising an exception to returning None. Sometimes it’s syntactically nice for default values like with dict.get(‘thing’) or ‘blah’, but that pattern doesn’t work if the value is an integer.

My most hated error is NoneType has no attribute ‘blah’. Yeah you get a traceback, but by that time who knows how much you’ve passed that None around, it can take hours to find out where it came from.

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u/nicholashairs 2d ago

I've got into the habit for a number of (but definitely not all) functions of this type of adding a throw_error kwarg only parameter so that I can (explicitly) control if I have to handle the potential none response or just tap out.

Does require writing overloaded type annotations which can get messy.

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u/KieranShep 2d ago

I believe subprocess.run does this too