Prevent usage of return, yield in try/finally

[hofrob]

Current problem

Code in finally blocks will always be executed. If there's a return inside the try, it's possible to overwrite this return value with a return in the finally.

def add(a: int, b: int) -> int:
    try:
        return a + b
    finally:
        return 0


if __name__ == "__main__":
    print(add(1, 2))  # those args will be ignored; "0" is printed

Desired solution

There should not be any return or yield statements in a finally block.

Additional context

https://docs.python.org/3/reference/compound_stmts.html#finally-clause

The return value of a function is determined by the last return statement executed.

[nickdrozd]

Sounds like a good idea to me. I did a quick search of a variety of repos and I couldn't find a single instance, so it seems to be a rare pattern.

[deepgohil]

I would like to solve this issue.

[Pierre-Sassoulas]

I assigned you to it @deepgohil :)

[deepgohil]

Can you tell me where I have to change the code? I want to know folder name

[Pierre-Sassoulas]

You can create or modify a checker in pylint/checkers, or create and modify an extension in pylint/extensions. This check feel like a nasty underhanded bug that could be a default checker of warning level imo.

Design by ChatGPT would indicate that it could be in the basic checker:

In what pylint checker should a check for 'return-superseded-by-finally' should go ?

The return-superseded-by-finally check in Pylint belongs to the "basic" category of checks. The "basic" checks are a set of checks that Pylint performs by default, without the need for any additional configuration. This check reports a warning when a return statement appears in a function that also has a finally block.

But I think that ChatGPT is basic and might be giving bad design advise here. I don't know where it should really go though. Or if it should really be called 'return-superseded-by-finally'.

[ollie-iterators]

What should be in the finally block instead of return?

[Pierre-Sassoulas]

What should be in the finally block instead of return?

The finally block should be removed for example like this:

def add(a: int, b: int) -> int:
    try:
        return a + b
    except:
        return 0

@deepgohil do you have an opinion regarding the name and the checker it should go in ? (maybe a new one ?)

[hofrob]

I'd say in general: finally blocks should not contain any yield or return statements. I would not make any suggestion on how to fix it.

I'm also wondering where to put this new rules 🙂

[Pierre-Sassoulas]

Regarding the name it could also be yield as @hofrob pointed out, so return-superseded-by-finally is not working. Maybe superseded-by-finally shadowed-by-finally, return-or-yield-in-finally ?

[nickdrozd]

return-or-yield-in-finally sounds the clearest, in the sense that it answers "what needs to be changed to make this warning go away".

[hofrob]

Since nobody is working on this, I'd try to come up with something. Let's see how this goes.