[BUG] try..finally/except and raises

[f-saez]

Bug description

Hi,

I've noticed a strange behavior with the last nightly.

from testing import assert_true

fn is_title(t : String) -> String:
    var result = String()
    if t.startswith("TITLE"):
        try:
            var t1 = t.split(" ")
            if len(t1)>1:
                result = t1[1]
        finally:
            pass
    return result

def main():
    assert_true(is_title("TITLE something"))

it doesn't compile, with the following error message :

note: try surrounding the call in a 'try' block
            var t1 = t.split("\"")

If I change finally by except

fn is_title(t : String) -> String:
    var result = String()
    if t.startswith("TITLE"):
        try:
            var t1 = t.split(" ")
            if len(t1)>1:
                result = t1[1]
        except:
            pass
    return result

everything's fine again.

Steps to reproduce

run the small piece of code

System information

- OS : Fedora 40 x86_64
- mojo -v => mojo 2024.8.2201 (f6a255dc)
- modular -v => modular 0.9.2 (b3079bd5)

[ematejska]

Confirming that this is a change in behavior since 24.4. To me this seems like a correct change to require that there is an except statement or that the function raises here but the error message doesn't look quite right. @Mogball for his view on this as well and what this needs.

[Mogball]

This is correct behaviour. We should improve the error message here, since it can be confusing. The try block needs an except region since the surrounding context is not raising.

[Moosems]

@Mogball What about requiring raises if there is no except clause? As of python 3.12 the following raises no syntax errors (even when you replace the raise with a pass) but has simply has no error handling:

try:
    raise Exception()
finally:
    pass

[Mogball]

@Mogball What about requiring raises if there is no except clause? As of python 3.12 the following raises no syntax errors (even when you replace the raise with a pass) but has simply has no error handling:

try:
    raise Exception()
finally:
    pass

In Python, this is equivalent to writing

try:
    raise Exception()
except:
    raise
finally:
    pass

I.e., the errors are passthrough the try-finally clause. In Mojo, the behaviour is the same.

def try_finally():
    try:
        raise "whoops"
    finally:
        pass

The problem in the example reported in the issue is that the try-finally clause itself is not in a context that can raise, thus it cannot implicitly re-raise. We should improve the error message here to be more specific!

[Moosems]

Makes sense, thanks for clarifying :)