Improve error messages for #[pyfunction] defined inside #[pymethods]

[csernazs]

Make error message more specific when #[pyfunction] is used in #[pymethods].

Effectively, this replaces the error message:

error: static method needs #[staticmethod] attribute

To:

functions inside #[pymethods] do not need to be annotated with #[pyfunction]

Fixes #4340

[davidhewitt]

Thanks very much for helping with this! I think there's a couple of further tweaks we should apply, and then potentially we can ship this in #4333

[davidhewitt]

I think if you make this check remove the attribute rather than just check for it, it will improve the error output by removing all the gibberish related to the pyfunction expansion.

Also, see what we recently did in #4288 - we should check for the the "namespaced" attribute here too.

[csernazs]

Hi David,

Thanks for the review!

I added clearing attrs and it removes the other error messages, but honestly I'm not sure if this is the correct solution. What do you think?

Regarding the namespaces, I'm reading the PR you linked.

Thanks again,
Zsolt

ps: this PR was created during europython 2024 sprint weekend.

[csernazs]

I was thinking about namespaces and I found the has_attribute function, so at the end the "if" can be updated to:

if has_attribute(&meth.attrs, "pyfunction")`

But this function is not public so I cannot use it in pyimpl.rs. I checked the references and it seems no code path runs through module.rs.

So if this is the way to solve (I mean, calling has_attribute). I think it can be done by:

• making this function public
• moving this function to some common source file (and making it public there).

Or something else. What do you think?

Zsolt

[davidhewitt]

I think you want both has_attribute and has_attribute_with_namespace. Perhaps move them to utils.rs and make them pub(crate)?

[csernazs]

I moved both functions, and I also had to move

pub(crate) enum IdentOrStr<'a> {
    Str(&'a str),
    Ident(syn::Ident),
}

as well to utils.rs.

[davidhewitt]

IMO .clear() is a bit too much because it might introduce other compile errors by removing needed attributes.

One option is to use .retain(), something like this pseudocode:

let mut error = None;
meth.attrs.retain(|attr| {
    if attr is pyfunction { 
        error = Some(err_spanned!(...));
        false
    } else {
        true
    }
});
error.map_or(Ok(()), Err)

[csernazs]

Thanks for the pseudo code, it helped a lot!

I make a copy of attrs to construct a 1-element slice here:

        let attrs = [attr.clone()];

idk what is the idiomatic way in rust to solve this issue ,but this is the best I could do.

There could be two additional function for has_attribute_with_namespace and has_attribute accepting just a single attribute but I felt it is easier to construct a one-element slice here.

Span needs to be adjusted a little bit as in retain the attrs is not yet removed at that point so rust points to the attribute instead if I use meth.span(), like this:

error: functions inside #[pymethods] do not need to be annotated with #[pyfunction]
  --> src/lib.rs:10:9
   |
10 |         #[pyfunction]
   |         ^

[davidhewitt]

Thanks, this looks good to me now!