#[classattr] functions should be able to take Python arguments

[davidhewitt]

Try to compile the following:

#[pyclass]
struct Singleton { }

#[pymethods]
impl Singleton {
    #[classattr]
    fn VALUE(py: Python<'_>) -> Py<Self> {
        Py::new(py, Singleton { }).unwrap()
    }
}

I expected this to compile, but instead the compiler emits:

error: class attribute methods cannot take arguments
  --> src/lib.rs:10:14
   |
10 |     fn VALUE(py: Python<'_>) -> Py<Self> {
   |              ^^

I think it'd be nice to change #[classattr] to allow Python argument (like #[getter], for example).

[mdaj06]

@davidhewitt i'd really like to implement this

[davidhewitt]

@mdaj06 please do! Do you need any tips to get started?

[mdaj06]

@davidhewitt i'll look into it and if I get blocked I'll reach out,if that's okay?

[birkenfeld]

Don't worry, we will try to help if you get stuck.

[mdaj06]

thanks!! @birkenfeld

[mdaj06]

i ran a few circles, trying to find out where to start. Could you point out the file i can start from?

[davidhewitt]

Sure thing.

The call to the #[classattr] function is generated in macro code at

pyo3/pyo3-macros-backend/src/pymethod.rs

Line 366 in 9dfeaa3

let mut ret = #cls::#name();

You will want to conditionally add py in there according to whether the #[classattr] function takes an argument. See the equivalent code for #[getter]:

pyo3/pyo3-macros-backend/src/pymethod.rs

Lines 489 to 500 in 9dfeaa3

	let (py_arg, args) = split_off_python_arg(&spec.args);
	ensure_spanned!(
	args.is_empty(),
	args[0].ty.span() => "getter function can only have one argument (of type pyo3::Python)"
	);
	let name = &spec.name;
	let fncall = if py_arg.is_some() {
	quote!(#cls::#name(_slf, _py))
	} else {
	quote!(#cls::#name(_slf))
	};

Let me know if you need more detail than that :)