Convert Python-created objects from rust library back to rust

[c-a-m-o]

I am trying to write a rust app that would be able to dynamically run python plugins. Such plugins should be able to access some of the types of the rust app and return instances of these types.

The question is, is it possible to get the rust and the python types to match such that I can write python functions that I could call from rust and that will return objects that I can convert to rust ?

What I tried was to create a rust lib containing a public type :

#[pyclass]
pub struct RustStruct {
    #[pyo3(get, set)]
    value: usize,
}

#[pymethods]
impl RustStruct {
    #[new]
    pub fn new(value: usize) -> Self {
        Self { value }
    }
}

I was able to build a wheel with maturin and install it on a local install of python.
The problem is when I try to call python from rust : I don't seem to be able to get back a rust object from a python-created one.

In a test module inside the rust library, I wrote this code :

fn test_struct() {
    prepare_freethreaded_python();
    Python::with_gil(|py| {
        const CODE: &str = r#"
from rustlib import RustStruct
def test_fn(value):
    return isinstance(value, RustStruct)
"#;
        let module = PyModule::from_code(py, CODE, "test_mod.py", "test_mod").unwrap();
        let test_fn = module.getattr("test_fn").unwrap();
        let py_res = test_fn.call1((RustStruct::new(12),)).unwrap();
        let res: bool = py_res.extract().unwrap();
        assert!(res);
    })
}

The test fails because the types are not the same. However, when I give to python an instance of the right type, I am able to get the class and then create an instance that I can export to rust

fn test_struct() {
    prepare_freethreaded_python();
    Python::with_gil(|py| {
        const CODE: &str = r#"
from rustlib import RustStruct
def test_fn(value):
    return type(value)(13)
"#;

        let module = PyModule::from_code(py, CODE, "test_mod.py", "test_mod").unwrap();
        let test_fn = module.getattr("test_fn").unwrap();
        let py_res = test_fn.call1((RustStruct::new(12),)).unwrap();
        let res: RustStruct = py_res.extract().unwrap();
        assert_eq!(res.value, 13)
    })
}

I asked the question on gitter two weeks ago, and I also found this previously asked question but it does not seem to be exactly the same problem.

Thank you