How do I add methods to exception type created with create_exception!()?

[tliron]

The types do not implement PyClass, so it's impossible to use #[pymethods] with them.

[tliron]

Well, I think I found a workaround.

There's this section in the documentation about "creating more complex exceptions", but it is incomplete. It's missing instructions on how to instantiate and return these, because they don't have the new_err function that exceptions created with the macros have.

Here's how I did it in my code:

#[pyclass(name = "Problem", extends=PyException, frozen)]
pub struct PyProblem {
    /// Inner problem.
    pub inner: Problem,
}

impl PyProblem {
    /// Constructor.
    pub fn new_err(inner: Problem) -> PyResult<PyErr> {
        Python::attach(|py| {
            let problem = Bound::new(py, Self { inner })?;
            Ok(PyErr::from_value(problem.into_any()))
        })
    }
}

This seems to work. However, I'm still unclear as to why create_exception! creates types that are not PyClass. Something else is going on here and I would like to understand the differences!

[davidhewitt]

Great question. In general we would like to move away from create_exception! and have #[pyclass] be the only way to create Python types.

There's design questions about things like new_err, e.g. #4186, which we'll solve eventually!