Handling an enum with an interior from external library

[AndrewJSchoen]

Hi all,

I am trying to make bindings for an enum that contains values from an external library, which itself unfortunately doesn't currently support PyO3 (nalgebra). Currently, I have the following enum:

#[repr(C)]
#[derive(Serialize,Deserialize,Clone,Debug)]
#[cfg_attr(feature = "pybindings", derive(FromPyObject))]
pub enum Goal {
    Translation(Translation3<f64>), // From nalgebra
    Rotation(UnitQuaternion<f64>), // From nalgebra
    Scalar(f64),
    Size(Vector3<f64>), // From nalgebra
    Ellipse(Ellipse), // My own struct
    RotationRange(RotationRange),  // My own struct
    ScalarRange(ScalarRange) // My own struct
}

#[cfg(feature = "pybindings")]
impl IntoPy<PyObject> for Goal {
    fn into_py(self, py: Python) -> PyObject {
        match self {
            Self::Translation(obj) => PyTranslation{value:obj}.into_py(py), // My own struct wraps the nalgebra version and converts that
            Self::Rotation(obj) => PyRotation{value:obj}.into_py(py),  // My own struct wraps the nalgebra version and converts that
            Self::Scalar(obj) => obj.into_py(py),
            Self::Size(obj) => PySize{value:obj}.into_py(py),  // My own struct wraps the nalgebra version and converts that
            Self::Ellipse(obj) => obj.into_py(py),
            Self::RotationRange(obj) => obj.into_py(py),
            Self::ScalarRange(obj) => obj.into_py(py),
        }
    }
}

// Example implementation for PyTranslation wrapper:
#[cfg_attr(feature = "pybindings", pyclass(name="Translation"))]
#[derive(Clone,Debug)]
pub struct PyTranslation {
    pub value: Translation3<f64>
}

As you can see, I created wrappers (PyTranslation, PyRotation, PySize) that do have PyO3 bindings. However, I still get an error that

the trait `PyClass` is not implemented for `Translation<f64, 3_usize>`

(That's the Translation3 from the external I wrap). Any suggestions on how to handle this? I also have WASM bindings where this all works, so ideally I don't want to break all those by wrapping these 3 external structs in something all the time.

Thanks for the help!

[adamreichold]

I think the error messages originate from

#[cfg_attr(feature = "pybindings", derive(FromPyObject))]

which tries to apply FromPyObject::extract to the enum's inner values. I think you will need a similar manual implement for FromPyObject as you have for IntoPy.

(Besides that, it might be of interest to you that the next version of the numpy crate will contain some integration with nalgebra, c.f. PyO3/rust-numpy#347 However, I fear that is still not sufficient for your use case but maybe you want to collaborate on that over at the rust-numpy project.)

[AndrewJSchoen]

That makes sense conceptually. Are there any examples of how to implement this from scratch? I couldn't find any in the documentation.

[adamreichold]

Not sure, but the main problem is probably recovering your PyTranslation etc. wrappers, i.e. extract a PyTranslation and then recover the inner Translation3<f64>, e.g.

impl<'source> FromPyObject<'source> for Goal {
  fn extract(ob: &'source PyAny) -> PyResult<Self> {
    if let Ok(ob) = PyTranslation::extract(ob) {
      return Ok(Self::Translation(ob.value));
    }

    if let Ok(ob) = PyRotation::extract(ob) {
      return Ok(Self::Rotation(ob.value));
    }

    ...
  }
}

[adamreichold]

(Generally speaking, the FromPyObject implemenation of an enum will usually compose existing FromPyObject implementations as alternatives.)

[AndrewJSchoen]

Got it, that makes sense. I think what I was doing was technically correct, but probably much more wasteful in terms of computation (checking attributes). Thanks!