Is there a way to return the same object from Rust side (singleton support)?

[lycantropos]

Imagine I have some Rust enumeration that I want to export to Python

#[pyclass(name = "MyEnum", module = "my_enum")]
#[derive(Clone)]
enum MyEnum {
 Value,
}

and I also want it to be constructible in Python

#[pymethods]
impl MyEnum {
    #[new]
    fn __new__() -> MyEnum {
        MyEnum::Value
    }
}

finally module definition could be simply

#[pymodule]
fn my_module(_py: Python, module: &PyModule) -> PyResult<()> {
    module.add_class::<MyEnum>()?;
    Ok(())
}

The problem is that I want to return the same object so that Python's is operator works (like for Python's enum.Enums), but I get

>>> from my_module import MyEnum
>>> MyEnum() is MyEnum.Value
False

is there a way to do that?

[davidhewitt]

Maybe something like this?

use pyo3::prelude::*;

#[pyclass]
struct Singleton { }

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

I don't think it's possible for #[new] in PyO3 to return an existing object (at the moment), though it really should be.

[lycantropos]

because now it seems to be making clones each time called

[davidhewitt]

I think you might be able to do this with a #[staticmethod] which looks up existing attributes.

Or alternatively, are you actually highlighting that there's a bug with #[pyclass] enum in that it doesn't return singletons?

[lycantropos]

it returns the same object when I use #[classattr], but when I try to return #[classattr] attribute from method it creates a new instance, here is my real case scenario:
https://github.com/lycantropos/rene/blob/ed3b09ddaebc79556cacd7cbc8ba0dd6835971ff/src/lib.rs#L40-L91

my end goal is to be able to construct enum variants with int value from Python's side so that it returns same Python objects, currently it creates new instances on each call, so after building project locally (e.g. with python setup.py build_ext --inplace) I'm getting

>>> from rene import Orientation
>>> Orintation(0) is Orientation.COLLINEAR
False

while it works for Python's enum.Enum classes

[davidhewitt]

Right, sorry for the long delay on this. I think you can achieve something like this by replacing

return Ok(PyOrientation::CLOCKWISE)

with

return PyOrientation::type_object(py).getattr("CLOCKWISE")

i.e. you need to return the already-constructed instance on the Python type object. There's probably something that can be done on PyO3's side to make this nicer, but it's not immediately obvious what.

[lycantropos]

I'm still getting a new object instead of already constructed one, is check returns False