How to turn str | None into a Rust Enum

[dedebenui]

In python it's very common to give str to functions to toggle some features (like the mode in open). In Rust, those should probably be enums, but pyo3 cannot derive FromPyObject for enums with empty variants yet (#417). Here is how I do it:

enum SomeEnum {
    DefaultVariant,
    Variant2,
    Variant3,
}

impl<T> TryFrom<Option<T>> for SomeEnum
where
    T: Deref<Target = str>,
{
    type Error = PyErr;
    fn try_from(value: Option<T>) -> Result<Self, Self::Error> {
        match value.as_deref() {
            None | Some("variant_1") => Ok(Self::DefaultVariant),
            Some("variant_2") => Ok(Self::Variant2),
            Some("variant_3") => Ok(Self::Variant3),
            _ => Err(PyErr::new::<PyValueError, _>("Invalid value")),
        }
    }
}

impl<'py> FromPyObject<'py> for SomeEnum {
    fn extract(ob: &'py PyAny) -> PyResult<Self> {
        Self::try_from(ob.extract::<Option<String>>()?)
    }
}

My main approach is to extract the string from the python object, and implement TryFrom for my enum for any type that can be dereferenced as str.

To avoid repeating some code, I would have liked to write this last implementation block as

impl<'py, E> FromPyObject<'py> for E
where
    E: TryFrom<Option<String>>,
{
    fn extract(ob: &'py PyAny) -> PyResult<E> {
        E::try_from(ob.extract::<Option<E>>()?)
    }
}

but I cannot write blanket implementations for traits that I didn't define.