Ergonomic API around capsules

[Tpt]

• A trait PyCapsuleType to give the capsule name for the type and encode that the type is pycapsule-safe
• Two wrapper types PyCapsuleValue and PyCapsuleValueRef that implement FromPyObject and IntoTypeObject wrapping respectively T and &T

Example of API goal:

#[pyfunction]
fn add_one(foo: PyCapsuleValueRef<'_, Foo>) -> PyCapsuleValue<Foo> {
    PyCapsuleValue(Foo { val: foo.val })
}

[ngoldbaum]

Just out of curiosity, why would someone want this? My understanding is that the PyCapsule API is there to serve use-cases like NumPy's C API where someone wants to expose a C API for a Python library that must be imported before it can be usable from C. Are you trying to enable a similar pattern with PyO3 or something else entirely?

[Tpt]

@ngoldbaum Yes! This is this kind of usecases. For example datafusion Python bindings relies a lot on capsule for extensibilities and method definition like:

    pub fn __datafusion_table_provider__<'py>(
        &self,
        py: Python<'py>,
        session: Bound<PyAny>,
    ) -> PyResult<Bound<'py, PyCapsule>> {

can become

    pub fn __datafusion_table_provider__(
        &self,
        session: PyCapsuleValueRef<'_, FFI_LogicalExtensionCodec>,
    ) -> PyResult<PyCapsuleValue<FFI_TableProvider>> {

[ngoldbaum]

Ah, TIL there's an Arrow PyCapsule interface: https://arrow.apache.org/docs/format/CDataInterface/PyCapsuleInterface.html.

[Tpt]

A follow up idea: auto implement this trait on all stable_abi::StableAbi implementations (we could try to build a name from the introspection data)