Pyo3: `#[new]` should support arrays

[c410-f3r]

Maybe a missing implementation?

use pyo3::prelude::*;

#[pyclass]
struct Foo {
  array: [i32; 2]
}

#[pymethods]
impl Foo {
  #[new]
  //pub fn new(array: [i32; 2]) -> Self {
  pub fn new(array: &[i32; 2]) -> Self {
    //Self { array }
    Self { array: *array }
  }
}

[davidhewitt]

Like with everything about arrays in Rust, we could write a solution to support arrays up to an arbitrary length (normally 32).

To do this for all sizes would require const generics being available.

A PR for this to support arrays up to length 32 is very welcome.

[c410-f3r]

I will gladly provide a PR for arrays (up to 32) using declarative macros with some mentoring

[davidhewitt]

That would be awesome, thanks! I'll find some time to write some quick mentoring notes either tomorrow or the day after.

[davidhewitt]

Mentoring notes, as promised:

Take a look at the implementation of FromPyObject for Vec<T>: https://github.com/PyO3/pyo3/blob/90b14fb36904ccbd91ce8adf121c38d23c8b4a4c/src/types/sequence.rs#L244

This is probably a good starting point. It uses specialization to do an optimization for Buffer objects, but we don't need that here - arrays of only 32 elements will be short enough that an optimization is kinda irrelevant. So you only need to care about the extract_sequence function.

As for the macro, you could structure it something like this:

macro_rules impl_array! {
    ($n:literal) => {
        impl<'a, T> FromPyObject<'a> for [T; $n]
            where T: FromPyObject<'a>
        {
            ...
        }
    };
}

impl_array!(0);
...  // call for each N
impl_array!(32);

You might be able to also make a trick so that impl_array! calls itself recursively, but that's a bit of fun you can play with yourself if you want.

If you're willing, it would be awesome to also have IntoPy<PyObject> implemented for arrays in the same way. See https://github.com/PyO3/pyo3/blob/90b14fb36904ccbd91ce8adf121c38d23c8b4a4c/src/types/list.rs#L190

@c410-f3r ping me if you have any further questions on this. The help is much appreciated.

[c410-f3r]

Thank you, @davidhewitt