Pass List from Python to Rust without cloning elements

[cswinter]

Is there a good way to receive a Python List without the elements getting cloned? (Context: entity-neural-network/ragged-buffer:src/monomorphs/f32.rs#71)

I've tried receiving a list and extracting each element, but the extract doesn't like returning a borrow:

    #[classmethod]
    fn cat(_cls: &PyType, buffers: &PyList, dim: usize) -> PyResult<Self> {
        let mut bufs = Vec::new();
        for buf in buffers.iter() {
            bufs.push(&buf.extract::<&RaggedBufferF32>()?.0);
        }
        Ok(RaggedBufferF32(RaggedBuffer::cat(&bufs, dim)?))
    }

[birkenfeld]

You can extract a &PyCell<RaggedBufferF32> and borrow it to get a &RaggedBufferF32.

[birkenfeld]

Even easier, you can extract PyRef<RaggedBufferF32> directly (which is what you would get from borrow).

[cswinter]

Nice, thank you! With PyRef, you don't even need the manual extract anymore:

    #[classmethod]
    fn cat(_cls: &PyType, buffers: Vec<PyRef<RaggedBufferF32>>, dim: usize) -> PyResult<Self> {
        Ok(RaggedBufferF32(RaggedBuffer::cat(
            &buffers.iter().map(|b| &b.0).collect::<Vec<_>>(),
            dim,
        )?))
    }

[birkenfeld]

Indeed, although now you're allocating one more intermediate Vec. Of course, this might not really matter.

[cswinter]

Yes, it doesn't matter too much in my case. With the current interface, you do need the second Vec as a backing array for the PyRef objects, but at least one vec alloc could be eliminated by changing the RaggedBuffer::cat to accepting PyRef<RaggedBufferF32> or AsRef<RaggedBufferF32 instead of borrows.