Implementing getter for vector of class instances (without copy)

[chrisjsewell]

Heya, given the following class, how can I expose the children attribute in Python?

#[pyclass]
pub struct Node {
    pub type_: String,
    pub children_: Vec<Node>,
}

I know I can do the following, but then it is always creating new copies of the nodes and I want just the references:

#[pyclass]
#[derive(Clone)]
pub struct Node {
    pub type_: String,
    pub children_: Vec<Node>,
}

#[pymethods]
impl Node {
    #[getter]
    fn children(&self) -> Vec<Node> {
        self.children_.clone()
    }
}

---

For bonus points, I would also like to implement a walk method to iterate over Node references in the tree

[adamreichold]

It depends, if you want to share access to the nodes with Python code, you probably to store it in Py<Node>. Vec<Py<Node> would still create a new vector, but it will not clone the nodes, just increase their reference counts. (This is basically also what native Python containers like PyList do.)

Additionally and orthogonally to that, you could turn Node itself into a Python container by implementing slots like __getitem__ etc. and have them return Py<Node>, but that is more a question of API ergonomics than where state is shared or not.

[chrisjsewell]

Thanks @adamreichold could you provide me an example of how this would work in the code above and how you "create" Py<Node>?

[adamreichold]

#[pyclass]
#[derive(Clone)]
pub struct Node {
    pub type_: String,
    pub children_: Vec<Py<Node>>,
}

#[pymethods]
impl Node {
    #[getter]
    fn children(&self) -> Vec<Py<Node>> {
        self.children_.clone()
    }

    fn add_child(&mut self, type_: String, py: Python<'_>) {
        let child = Self {
            type_,
            children_: Vec::new(),
        };

        self.children_.push(Py::new(py, child).unwrap());
    }
}

[chrisjsewell]

Awesome, thanks! that seems to work 😄

Just to note, unless I'm missing something I think with this implementation the Clone and separate getter are no longer required, i.e.

#[pyclass]
pub struct Node {
    pub type_: String,

	#[pyo3(get)]
    pub children: Vec<Py<Node>>,
}

#[pymethods]
impl Node {
    fn add_child(&mut self, type_: String, py: Python<'_>) {
        let child = Self {
            type_,
            children: Vec::new(),
        };

        self.children.push(Py::new(py, child).unwrap());
    }
}

[chrisjsewell]

@adamreichold another quick question 😅

how can you return a reference of self from a method?

#[pyclass]
pub struct MyClass {}

#[pymethods]
impl MyClass {
	fn change_state(&mut self) -> ?? {
        // change state ...
        return self
    }
}

e.g. to chain method calls together on the python side

my_class = MyClass().change_state().change_state()

[adamreichold]

Instead of &self, take slf: Py<Self> as the receiver and return Py<Self> as well. (Use slf.borrow(py) to get access to &self in that mode.)

[chrisjsewell]

thats the one, thanks again!