Python cannot change values in an internal/nested struct

[sdfgeoff]

Bug Description

Using the #[pyo3(get, set)] only works if python directly owns the class, but not if the struct is contained inside another rust/py03 object.

The same issue is present if using a function (in the inner class) to get/set the variable. I suspect that python is cloning the inner object and throwing it away.

Steps to Reproduce

Consider the Rust structs:

#[pyclass]
struct PainterCore {
    #[pyo3(get, set)]
    pub test1: f32,
    #[pyo3(get, set)]
    pub inner: InnerStruct,
}

#[pymethods]
impl PainterCore {
    #[new]
    pub fn new() -> PyResult<Self> {
        Ok(Self {
            test1: 0.0,
            inner: InnerStruct{
                test2: 0.0
            }
        })
    }
}

#[pyclass]
#[derive(Clone)]
struct InnerStruct {
    #[pyo3(get, set)]
    test2: f32,
}

And consider the python:

        self.core = painter_core.PainterCore()

        print(self.core.test1, self.core.inner.test2)
        self.core.test1 = 5.0
        self.core.inner.test2 = 5.0
        print(self.core.test1, self.core.inner.test2)

The expected output from the first print is: 0.0 0.0 because variables test1 and test2 have been initialised to zero. The expected output from the second print is 5.0 5.0 because both variables have been set to 5.0

The actual output of the second print is 5.0 0.0

Backtrace

No response

Your operating system and version

Archlinux

Your Python version (python --version)

3.9.7

Your Rust version (rustc --version)

rustc 1.55.0 (c8dfcfe04 2021-09-06)

Your PyO3 version

0.14.5

How did you install python? Did you use a virtualenv?

sudo pacman -S python

# Makefile
VENV_START = ${PYTHON} -m venv .env; source .env/bin/activate;

setup:
	${VENV_START} \
	pip install -r requirements.txt; \

# requirements.txt
maturin

Additional Info

No response

[birkenfeld]

The attribute access self.core.inner returns a copy of the object; any changes you make will not be reflected there.

If you need to return a reference instead, the struct member has to be Py<InnerStruct>.

[mejrs]

See also https://pyo3.rs/v0.15.0/faq.html#pyo3get-clones-my-field

[Trolldemorted]

I have just run into this and it was quite unexpected. Shouldn't this be mentioned in the documentation to classes?

This is kind of a footgun, and neither the class documentation nor the conversion table mention it.