Pass a mutable reference

[therealevahill]

Hi,

at first, thank your great work. It is really a pleasure to work with PyO3.

I have one question, I did not find an answer for it.
How can I pass a mutalbe reference u16 from a python function call into PyO3 rust?

#[pyfunction]
fn foo(value: &mut u16) {
    *value = 42;
}

value = 0
foo(value)
print(value)  # would expect to print 42 here.

I tried lots of stuff with PyCell and Bound, but did not get it working.

[alex]

You can't. Integers are immutable in Python, and Python does not support passing reference-to-variables in the way your example shows.

[therealevahill]

Is it then possible with a struct?

[alex]

Yes, it's possible to receive a mutable reference to a pyclass struct: https://pyo3.rs/v0.21.2/class#bound-and-interior-mutability

[therealevahill]

You said, that it is not possible in the way the example shows. What are the other possibilities?
With ctypes it works like this, if calling a dll/so.

pointer_to_u16 = ctypes.pointer(ctypes.c_uint16(0))
foo(pointer_to_u16)
content = pointer_to_u16.contents.value

[therealevahill]

Yes, it's possible to receive a mutable reference to a pyclass struct: https://pyo3.rs/v0.21.2/class#bound-and-interior-mutability

Doing like this works: Thank you very much @alex

use pyo3::prelude::*;

#[pyclass]
struct MyClass {
    pub inner: i32,
}

#[pymethods]
impl MyClass {
    #[new]
    fn new(value: i32) -> Self {
        MyClass { inner: value }
    }

    fn show(&self) {
        println!("MyClass: {}", self.inner);
    }
}


#[pyfunction]
fn adapt(x: &mut MyClass) {
    x.inner = 42;
}

#[pymodule]
fn foo_rs(m: &Bound<'_, PyModule>) -> PyResult<()> {
    m.add_class::<MyClass>()?;
    m.add_function(wrap_pyfunction!(adapt, m)?)?;
    Ok(())
}

from foo_rs import adapt, MyClass

c = MyClass(37)
c.show()
adapt(c)
c.show()

[therealevahill]

@alex Could you be so kind, and tell me, what the other possibilies would look like?

You said, that it is not possible in the way the example shows. What are the other possibilities?
With ctypes it works like this, if calling a dll/so.

pointer_to_u16 = ctypes.pointer(ctypes.c_uint16(0))
foo(pointer_to_u16)
content = pointer_to_u16.contents.value