How to pass a callback function from Rust to Python with pyo3

[vrrashkov]

Here is an example what I would like to achieve, of course this is just a psudo code that doesn't work.

Python class with a function that should be called from rust with a callback parameter.

class Test:
    
    def test_fn(callback):
        a = 1
        print(f"value a from python {a}")
        callback(a)
        b = 2
        print(f"value b from python {b}")
        callback(b)

Rust that just loads a class and calls a method and passing our callback function.

fn my_callback(value: u32) {
    println!("value from rust: {}", value);
}

pub fn cllback_test() -> PyResult<()> {
    let py_test_path = include_str!(concat!(
        env!("CARGO_MANIFEST_DIR"),
        "/test.py"
    ));
    Python::with_gil(|py| -> PyResult<()> {
        let test_class = PyModule::from_code(py, py_test_path, "", "")?
            .getattr("Test")?;

            test_class.call_method1("test_fn", (my_callback))?;
        Ok(())
    })?;

    Ok(())
}

The main Idea is that I want to be able to emit/callback some values at some point from python, I don't want to return them at the end of the function but use callback so I can emit multiple values at different times. Maybe this is not the correct approach but I wanted to give an example to what I would like to achieve.

Also I am new to pyo3 so currently I am not using maturin, just the pyo3 library and if possible would like to stick as similar as the way I am showing if possible, if it is not would appreciate any tips.

[mejrs]

you can make a closure with https://docs.rs/pyo3/latest/pyo3/types/struct.PyCFunction.html#method.new_closure or create a pyclass that implements __call__.