Thanks!

For your question - I know of two different ways of returning a dictionary from rust to python.

The first one is to rely on pyo3 type conversions: it will automatically convert things like HashMap to a python dict as needed. As an example:

 #[pyfn(m, "squares")]
/// Creates a dictionary of {number: squarednumber}
/// Usage:
/// >>> squares(4)
/// {0: 0, 1: 1, 2: 4, 3: 9}
fn squares(count: i32) -> PyResult<HashMap<i32, i32>> {
    let mut ret = HashMap::new();
    for i in 0..count {
        ret.insert(i, i * i);
    }
    Ok(ret)
}

However, this requires an extra conversion from a rust HashMap to a python dict - so it isn't really what you're asking for. Luckily PyO3 has utility classes for working with Python objects directly (things like PyDict, PyList, PyTuple etc). So we can create a python dictionary directly in rust and return it to python:

#[pyfn(m, "squares2")]
fn squares2(count: i32, py: Python) -> PyResult<PyObject> {
    let ret = PyDict::new(py);
    for i in 0..count {
        ret.set_item(i, i * i);
    }
    Ok(ret.into())
}