Pyo3 supports two ways to define a function in python. Both require registering the function to a module

One way is defining the function in the module definition.

#![feature(proc_macro)]

extern crate pyo3;
use pyo3::prelude::*;


#[pymodinit]
fn rust2py(py: Python, m: &PyModule) -> PyResult<()> {

    // Note that the `#[pyfn()]` annotation automatically converts the arguments from
    // Python objects to Rust values; and the Rust return value back into a Python object.
    #[pyfn(m, "sum_as_string")]
    fn sum_as_string_py(_py: Python, a:i64, b:i64) -> PyResult<String> {
       Ok(format!("{}", a + b).to_string())
    }

    Ok(())
}

# fn main() {}

The other is annotating a function with #[py::function] and then adding it to the module using the add_function_to_module! macro, which takes the module as first parameter, the function name as second and an instance of Python as third.

#![feature(specialization, use_extern_macros)]

#[macro_use]
extern crate pyo3;
use pyo3::prelude::*;

#[pyfunction]
fn double(x: usize) -> usize {
    x * 2
}

#[pymodinit]
fn module_with_functions(py: Python, m: &PyModule) -> PyResult<()> {
    m.add_function(wrap_function!(double)).unwrap();

    Ok(())
}

# fn main() {}