Function signatures

The #[pyfunction] attribute also accepts parameters to control how the generated Python function accepts arguments. Just like in Python, arguments can be positional-only, keyword-only, or accept either. *args lists and **kwargs dicts can also be accepted. These parameters also work for #[pymethods] which will be introduced in the Python Classes section of the guide.

Like Python, by default PyO3 accepts all arguments as either positional or keyword arguments. The extra arguments to #[pyfunction] modify this behaviour. For example, below is a function that accepts arbitrary keyword arguments (**kwargs in Python syntax) and returns the number that was passed:

fn main() {
use pyo3::prelude::*;
use pyo3::types::PyDict;

fn num_kwds(kwds: Option<&PyDict>) -> usize {
    kwds.map_or(0, |dict| dict.len())

fn module_with_functions(py: Python<'_>, m: &PyModule) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(num_kwds, m)?).unwrap();

The following parameters can be passed to the #[pyfunction] attribute:

  • "/": positional-only arguments separator, each parameter defined before "/" is a positional-only parameter. Corresponds to python's def meth(arg1, arg2, ..., /, argN..).
  • "*": var arguments separator, each parameter defined after "*" is a keyword-only parameter. Corresponds to python's def meth(*, arg1.., arg2=..).
  • args="*": "args" is var args, corresponds to Python's def meth(*args). Type of the args parameter has to be &PyTuple.
  • kwargs="**": "kwargs" receives keyword arguments, corresponds to Python's def meth(**kwargs). The type of the kwargs parameter has to be Option<&PyDict>.
  • arg="Value": arguments with default value. Corresponds to Python's def meth(arg=Value). If the arg argument is defined after var arguments, it is treated as a keyword-only argument. Note that Value has to be valid rust code, PyO3 just inserts it into the generated code unmodified.


fn main() {
use pyo3::prelude::*;
use pyo3::types::{PyDict, PyTuple};

struct MyClass {
    num: i32,
impl MyClass {
    #[args(num = "-1")]
    fn new(num: i32) -> Self {
        MyClass { num }

        num = "10",
        py_args = "*",
        name = "\"Hello\"",
        py_kwargs = "**"
    fn method(
        &mut self,
        num: i32,
        name: &str,
        py_args: &PyTuple,
        py_kwargs: Option<&PyDict>,
    ) -> PyResult<String> {
        self.num = num;
            "py_args={:?}, py_kwargs={:?}, name={}, num={}",
            py_args, py_kwargs, name, self.num

    fn make_change(&mut self, num: i32) -> PyResult<String> {
        self.num = num;
        Ok(format!("num={}", self.num))

N.B. the position of the "/" and "*" arguments (if included) control the system of handling positional and keyword arguments. In Python:

import mymodule

mc = mymodule.MyClass()
print(mc.method(44, False, "World", 666, x=44, y=55))
print(mc.method(num=-1, name="World"))
print(mc.make_change(44, False))

Produces output:

py_args=('World', 666), py_kwargs=Some({'x': 44, 'y': 55}), name=Hello, num=44
py_args=(), py_kwargs=None, name=World, num=-1