Python Functions

PyO3 supports two ways to define a free function in Python. Both require registering the function to a module.

One way is annotating a function with #[pyfunction] and then adding it to the module using the wrap_pyfunction! macro.

use pyo3::prelude::*;

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

#[pymodule]
fn module_with_functions(py: Python, m: &PyModule) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(double, m)?)?;
    Ok(())
}

fn main() {}

Alternatively there is a shorthand; the function can be placed inside the module definition and annotated with #[pyfn], as below:

use pyo3::prelude::*;

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

    #[pyfn(m)]
    fn sum_as_string(_py: Python, a:i64, b:i64) -> PyResult<String> {
        Ok(format!("{}", a + b))
    }

    Ok(())
}

fn main() {}

#[pyfn(m)] is just syntax sugar for #[pyfunction], and takes all the same options documented in the rest of this chapter. The code above is expanded to the following:

use pyo3::prelude::*;

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

    #[pyfunction]
    fn sum_as_string(_py: Python, a:i64, b:i64) -> PyResult<String> {
        Ok(format!("{}", a + b))
    }

    m.add_function(wrap_pyfunction!(sum_as_string, m)?)?;

    Ok(())
}

fn main() {}

Function options

The #[pyo3] attribute can be used to modify properties of the generated Python function. It can take any combination of the following options:

#[pyo3(name = "...")]

Overrides the name generated in Python:


#![allow(unused)]
fn main() {
use pyo3::prelude::*;

#[pyfunction]
#[pyo3(name = "no_args")]
fn no_args_py() -> usize { 42 }

#[pymodule]
fn module_with_functions(py: Python, m: &PyModule) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(no_args_py, m)?)?;
    Ok(())
}

Python::with_gil(|py| {
    let m = pyo3::wrap_pymodule!(module_with_functions)(py);
    assert!(m.getattr(py, "no_args").is_ok());
    assert!(m.getattr(py, "no_args_py").is_err());
});
}

The #[pyfunction] attribute supports specifying details of argument parsing. The details are given in the section "Method arguments" of the Classes chapter. Here is an example for a function that accepts arbitrary keyword arguments (**kwargs in Python syntax) and returns the number that was passed:

use pyo3::prelude::*;
use pyo3::types::PyDict;

#[pyfunction(kwds="**")]
fn num_kwds(kwds: Option<&PyDict>) -> usize {
    kwds.map_or(0, |dict| dict.len())
}

#[pymodule]
fn module_with_functions(py: Python, m: &PyModule) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(num_kwds, m)?).unwrap();
    Ok(())
}

fn main() {}

Making the function signature available to Python

In order to make the function signature available to Python to be retrieved via inspect.signature, use the #[pyo3(text_signature)] annotation as in the example below. The / signifies the end of positional-only arguments. (This is not a feature of this library in particular, but the general format used by CPython for annotating signatures of built-in functions.)


#![allow(unused)]
fn main() {
use pyo3::prelude::*;

/// This function adds two unsigned 64-bit integers.
#[pyfunction]
#[pyo3(text_signature = "(a, b, /)")]
fn add(a: u64, b: u64) -> u64 {
    a + b
}
}

This also works for classes and methods:


#![allow(unused)]
fn main() {
use pyo3::prelude::*;
use pyo3::types::PyType;

// it works even if the item is not documented:

#[pyclass]
#[pyo3(text_signature = "(c, d, /)")]
struct MyClass {}

#[pymethods]
impl MyClass {
    // the signature for the constructor is attached
    // to the struct definition instead.
    #[new]
    fn new(c: i32, d: &str) -> Self {
        Self {}
    }
    // the self argument should be written $self
    #[pyo3(text_signature = "($self, e, f)")]
    fn my_method(&self, e: i32, f: i32) -> i32 {
        e + f
    }
    #[classmethod]
    #[pyo3(text_signature = "(cls, e, f)")]
    fn my_class_method(cls: &PyType, e: i32, f: i32) -> i32 {
        e + f
    }
    #[staticmethod]
    #[pyo3(text_signature = "(e, f)")]
    fn my_static_method(e: i32, f: i32) -> i32 {
        e + f
    }
}
}

Note that text_signature on classes is not compatible with compilation in abi3 mode until Python 3.10 or greater.

Making the function signature available to Python (old method)

Alternatively, simply make sure the first line of your docstring is formatted like in the following example. Please note that the newline after the -- is mandatory. The / signifies the end of positional-only arguments.

#[pyo3(text_signature)] should be preferred, since it will override automatically generated signatures when those are added in a future version of PyO3.


#![allow(unused)]
fn main() {
use pyo3::prelude::*;

/// add(a, b, /)
/// --
///
/// This function adds two unsigned 64-bit integers.
#[pyfunction]
fn add(a: u64, b: u64) -> u64 {
    a + b
}

// a function with a signature but without docs. Both blank lines after the `--` are mandatory.

/// sub(a, b, /)
/// --
///
///
#[pyfunction]
fn sub(a: u64, b: u64) -> u64 {
    a - b
}
}

When annotated like this, signatures are also correctly displayed in IPython.

>>> pyo3_test.add?
Signature: pyo3_test.add(a, b, /)
Docstring: This function adds two unsigned 64-bit integers.
Type:      builtin_function_or_method

Closures

Currently, there are no conversions between Fns in Rust and callables in Python. This would definitely be possible and very useful, so contributions are welcome. In the meantime, you can do the following:

Calling Python functions in Rust

You can pass Python def'd functions and built-in functions to Rust functions PyFunction corresponds to regular Python functions while PyCFunction describes built-ins such as repr().

You can also use PyAny::is_callable to check if you have a callable object. is_callable will return true for functions (including lambdas), methods and objects with a __call__ method. You can call the object with PyAny::call with the args as first parameter and the kwargs (or None) as second parameter. There are also PyAny::call0 with no args and PyAny::call1 with only positional args.

Calling Rust functions in Python

If you have a static function, you can expose it with #[pyfunction] and use wrap_pyfunction! to get the corresponding PyCFunction. For dynamic functions, e.g. lambdas and functions that were passed as arguments, you must put them in some kind of owned container, e.g. a Box. (A long-term solution will be a special container similar to wasm-bindgen's Closure). You can then use a #[pyclass] struct with that container as a field as a way to pass the function over the FFI barrier. You can even make that class callable with __call__ so it looks like a function in Python code.

Accessing the module of a function

It is possible to access the module of a #[pyfunction] in the function body by using #[pyo3(pass_module)] option:

use pyo3::prelude::*;

#[pyfunction]
#[pyo3(pass_module)]
fn pyfunction_with_module(module: &PyModule) -> PyResult<&str> {
    module.name()
}

#[pymodule]
fn module_with_fn(py: Python, m: &PyModule) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(pyfunction_with_module, m)?)
}

fn main() {}

If pass_module is set, the first argument must be the &PyModule. It is then possible to use the module in the function body.

Accessing the FFI functions

In order to make Rust functions callable from Python, PyO3 generates an extern "C" function whose exact signature depends on the Rust signature. (PyO3 chooses the optimal Python argument passing convention.) It then embeds the call to the Rust function inside this FFI-wrapper function. This wrapper handles extraction of the regular arguments and the keyword arguments from the input PyObjects.

The wrap_pyfunction macro can be used to directly get a PyCFunction given a #[pyfunction] and a PyModule: wrap_pyfunction!(rust_fun, module).

PyO3 user guide