Python Functions

The #[pyfunction] attribute is used to define a Python function from a Rust function. Once defined, the function needs to be added to a module using the wrap_pyfunction! macro.

The following example defines a function called double in a Python module called my_extension:


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

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

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

This chapter of the guide explains full usage of the #[pyfunction] attribute. The following topics are covered:

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 exposed to Python.

    In the following example, the Rust function no_args_py will be added to the Python module module_with_functions as the Python function no_args:

    
#![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());
});
}

  • #[pyo3(text_signature = "...")]

    Sets the function signature visible in Python tooling (such as via inspect.signature).

    The example below creates a function add which has a signature describing two positional-only arguments a and b.

    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
}

fn main() -> PyResult<()> {
    Python::with_gil(|py| {
        let fun = pyo3::wrap_pyfunction!(add, py)?;

        let doc: String = fun.getattr("__doc__")?.extract()?;
        assert_eq!(doc, "This function adds two unsigned 64-bit integers.");

        let inspect = PyModule::import(py, "inspect")?.getattr("signature")?;
        let sig: String = inspect
            .call1((fun,))?
            .call_method0("__str__")?
            .extract()?;
        assert_eq!(sig, "(a, b, /)");

        Ok(())
    })
}

  • #[pyo3(pass_module)]

    Set this option to make PyO3 pass the containing module as the first argument to the function. It is then possible to use the module in the function body. The first argument must be of type &PyModule.

    The following example creates a function pyfunction_with_module which returns the containing module's name (i.e. module_with_fn):

    
#![allow(unused)]
fn main() {
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)?)
}
}

Argument parsing

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:


#![allow(unused)]
fn main() {
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(())
}
}

Per-argument options

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

  • #[pyo3(from_py_with = "...")]

    Set this on an option to specify a custom function to convert the function argument from Python to the desired Rust type, instead of using the default FromPyObject extraction. The function signature must be fn(&PyAny) -> PyResult<T> where T is the Rust type of the argument.

    The following example uses from_py_with to convert the input Python object to its length:

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

fn get_length(obj: &PyAny) -> PyResult<usize> {
    let length = obj.len()?;
    Ok(length)
}

#[pyfunction]
fn object_length(
    #[pyo3(from_py_with = "get_length")] argument: usize
) -> usize {
    argument
}

Python::with_gil(|py| {
    let f = pyo3::wrap_pyfunction!(object_length)(py).unwrap();
    assert_eq!(f.call1((vec![1, 2, 3],)).unwrap().extract::<usize>().unwrap(), 3);
});
}

Advanced function patterns

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() {
#![allow(dead_code)]
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 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).

#[pyfn] shorthand

There is a shorthand to #[pyfunction] and wrap_pymodule!: the function can be placed inside the module definition and annotated with #[pyfn]. To simplify PyO3, it is expected that #[pyfn] may be removed in a future release (See #694).

An example of #[pyfn] is below:


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

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

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

    Ok(())
}
}

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


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

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

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

    m.add_function(wrap_pyfunction!(double, m)?)?;
    Ok(())
}
}