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.

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

#[pyo3::pymodule]
mod my_extension {
    use pyo3::prelude::*;

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

This chapter of the guide explains full usage of the #[pyfunction] attribute. In this first section, the following topics are covered:

Function options
Per-argument options
Advanced function patterns

There are also additional sections on the following topics:

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:

use pyo3::prelude::*;
#[pyo3::pymodule]
mod module_with_functions {
    use pyo3::prelude::*;

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

Python::attach(|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(signature = (...))]

Defines the function signature in Python. See Function Signatures.
#[pyo3(text_signature = "...")]

Overrides the PyO3-generated function signature visible in Python tooling (such as via inspect.signature). See the corresponding topic in the Function Signatures subchapter.
#[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 &Bound<'_, PyModule>, Bound<'_, PyModule>, or Py<PyModule>.

The following example creates a function pyfunction_with_module which returns the containing module's name (i.e. module_with_fn):
```
#[pyo3::pymodule]
mod module_with_fn {
    use pyo3::prelude::*;
    use pyo3::types::PyString;

    #[pyfunction]
    #[pyo3(pass_module)]
    fn pyfunction_with_module<'py>(
        module: &Bound<'py, PyModule>,
    ) -> PyResult<Bound<'py, PyString>> {
        module.name()
    }
}
```

#[pyo3(warn(message = "...", category = ...))]

This option is used to display a warning when the function is used in Python. It is equivalent to warnings.warn(message, category). The message parameter is a string that will be displayed when the function is called, and the category parameter is optional and has to be a subclass of Warning. When the category parameter is not provided, the warning will be defaulted to UserWarning.

Note: when used with #[pymethods], this attribute does not work with #[classattr] nor __traverse__ magic method.

The following are examples of using the #[pyo3(warn)] attribute:

use pyo3::prelude::*;

#[pymodule]
mod raising_warning_fn {
    use pyo3::prelude::pyfunction;
    use pyo3::exceptions::PyFutureWarning;

    #[pyfunction]
    #[pyo3(warn(message = "This is a warning message"))]
    fn function_with_warning() -> usize {
        42
    }

    #[pyfunction]
    #[pyo3(warn(message = "This function is warning with FutureWarning", category = PyFutureWarning))]
    fn function_with_warning_and_custom_category() -> usize {
        42
    }
}

use pyo3::exceptions::{PyFutureWarning, PyUserWarning};
use pyo3::types::{IntoPyDict, PyList};
use pyo3::PyTypeInfo;

fn catch_warning(py: Python<'_>, f: impl FnOnce(&Bound<'_, PyList>) -> ()) -> PyResult<()> {
    let warnings = py.import("warnings")?;
    let kwargs = [("record", true)].into_py_dict(py)?;
    let catch_warnings = warnings
        .getattr("catch_warnings")?
        .call((), Some(&kwargs))?;
    let list = catch_warnings.call_method0("__enter__")?.cast_into()?;
    warnings.getattr("simplefilter")?.call1(("always",))?;  // show all warnings
    f(&list);
    catch_warnings
        .call_method1("__exit__", (py.None(), py.None(), py.None()))
        .unwrap();
    Ok(())
}

macro_rules! assert_warnings {
    ($py:expr, $body:expr, [$(($category:ty, $message:literal)),+] $(,)? ) => {
        catch_warning($py, |list| {
            $body;
            let expected_warnings = [$((<$category as PyTypeInfo>::type_object($py), $message)),+];
            assert_eq!(list.len(), expected_warnings.len());
            for (warning, (category, message)) in list.iter().zip(expected_warnings) {
                assert!(warning.getattr("category").unwrap().is(&category));
                assert_eq!(
                    warning.getattr("message").unwrap().str().unwrap().to_string_lossy(),
                    message
                );
            }
        }).unwrap();
    };
}

Python::attach(|py| {
    assert_warnings!(
        py,
        {
            let m = pyo3::wrap_pymodule!(raising_warning_fn)(py);
            let f1 = m.getattr(py, "function_with_warning").unwrap();
            let f2 = m.getattr(py, "function_with_warning_and_custom_category").unwrap();
            f1.call0(py).unwrap();
            f2.call0(py).unwrap();
        },
        [
            (PyUserWarning, "This is a warning message"),
            (
                PyFutureWarning,
                "This function is warning with FutureWarning"
            )
        ]
    );
});

When the functions are called as the following, warnings will be displayed.

import warnings
from raising_warning_fn import function_with_warning, function_with_warning_and_custom_category

function_with_warning()
function_with_warning_and_custom_category()

The warning output will be:

UserWarning: This is a warning message
FutureWarning: This function is warning with FutureWarning

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(&Bound<'_, 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:

use pyo3::prelude::*;

fn get_length(obj: &Bound<'_, PyAny>) -> PyResult<usize> {
    obj.len()
}

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

Python::attach(|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

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 Bound<'_, 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 Bound<'_, PyAny>::call with the args as first parameter and the kwargs (or None) as second parameter. There are also Bound<'_, PyAny>::call0 with no args and Bound<'_, PyAny>::call1 with only positional args.

Calling Rust functions in Python

The ways to convert a Rust function into a Python object vary depending on the function:

Named functions, e.g. fn foo(): add #[pyfunction] and then use wrap_pyfunction! to get the corresponding PyCFunction.
Anonymous functions (or closures), e.g. foo: fn() either:
- use a #[pyclass] struct which stores the function as a field and implement __call__ to call the stored function.
- use PyCFunction::new_closure to create an object directly from the function.

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 Bound<PyCFunction> given a #[pyfunction] and a Bound<PyModule>: wrap_pyfunction!(rust_fun, module).

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