PyO3 user guide

Conversion traits

PyO3 provides some handy traits to convert between Python types and Rust types.

.extract() and the FromPyObject trait

The easiest way to convert a Python object to a Rust value is using .extract(). It returns a PyResult with a type error if the conversion fails, so usually you will use something like

use pyo3::prelude::*;
use pyo3::types::PyList;
fn main() -> PyResult<()> {
    Python::with_gil(|py| {
        let list = PyList::new(py, b"foo")?;
let v: Vec<i32> = list.extract()?;
        assert_eq!(&v, &[102, 111, 111]);
        Ok(())
    })
}

This method is available for many Python object types, and can produce a wide variety of Rust types, which you can check out in the implementor list of FromPyObject.

FromPyObject is also implemented for your own Rust types wrapped as Python objects (see the chapter about classes). There, in order to both be able to operate on mutable references and satisfy Rust's rules of non-aliasing mutable references, you have to extract the PyO3 reference wrappers PyRef and PyRefMut. They work like the reference wrappers of std::cell::RefCell and ensure (at runtime) that Rust borrows are allowed.

Deriving FromPyObject

FromPyObject can be automatically derived for many kinds of structs and enums if the member types themselves implement FromPyObject. This even includes members with a generic type T: FromPyObject. Derivation for empty enums, enum variants and structs is not supported.

Deriving FromPyObject for structs

The derivation generates code that will attempt to access the attribute my_string on the Python object, i.e. obj.getattr("my_string"), and call extract() on the attribute.

use pyo3::prelude::*;
use pyo3_ffi::c_str;

#[derive(FromPyObject)]
struct RustyStruct {
    my_string: String,
}

fn main() -> PyResult<()> {
    Python::with_gil(|py| -> PyResult<()> {
        let module = PyModule::from_code(
            py,
            c_str!("class Foo:
            def __init__(self):
                self.my_string = 'test'"),
            c_str!(""),
            c_str!(""),
        )?;

        let class = module.getattr("Foo")?;
        let instance = class.call0()?;
        let rustystruct: RustyStruct = instance.extract()?;
        assert_eq!(rustystruct.my_string, "test");
        Ok(())
    })
}

By setting the #[pyo3(item)] attribute on the field, PyO3 will attempt to extract the value by calling the get_item method on the Python object.

use pyo3::prelude::*;

#[derive(FromPyObject)]
struct RustyStruct {
    #[pyo3(item)]
    my_string: String,
}

use pyo3::types::PyDict;
fn main() -> PyResult<()> {
    Python::with_gil(|py| -> PyResult<()> {
        let dict = PyDict::new(py);
        dict.set_item("my_string", "test")?;

        let rustystruct: RustyStruct = dict.extract()?;
        assert_eq!(rustystruct.my_string, "test");
        Ok(())
    })
}

The argument passed to getattr and get_item can also be configured:

use pyo3::prelude::*;
use pyo3_ffi::c_str;

#[derive(FromPyObject)]
struct RustyStruct {
    #[pyo3(item("key"))]
    string_in_mapping: String,
    #[pyo3(attribute("name"))]
    string_attr: String,
}

fn main() -> PyResult<()> {
    Python::with_gil(|py| -> PyResult<()> {
        let module = PyModule::from_code(
            py,
            c_str!("class Foo(dict):
            def __init__(self):
                self.name = 'test'
                self['key'] = 'test2'"),
            c_str!(""),
            c_str!(""),
        )?;

        let class = module.getattr("Foo")?;
        let instance = class.call0()?;
        let rustystruct: RustyStruct = instance.extract()?;
		assert_eq!(rustystruct.string_attr, "test");
        assert_eq!(rustystruct.string_in_mapping, "test2");

        Ok(())
    })
}

This tries to extract string_attr from the attribute name and string_in_mapping from a mapping with the key "key". The arguments for attribute are restricted to non-empty string literals while item can take any valid literal that implements ToBorrowedObject.

You can use #[pyo3(from_item_all)] on a struct to extract every field with get_item method. In this case, you can't use #[pyo3(attribute)] or barely use #[pyo3(item)] on any field. However, using #[pyo3(item("key"))] to specify the key for a field is still allowed.

use pyo3::prelude::*;

#[derive(FromPyObject)]
#[pyo3(from_item_all)]
struct RustyStruct {
    foo: String,
    bar: String,
    #[pyo3(item("foobar"))]
    baz: String,
}

fn main() -> PyResult<()> {
    Python::with_gil(|py| -> PyResult<()> {
        let py_dict = py.eval(pyo3::ffi::c_str!("{'foo': 'foo', 'bar': 'bar', 'foobar': 'foobar'}"), None, None)?;
        let rustystruct: RustyStruct = py_dict.extract()?;
		  assert_eq!(rustystruct.foo, "foo");
        assert_eq!(rustystruct.bar, "bar");
        assert_eq!(rustystruct.baz, "foobar");

        Ok(())
    })
}

Deriving FromPyObject for tuple structs

Tuple structs are also supported but do not allow customizing the extraction. The input is always assumed to be a Python tuple with the same length as the Rust type, the nth field is extracted from the nth item in the Python tuple.

use pyo3::prelude::*;

#[derive(FromPyObject)]
struct RustyTuple(String, String);

use pyo3::types::PyTuple;
fn main() -> PyResult<()> {
    Python::with_gil(|py| -> PyResult<()> {
        let tuple = PyTuple::new(py, vec!["test", "test2"])?;

        let rustytuple: RustyTuple = tuple.extract()?;
        assert_eq!(rustytuple.0, "test");
        assert_eq!(rustytuple.1, "test2");

        Ok(())
    })
}

Tuple structs with a single field are treated as wrapper types which are described in the following section. To override this behaviour and ensure that the input is in fact a tuple, specify the struct as

use pyo3::prelude::*;

#[derive(FromPyObject)]
struct RustyTuple((String,));

use pyo3::types::PyTuple;
fn main() -> PyResult<()> {
    Python::with_gil(|py| -> PyResult<()> {
        let tuple = PyTuple::new(py, vec!["test"])?;

        let rustytuple: RustyTuple = tuple.extract()?;
        assert_eq!((rustytuple.0).0, "test");

        Ok(())
    })
}

Deriving FromPyObject for wrapper types

The pyo3(transparent) attribute can be used on structs with exactly one field. This results in extracting directly from the input object, i.e. obj.extract(), rather than trying to access an item or attribute. This behaviour is enabled per default for newtype structs and tuple-variants with a single field.

use pyo3::prelude::*;

#[derive(FromPyObject)]
struct RustyTransparentTupleStruct(String);

#[derive(FromPyObject)]
#[pyo3(transparent)]
struct RustyTransparentStruct {
    inner: String,
}

use pyo3::types::PyString;
fn main() -> PyResult<()> {
    Python::with_gil(|py| -> PyResult<()> {
        let s = PyString::new(py, "test");

        let tup: RustyTransparentTupleStruct = s.extract()?;
        assert_eq!(tup.0, "test");

        let stru: RustyTransparentStruct = s.extract()?;
        assert_eq!(stru.inner, "test");

        Ok(())
    })
}

Deriving FromPyObject for enums

The FromPyObject derivation for enums generates code that tries to extract the variants in the order of the fields. As soon as a variant can be extracted successfully, that variant is returned. This makes it possible to extract Python union types like str | int.

The same customizations and restrictions described for struct derivations apply to enum variants, i.e. a tuple variant assumes that the input is a Python tuple, and a struct variant defaults to extracting fields as attributes but can be configured in the same manner. The transparent attribute can be applied to single-field-variants.

use pyo3::prelude::*;
use pyo3_ffi::c_str;

#[derive(FromPyObject)]
#[derive(Debug)]
enum RustyEnum<'py> {
    Int(usize),                    // input is a positive int
    String(String),                // input is a string
    IntTuple(usize, usize),        // input is a 2-tuple with positive ints
    StringIntTuple(String, usize), // input is a 2-tuple with String and int
    Coordinates3d {
        // needs to be in front of 2d
        x: usize,
        y: usize,
        z: usize,
    },
    Coordinates2d {
        // only gets checked if the input did not have `z`
        #[pyo3(attribute("x"))]
        a: usize,
        #[pyo3(attribute("y"))]
        b: usize,
    },
    #[pyo3(transparent)]
    CatchAll(Bound<'py, PyAny>), // This extraction never fails
}

use pyo3::types::{PyBytes, PyString};
fn main() -> PyResult<()> {
    Python::with_gil(|py| -> PyResult<()> {
        {
            let thing = 42_u8.into_pyobject(py)?;
            let rust_thing: RustyEnum<'_> = thing.extract()?;

            assert_eq!(
                42,
                match rust_thing {
                    RustyEnum::Int(i) => i,
                    other => unreachable!("Error extracting: {:?}", other),
                }
            );
        }
        {
            let thing = PyString::new(py, "text");
            let rust_thing: RustyEnum<'_> = thing.extract()?;

            assert_eq!(
                "text",
                match rust_thing {
                    RustyEnum::String(i) => i,
                    other => unreachable!("Error extracting: {:?}", other),
                }
            );
        }
        {
            let thing = (32_u8, 73_u8).into_pyobject(py)?;
            let rust_thing: RustyEnum<'_> = thing.extract()?;

            assert_eq!(
                (32, 73),
                match rust_thing {
                    RustyEnum::IntTuple(i, j) => (i, j),
                    other => unreachable!("Error extracting: {:?}", other),
                }
            );
        }
        {
            let thing = ("foo", 73_u8).into_pyobject(py)?;
            let rust_thing: RustyEnum<'_> = thing.extract()?;

            assert_eq!(
                (String::from("foo"), 73),
                match rust_thing {
                    RustyEnum::StringIntTuple(i, j) => (i, j),
                    other => unreachable!("Error extracting: {:?}", other),
                }
            );
        }
        {
            let module = PyModule::from_code(
                py,
                c_str!("class Foo(dict):
            def __init__(self):
                self.x = 0
                self.y = 1
                self.z = 2"),
                c_str!(""),
                c_str!(""),
            )?;

            let class = module.getattr("Foo")?;
            let instance = class.call0()?;
            let rust_thing: RustyEnum<'_> = instance.extract()?;

            assert_eq!(
                (0, 1, 2),
                match rust_thing {
                    RustyEnum::Coordinates3d { x, y, z } => (x, y, z),
                    other => unreachable!("Error extracting: {:?}", other),
                }
            );
        }

        {
            let module = PyModule::from_code(
                py,
                c_str!("class Foo(dict):
            def __init__(self):
                self.x = 3
                self.y = 4"),
                c_str!(""),
                c_str!(""),
            )?;

            let class = module.getattr("Foo")?;
            let instance = class.call0()?;
            let rust_thing: RustyEnum<'_> = instance.extract()?;

            assert_eq!(
                (3, 4),
                match rust_thing {
                    RustyEnum::Coordinates2d { a, b } => (a, b),
                    other => unreachable!("Error extracting: {:?}", other),
                }
            );
        }

        {
            let thing = PyBytes::new(py, b"text");
            let rust_thing: RustyEnum<'_> = thing.extract()?;

            assert_eq!(
                b"text",
                match rust_thing {
                    RustyEnum::CatchAll(ref i) => i.downcast::<PyBytes>()?.as_bytes(),
                    other => unreachable!("Error extracting: {:?}", other),
                }
            );
        }
        Ok(())
    })
}

If none of the enum variants match, a PyTypeError containing the names of the tested variants is returned. The names reported in the error message can be customized through the #[pyo3(annotation = "name")] attribute, e.g. to use conventional Python type names:

use pyo3::prelude::*;

#[derive(FromPyObject)]
#[derive(Debug)]
enum RustyEnum {
    #[pyo3(transparent, annotation = "str")]
    String(String),
    #[pyo3(transparent, annotation = "int")]
    Int(isize),
}

fn main() -> PyResult<()> {
    Python::with_gil(|py| -> PyResult<()> {
        {
            let thing = 42_u8.into_pyobject(py)?;
            let rust_thing: RustyEnum = thing.extract()?;

            assert_eq!(
                42,
                match rust_thing {
                    RustyEnum::Int(i) => i,
                    other => unreachable!("Error extracting: {:?}", other),
                }
            );
        }

        {
            let thing = "foo".into_pyobject(py)?;
            let rust_thing: RustyEnum = thing.extract()?;

            assert_eq!(
                "foo",
                match rust_thing {
                    RustyEnum::String(i) => i,
                    other => unreachable!("Error extracting: {:?}", other),
                }
            );
        }

        {
            let thing = b"foo".into_pyobject(py)?;
            let error = thing.extract::<RustyEnum>().unwrap_err();
            assert!(error.is_instance_of::<pyo3::exceptions::PyTypeError>(py));
        }

        Ok(())
    })
}

If the input is neither a string nor an integer, the error message will be: "'<INPUT_TYPE>' cannot be converted to 'str | int'".

#[derive(FromPyObject)] Container Attributes

• pyo3(transparent)
  • extract the field directly from the object as obj.extract() instead of get_item() or getattr()
  • Newtype structs and tuple-variants are treated as transparent per default.
  • only supported for single-field structs and enum variants
• pyo3(annotation = "name")
  • changes the name of the failed variant in the generated error message in case of failure.
  • e.g. pyo3("int") reports the variant's type as int.
  • only supported for enum variants

#[derive(FromPyObject)] Field Attributes

• pyo3(attribute), pyo3(attribute("name"))
  • retrieve the field from an attribute, possibly with a custom name specified as an argument
  • argument must be a string-literal.
• pyo3(item), pyo3(item("key"))
  • retrieve the field from a mapping, possibly with the custom key specified as an argument.
  • can be any literal that implements ToBorrowedObject
• pyo3(from_py_with = "...")
  • apply a custom function to convert the field from Python the desired Rust type.
  • the argument must be the name of the function as a string.
  • the function signature must be fn(&Bound<PyAny>) -> PyResult<T> where T is the Rust type of the argument.

IntoPyObject

This trait defines the to-python conversion for a Rust type. All types in PyO3 implement this trait, as does a #[pyclass] which doesn't use extends.

Occasionally you may choose to implement this for custom types which are mapped to Python types without having a unique python type.

derive macro

IntoPyObject can be implemented using our derive macro. Both structs and enums are supported.

structs will turn into a PyDict using the field names as keys, tuple structs will turn convert into PyTuple with the fields in declaration order.

#![allow(dead_code)]
use pyo3::prelude::*;
use std::collections::HashMap;
use std::hash::Hash;

// structs convert into `PyDict` with field names as keys
#[derive(IntoPyObject)]
struct Struct { 
    count: usize,
    obj: Py<PyAny>,
}

// tuple structs convert into `PyTuple`
// lifetimes and generics are supported, the impl will be bounded by
// `K: IntoPyObject, V: IntoPyObject`
#[derive(IntoPyObject)]
struct Tuple<'a, K: Hash + Eq, V>(&'a str, HashMap<K, V>);

For structs with a single field (newtype pattern) the #[pyo3(transparent)] option can be used to forward the implementation to the inner type.

#![allow(dead_code)]
use pyo3::prelude::*;

// newtype tuple structs are implicitly `transparent`
#[derive(IntoPyObject)]
struct TransparentTuple(PyObject); 

#[derive(IntoPyObject)]
#[pyo3(transparent)]
struct TransparentStruct<'py> { 
    inner: Bound<'py, PyAny>, // `'py` lifetime will be used as the Python lifetime
}

For enums each variant is converted according to the rules for structs above.

#![allow(dead_code)]
use pyo3::prelude::*;
use std::collections::HashMap;
use std::hash::Hash;

#[derive(IntoPyObject)]
enum Enum<'a, 'py, K: Hash + Eq, V> { // enums are supported and convert using the same
    TransparentTuple(PyObject),       // rules on the variants as the structs above
    #[pyo3(transparent)]
    TransparentStruct { inner: Bound<'py, PyAny> },
    Tuple(&'a str, HashMap<K, V>),
    Struct { count: usize, obj: Py<PyAny> }
}

Additionally IntoPyObject can be derived for a reference to a struct or enum using the IntoPyObjectRef derive macro. All the same rules from above apply as well.

manual implementation

If the derive macro is not suitable for your use case, IntoPyObject can be implemented manually as demonstrated below.

use pyo3::prelude::*;
#[allow(dead_code)]
struct MyPyObjectWrapper(PyObject);

impl<'py> IntoPyObject<'py> for MyPyObjectWrapper {
    type Target = PyAny; // the Python type
    type Output = Bound<'py, Self::Target>; // in most cases this will be `Bound`
    type Error = std::convert::Infallible; // the conversion error type, has to be convertable to `PyErr`

    fn into_pyobject(self, py: Python<'py>) -> Result<Self::Output, Self::Error> {
        Ok(self.0.into_bound(py))
    }
}

// equivalent to former `ToPyObject` implementations 
impl<'a, 'py> IntoPyObject<'py> for &'a MyPyObjectWrapper {
    type Target = PyAny;
    type Output = Borrowed<'a, 'py, Self::Target>; // `Borrowed` can be used to optimized reference counting
    type Error = std::convert::Infallible;

    fn into_pyobject(self, py: Python<'py>) -> Result<Self::Output, Self::Error> {
        Ok(self.0.bind_borrowed(py))
    }
}

IntoPy<T>

This trait defines the to-python conversion for a Rust type. It is usually implemented as IntoPy<PyObject>, which is the trait needed for returning a value from #[pyfunction] and #[pymethods].

All types in PyO3 implement this trait, as does a #[pyclass] which doesn't use extends.

Occasionally you may choose to implement this for custom types which are mapped to Python types without having a unique python type.

use pyo3::prelude::*;
#[allow(dead_code)]
struct MyPyObjectWrapper(PyObject);

#[allow(deprecated)]
impl IntoPy<PyObject> for MyPyObjectWrapper {
    fn into_py(self, py: Python<'_>) -> PyObject {
        self.0
    }
}

BoundObject for conversions that may be Bound or Borrowed

IntoPyObject::into_py_object returns either Bound or Borrowed depending on the implementation for a concrete type. For example, the IntoPyObject implementation for u32 produces a Bound<'py, PyInt> and the bool implementation produces a Borrowed<'py, 'py, PyBool>:

use pyo3::prelude::*;
use pyo3::IntoPyObject;
use pyo3::types::{PyBool, PyInt};

let ints: Vec<u32> = vec![1, 2, 3, 4];
let bools = vec![true, false, false, true];

Python::with_gil(|py| {
    let ints_as_pyint: Vec<Bound<'_, PyInt>> = ints
        .iter()
        .map(|x| Ok(x.into_pyobject(py)?))
        .collect::<PyResult<_>>()
        .unwrap();

    let bools_as_pybool: Vec<Borrowed<'_, '_, PyBool>> = bools
        .iter()
        .map(|x| Ok(x.into_pyobject(py)?))
        .collect::<PyResult<_>>()
        .unwrap();
});

In this example if we wanted to combine ints_as_pyints and bools_as_pybool into a single Vec<Py<PyAny>> to return from the with_gil closure, we would have to manually convert the concrete types for the smart pointers and the python types.

Instead, we can write a function that generically converts vectors of either integers or bools into a vector of Py<PyAny> using the BoundObject trait:

use pyo3::prelude::*;
use pyo3::BoundObject;
use pyo3::IntoPyObject;

let bools = vec![true, false, false, true];
let ints = vec![1, 2, 3, 4];

fn convert_to_vec_of_pyobj<'py, T>(py: Python<'py>, the_vec: Vec<T>) -> PyResult<Vec<Py<PyAny>>>
where
   T: IntoPyObject<'py> + Copy
{
    the_vec.iter()
        .map(|x| {
            Ok(
                x.into_pyobject(py)
                .map_err(Into::into)?
                .into_any()
                .unbind()
            )
        })
        .collect()
}

let vec_of_pyobjs: Vec<Py<PyAny>> = Python::with_gil(|py| {
    let mut bools_as_pyany = convert_to_vec_of_pyobj(py, bools).unwrap();
    let mut ints_as_pyany = convert_to_vec_of_pyobj(py, ints).unwrap();
    let mut result: Vec<Py<PyAny>> = vec![];
    result.append(&mut bools_as_pyany);
    result.append(&mut ints_as_pyany);
    result
});

In the example above we used BoundObject::into_any and BoundObject::unbind to manipulate the python types and smart pointers into the result type we wanted to produce from the function.

The ToPyObject trait

ToPyObject is a conversion trait that allows various objects to be converted into PyObject. IntoPy<PyObject> serves the same purpose, except that it consumes self.