Class customizations

Python's object model defines several protocols for different object behavior, like sequence, mapping or number protocols. PyO3 defines separate traits for each of them. To provide specific Python object behavior, you need to implement the specific trait for your struct. Important note, each protocol implementation block has to be annotated with the #[pyproto] attribute.

All #[pyproto] methods which can be defined below can return T instead of PyResult<T> if the method implementation is infallible. In addition, if the return type is (), it can be omitted altogether.

Basic object customization

The PyObjectProtocol trait provides several basic customizations.

Attribute access

To customize object attribute access, define the following methods:

  • fn __getattr__(&self, name: FromPyObject) -> PyResult<impl IntoPy<PyObject>>
  • fn __setattr__(&mut self, name: FromPyObject, value: FromPyObject) -> PyResult<()>
  • fn __delattr__(&mut self, name: FromPyObject) -> PyResult<()>

Each method corresponds to Python's self.attr, self.attr = value and del self.attr code.

String Conversions

  • fn __repr__(&self) -> PyResult<impl ToPyObject<ObjectType=PyString>>

  • fn __str__(&self) -> PyResult<impl ToPyObject<ObjectType=PyString>>

    Possible return types for __str__ and __repr__ are PyResult<String> or PyResult<PyString>.

  • fn __bytes__(&self) -> PyResult<PyBytes>

    Provides the conversion to bytes.

  • fn __format__(&self, format_spec: &str) -> PyResult<impl ToPyObject<ObjectType=PyString>>

    Special method that is used by the format() builtin and the str.format() method. Possible return types are PyResult<String> or PyResult<PyString>.

Comparison operators

  • fn __richcmp__(&self, other: impl FromPyObject, op: CompareOp) -> PyResult<impl ToPyObject>

    Overloads Python comparison operations (==, !=, <, <=, >, and >=). The op argument indicates the comparison operation being performed. The return type will normally be PyResult<bool>, but any Python object can be returned. If other is not of the type specified in the signature, the generated code will automatically return NotImplemented.

  • fn __hash__(&self) -> PyResult<impl PrimInt>

    Objects that compare equal must have the same hash value. The return type must be PyResult<T> where T is one of Rust's primitive integer types.

Other methods

  • fn __bool__(&self) -> PyResult<bool>

    Determines the "truthyness" of the object.

Emulating numeric types

The PyNumberProtocol trait allows emulate numeric types.

  • fn __add__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __sub__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __mul__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __matmul__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __truediv__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __floordiv__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __mod__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __divmod__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __pow__(lhs: impl FromPyObject, rhs: impl FromPyObject, modulo: Option<impl FromPyObject>) -> PyResult<impl ToPyObject>
  • fn __lshift__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rshift__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __and__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __or__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __xor__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>

These methods are called to implement the binary arithmetic operations (+, -, *, @, /, //, %, divmod(), pow() and **, <<, >>, &, ^, and |).

If rhs is not of the type specified in the signature, the generated code will automatically return NotImplemented. This is not the case for lhs which must match signature or else raise a TypeError.

The reflected operations are also available:

  • fn __radd__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rsub__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rmul__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rmatmul__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rtruediv__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rfloordiv__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rmod__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rdivmod__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rpow__(lhs: impl FromPyObject, rhs: impl FromPyObject, modulo: Option<impl FromPyObject>) -> PyResult<impl ToPyObject>
  • fn __rlshift__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rrshift__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rand__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __ror__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>
  • fn __rxor__(lhs: impl FromPyObject, rhs: impl FromPyObject) -> PyResult<impl ToPyObject>

The code generated for these methods expect that all arguments match the signature, or raise a TypeError.

Note: Currently implementing the method for a binary arithmetic operations (e.g, __add__) shadows the reflected operation (e.g, __radd__). This is being addressed in #844. to make these methods

This trait also has support the augmented arithmetic assignments (+=, -=, *=, @=, /=, //=, %=, **=, <<=, >>=, &=, ^=, |=):

  • fn __iadd__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __isub__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __imul__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __imatmul__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __itruediv__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __ifloordiv__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __imod__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __ipow__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __ilshift__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __irshift__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __iand__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __ior__(&'p mut self, other: impl FromPyObject) -> PyResult<()>
  • fn __ixor__(&'p mut self, other: impl FromPyObject) -> PyResult<()>

The following methods implement the unary arithmetic operations (-, +, abs() and ~):

  • fn __neg__(&'p self) -> PyResult<impl ToPyObject>
  • fn __pos__(&'p self) -> PyResult<impl ToPyObject>
  • fn __abs__(&'p self) -> PyResult<impl ToPyObject>
  • fn __invert__(&'p self) -> PyResult<impl ToPyObject>

Support for coercions:

  • fn __complex__(&'p self) -> PyResult<impl ToPyObject>
  • fn __int__(&'p self) -> PyResult<impl ToPyObject>
  • fn __float__(&'p self) -> PyResult<impl ToPyObject>

Other:

  • fn __index__(&'p self) -> PyResult<impl ToPyObject>
  • fn __round__(&'p self, ndigits: Option<impl FromPyObject>) -> PyResult<impl ToPyObject>

Garbage Collector Integration

If your type owns references to other Python objects, you will need to integrate with Python's garbage collector so that the GC is aware of those references. To do this, implement the PyGCProtocol trait for your struct. It includes two methods __traverse__ and __clear__. These correspond to the slots tp_traverse and tp_clear in the Python C API. __traverse__ must call visit.call() for each reference to another Python object. __clear__ must clear out any mutable references to other Python objects (thus breaking reference cycles). Immutable references do not have to be cleared, as every cycle must contain at least one mutable reference. Example:


#![allow(unused)]
fn main() {
extern crate pyo3;

use pyo3::prelude::*;
use pyo3::PyTraverseError;
use pyo3::gc::{PyGCProtocol, PyVisit};

#[pyclass]
struct ClassWithGCSupport {
    obj: Option<PyObject>,
}

#[pyproto]
impl PyGCProtocol for ClassWithGCSupport {
    fn __traverse__(&self, visit: PyVisit) -> Result<(), PyTraverseError> {
        if let Some(obj) = &self.obj {
            visit.call(obj)?
        }
        Ok(())
    }

    fn __clear__(&mut self) {
        // Clear reference, this decrements ref counter.
        self.obj = None;
    }
}
}

Special protocol trait implementations have to be annotated with the #[pyproto] attribute.

It is also possible to enable GC for custom classes using the gc parameter of the pyclass attribute. i.e. #[pyclass(gc)]. In that case instances of custom class participate in Python garbage collection, and it is possible to track them with gc module methods. When using the gc parameter, it is required to implement the PyGCProtocol trait, failure to do so will result in an error at compile time:

#[pyclass(gc)]
struct GCTracked {} // Fails because it does not implement PyGCProtocol

Iterator Types

Iterators can be defined using the PyIterProtocol trait. It includes two methods __iter__ and __next__:

  • fn __iter__(slf: PyRefMut<Self>) -> PyResult<impl IntoPy<PyObject>>
  • fn __next__(slf: PyRefMut<Self>) -> PyResult<Option<impl IntoPy<PyObject>>>

Returning None from __next__ indicates that that there are no further items. These two methods can be take either PyRef<Self> or PyRefMut<Self> as their first argument, so that mutable borrow can be avoided if needed.

Example:


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

#[pyclass]
struct MyIterator {
    iter: Box<Iterator<Item = PyObject> + Send>,
}

#[pyproto]
impl PyIterProtocol for MyIterator {
    fn __iter__(slf: PyRef<Self>) -> PyRef<Self> {
        slf
    }
    fn __next__(mut slf: PyRefMut<Self>) -> Option<PyObject> {
        slf.iter.next()
    }
}
}

In many cases you'll have a distinction between the type being iterated over (i.e. the iterable) and the iterator it provides. In this case, you should implement PyIterProtocol for both the iterable and the iterator, but the iterable only needs to support __iter__() while the iterator must support both __iter__() and __next__(). The default implementations in PyIterProtocol will ensure that the objects behave correctly in Python. For example:


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

#[pyclass]
struct Iter {
    inner: std::vec::IntoIter<usize>,
}

#[pyproto]
impl PyIterProtocol for Iter {
    fn __iter__(slf: PyRef<Self>) -> PyRef<Self> {
        slf
    }

    fn __next__(mut slf: PyRefMut<Self>) -> Option<usize> {
        slf.inner.next()
    }
}

#[pyclass]
struct Container {
    iter: Vec<usize>,
}

#[pyproto]
impl PyIterProtocol for Container {
    fn __iter__(slf: PyRef<Self>) -> PyResult<Py<Iter>> {
        let iter = Iter {
            inner: slf.iter.clone().into_iter(),
        };
        Py::new(slf.py(), iter)
    }
}

let gil = Python::acquire_gil();
let py = gil.python();
let inst = pyo3::PyCell::new(
    py,
    Container {
        iter: vec![1, 2, 3, 4],
    },
)
.unwrap();
pyo3::py_run!(py, inst, "assert list(inst) == [1, 2, 3, 4]");
pyo3::py_run!(py, inst, "assert list(iter(iter(inst))) == [1, 2, 3, 4]");
}

For more details on Python's iteration protocols, check out the "Iterator Types" section of the library documentation.

Returning a value from iteration

This guide has so far shown how to use Option<T> to implement yielding values during iteration. In Python a generator can also return a value. To express this in Rust, PyO3 provides the IterNextOutput enum to both Yield values and Return a final value - see its docs for further details and an example.