PyO3 user guide

Class customizations

PyO3 uses the #[pyproto] attribute in combination with special traits to implement certain protocol (aka __dunder__) methods of Python classes. The special traits are listed in this chapter of the guide. See also the documentation for the pyo3::class module.

Python's object model defines several protocols for different object behavior, such as the sequence, mapping, and number protocols. You may be familiar with implementing these protocols in Python classes by "dunder" methods, such as __str__ or __repr__.

In the Python C-API which PyO3 is dependent upon, many of these protocol methods have to be provided into special "slots" on the class type object. To fill these slots PyO3 uses the #[pyproto] attribute in combination with special traits.

All #[pyproto] methods 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.

There are many "dunder" methods which are not included in any of PyO3's protocol traits, such as __dir__. These methods can be implemented in #[pymethods] as already covered in the previous section.

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

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 can be implemented to 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.

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

Other:

• fn __index__(&'p self) -> PyResult<impl ToPyObject>

Emulating sequential containers (such as lists or tuples)

The PySequenceProtocol trait can be implemented to emulate sequential container types.

For a sequence, the keys are the integers k for which 0 <= k < N, where N is the length of the sequence.

• fn __len__(&self) -> PyResult<usize>

  Implements the built-in function len() for the sequence.

• fn __getitem__(&self, idx: isize) -> PyResult<impl ToPyObject>

  Implements evaluation of the self[idx] element. If the idx value is outside the set of indexes for the sequence, IndexError should be raised.

  Note: Negative integer indexes are handled as follows: if __len__() is defined, it is called and the sequence length is used to compute a positive index, which is passed to __getitem__(). If __len__() is not defined, the index is passed as is to the function.

• fn __setitem__(&mut self, idx: isize, value: impl FromPyObject) -> PyResult<()>

  Implements assignment to the self[idx] element. Same note as for __getitem__(). Should only be implemented if sequence elements can be replaced.

• fn __delitem__(&mut self, idx: isize) -> PyResult<()>

  Implements deletion of the self[idx] element. Same note as for __getitem__(). Should only be implemented if sequence elements can be deleted.

• fn __contains__(&self, item: impl FromPyObject) -> PyResult<bool>

  Implements membership test operators. Should return true if item is in self, false otherwise. For objects that don’t define __contains__(), the membership test simply traverses the sequence until it finds a match.

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

  Concatenates two sequences. Used by the + operator, after trying the numeric addition via the PyNumberProtocol trait method.

• fn __repeat__(&self, count: isize) -> PyResult<impl ToPyObject>

  Repeats the sequence count times. Used by the * operator, after trying the numeric multiplication via the PyNumberProtocol trait method.

• fn __inplace_concat__(&mut self, other: impl FromPyObject) -> PyResult<Self>

  Concatenates two sequences in place. Returns the modified first operand. Used by the += operator, after trying the numeric in place addition via the PyNumberProtocol trait method.

• fn __inplace_repeat__(&mut self, count: isize) -> PyResult<Self>

  Repeats the sequence count times in place. Returns the modified first operand. Used by the *= operator, after trying the numeric in place multiplication via the PyNumberProtocol trait method.

Emulating mapping containers (such as dictionaries)

The PyMappingProtocol trait allows to emulate mapping container types.

For a mapping, the keys may be Python objects of arbitrary type.

• fn __len__(&self) -> PyResult<usize>

  Implements the built-in function len() for the mapping.

• fn __getitem__(&self, key: impl FromPyObject) -> PyResult<impl ToPyObject>

  Implements evaluation of the self[key] element. If key is of an inappropriate type, TypeError may be raised; if key is missing (not in the container), KeyError should be raised.

• fn __setitem__(&mut self, key: impl FromPyObject, value: impl FromPyObject) -> PyResult<()>

  Implements assignment to the self[key] element or insertion of a new key mapping to value. Should only be implemented if the mapping support changes to the values for keys, or if new keys can be added. The same exceptions should be raised for improper key values as for the __getitem__() method.

• fn __delitem__(&mut self, key: impl FromPyObject) -> PyResult<()>

  Implements deletion of the self[key] element. Should only be implemented if the mapping supports removal of keys. The same exceptions should be raised for improper key values as for the __getitem__() method.

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() {
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)
    }
}

Python::with_gil(|py| {
    let container = Container { iter: vec![1, 2, 3, 4] };
    let inst = pyo3::PyCell::new(py, container).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.