Basic object customization

Recall the Number class from the previous chapter:

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

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    #[new]
    fn new(value: i32) -> Self {
        Self(value)
    }
}

#[pymodule]
fn my_module(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
    m.add_class::<Number>()?;
    Ok(())
}
}

At this point Python code can import the module, access the class and create class instances - but nothing else.

from my_module import Number

n = Number(5)
print(n)

<builtins.Number object at 0x000002B4D185D7D0>

String representations

It can't even print an user-readable representation of itself! We can fix that by defining the __repr__ and __str__ methods inside a #[pymethods] block. We do this by accessing the value contained inside Number.

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

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    // For `__repr__` we want to return a string that Python code could use to recreate
    // the `Number`, like `Number(5)` for example.
    fn __repr__(&self) -> String {
        // We use the `format!` macro to create a string. Its first argument is a
        // format string, followed by any number of parameters which replace the
        // `{}`'s in the format string.
        //
        //                       👇 Tuple field access in Rust uses a dot
        format!("Number({})", self.0)
    }
    // `__str__` is generally used to create an "informal" representation, so we
    // just forward to `i32`'s `ToString` trait implementation to print a bare number.
    fn __str__(&self) -> String {
        self.0.to_string()
    }
}
}

Accessing the class name

In the __repr__, we used a hard-coded class name. This is sometimes not ideal, because if the class is subclassed in Python, we would like the repr to reflect the subclass name. This is typically done in Python code by accessing self.__class__.__name__. In order to be able to access the Python type information and the Rust struct, we need to use a PyCell as the self argument.

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

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    fn __repr__(slf: &PyCell<Self>) -> PyResult<String> {
        // This is the equivalent of `self.__class__.__name__` in Python.
        let class_name: &str = slf.get_type().name()?;
        // To access fields of the Rust struct, we need to borrow the `PyCell`.
        Ok(format!("{}({})", class_name, slf.borrow().0))
    }
}
}

Hashing

Let's also implement hashing. We'll just hash the i32. For that we need a Hasher. The one provided by std is DefaultHasher, which uses the SipHash algorithm.

#![allow(unused)]
fn main() {
use std::collections::hash_map::DefaultHasher;

// Required to call the `.hash` and `.finish` methods, which are defined on traits.
use std::hash::{Hash, Hasher};

use pyo3::prelude::*;

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    fn __hash__(&self) -> u64 {
        let mut hasher = DefaultHasher::new();
        self.0.hash(&mut hasher);
        hasher.finish()
    }
}
}

Note: When implementing __hash__ and comparisons, it is important that the following property holds:

k1 == k2 -> hash(k1) == hash(k2)

In other words, if two keys are equal, their hashes must also be equal. In addition you must take care that your classes' hash doesn't change during its lifetime. In this tutorial we do that by not letting Python code change our Number class. In other words, it is immutable.

By default, all #[pyclass] types have a default hash implementation from Python. Types which should not be hashable can override this by setting __hash__ to None. This is the same mechanism as for a pure-Python class. This is done like so:

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

#[pymethods]
impl NotHashable {
    #[classattr]
    const __hash__: Option<Py<PyAny>> = None;
}
}

Comparisons

Unlike in Python, PyO3 does not provide the magic comparison methods you might expect like __eq__, __lt__ and so on. Instead you have to implement all six operations at once with __richcmp__. This method will be called with a value of CompareOp depending on the operation.

#![allow(unused)]
fn main() {
use pyo3::class::basic::CompareOp;

use pyo3::prelude::*;

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    fn __richcmp__(&self, other: &Self, op: CompareOp) -> PyResult<bool> {
        match op {
            CompareOp::Lt => Ok(self.0 < other.0),
            CompareOp::Le => Ok(self.0 <= other.0),
            CompareOp::Eq => Ok(self.0 == other.0),
            CompareOp::Ne => Ok(self.0 != other.0),
            CompareOp::Gt => Ok(self.0 > other.0),
            CompareOp::Ge => Ok(self.0 >= other.0),
        }
    }
}
}

If you obtain the result by comparing two Rust values, as in this example, you can take a shortcut using CompareOp::matches:

#![allow(unused)]
fn main() {
use pyo3::class::basic::CompareOp;

use pyo3::prelude::*;

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    fn __richcmp__(&self, other: &Self, op: CompareOp) -> bool {
        op.matches(self.0.cmp(&other.0))
    }
}
}

It checks that the std::cmp::Ordering obtained from Rust's Ord matches the given CompareOp.

Alternatively, if you want to leave some operations unimplemented, you can return py.NotImplemented() for some of the operations:

#![allow(unused)]
fn main() {
use pyo3::class::basic::CompareOp;

use pyo3::prelude::*;

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    fn __richcmp__(&self, other: &Self, op: CompareOp, py: Python<'_>) -> PyObject {
        match op {
            CompareOp::Eq => (self.0 == other.0).into_py(py),
            CompareOp::Ne => (self.0 != other.0).into_py(py),
            _ => py.NotImplemented(),
        }
    }
}
}

Truthyness

We'll consider Number to be True if it is nonzero:

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

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    fn __bool__(&self) -> bool {
        self.0 != 0
    }
}
}

Final code

#![allow(unused)]
fn main() {
use std::collections::hash_map::DefaultHasher;
use std::hash::{Hash, Hasher};

use pyo3::prelude::*;
use pyo3::class::basic::CompareOp;

#[pyclass]
struct Number(i32);

#[pymethods]
impl Number {
    #[new]
    fn new(value: i32) -> Self {
        Self(value)
    }

    fn __repr__(slf: &PyCell<Self>) -> PyResult<String> {
        let class_name: &str = slf.get_type().name()?;
        Ok(format!("{}({})", class_name, slf.borrow().0))
    }

    fn __str__(&self) -> String {
        self.0.to_string()
    }

    fn __hash__(&self) -> u64 {
        let mut hasher = DefaultHasher::new();
        self.0.hash(&mut hasher);
        hasher.finish()
    }

    fn __richcmp__(&self, other: &Self, op: CompareOp) -> PyResult<bool> {
        match op {
            CompareOp::Lt => Ok(self.0 < other.0),
            CompareOp::Le => Ok(self.0 <= other.0),
            CompareOp::Eq => Ok(self.0 == other.0),
            CompareOp::Ne => Ok(self.0 != other.0),
            CompareOp::Gt => Ok(self.0 > other.0),
            CompareOp::Ge => Ok(self.0 >= other.0),
        }
    }

    fn __bool__(&self) -> bool {
        self.0 != 0
    }
}

#[pymodule]
fn my_module(_py: Python<'_>, m: &PyModule) -> PyResult<()> {
    m.add_class::<Number>()?;
    Ok(())
}
}