Constructors

In the previous section (and its exercise) we relied on a #[pyfunction] as the constructor for the #[pyclass] we defined. Without new_wallet, we wouldn’t have been able to create new Wallet instances from Python.
Let’s now explore how to define a constructor directly within the #[pyclass] itself.

Defining a constructor

You can add a constructor to your #[pyclass] using the #[new] attribute on a method. Here’s an example:

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

#[pyclass]
struct Wallet {
    #[pyo3(get, set)]
    balance: i32,
}

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

A Rust method annotated with #[new] is equivalent to the __new__ method in Python. At the moment there is no way to define the __init__ method in Rust.
The impl block containing the constructor must also be annotated with the #[pymethods] attribute for #[new] to work as expected.

Signature

Everything we learned about arguments in the context of #[pyfunction]s applies to constructors as well.
In terms of output type, you can return Self if the constructor is infallible, or PyResult<Self> if it can fail.

Exercise

The exercise for this section is located in 02_classes/01_constructors