I am using PyO3 (v0.22.2) / Rust to write something a little similar to defaultdict, but with an integer default rather than a callback:

>>> from my_package.pyo3_ext import PyDefaultDict

>>> d = PyDefaultDict(42, a=1, b=2)

>>> d['a']
1

>>> d['z']
42

I am trying to subclass the built-in Python dict with extends=PyDict, as per examples:

use pyo3::prelude::*;
use pyo3::types::PyDict;

#[pyclass(extends=PyDict)]
pub struct PyDefaultDict {
    x: i32,
}

#[pymethods]
impl PyDefaultDict {
    #[new]
    #[pyo3(signature = (x, *_args, **_kwargs))]
    fn new(x: i32, _args: &Bound<'_, PyAny>, _kwargs: Option<&Bound<'_, PyAny>>) -> Self {
        Self { x }
    }

    fn get_x(&self) -> i32 {
        self.x
    }
}

This compiles, but when I try to call the constructor in Python, I get the following error:

>>> x = PyDefaultDict(42)

TypeError: 'int' object is not iterable

I am referring to the PyO3 user guide::Inheritance section. This actually has an example of ensuring that _args and _kwargs are passed through to the base class, but it does not demonstrate how to provide a custom __init__() parameter via new().

Note that even if I remove x from my struct, so that all I’m doing is accepting an i32 parameter in new(), and not trying to assign it to anything, I still get the same error.

I also tried accepting a PyAny and then extracting to an i32:

    fn new(x: &Bound<'_, PyAny>, _args: &Bound<'_, PyAny>, _kwargs: Option<&Bound<'_, PyAny>>) -> Self {
        Self { _x: x.extract().unwrap() }
    }

Alas, this gives me the same error.

What’s the little piece of knowledge I am missing here, to help solve this?

Note, the base example, without the new parameter x, works just like a normal dict, so I’m basing my work on this:

#[pyclass(extends=PyDict)]
pub struct PyDefaultDict {}

#[pymethods]
impl PyDefaultDict {
    #[new]
    #[pyo3(signature = (*_args, **_kwargs))]
    fn new(_args: &Bound<'_, PyAny>, _kwargs: Option<&Bound<'_, PyAny>>) -> Self {
        Self {}
    }
}