Module 03 — First PyO3 Binding

Goal

Expose a pure Rust function to Python so that:

from open_engineering_kernel import normalize_identifier
assert normalize_identifier("  OE.PICO.LAMP  ") == "oe.pico.lamp"

Conceptual flow

Python
   │  function call
   ▼
PyO3
   │
   ▼
Rust

The binding (simplified)

use pyo3::prelude::*;
use pyo3::types::PyModule;
use pyo3::PyResult;

#[pyfunction]
fn normalize_identifier(value: &str) -> String {
    value.trim().to_lowercase()
}

#[pymodule]
fn open_engineering_kernel(m: &Bound<'_, PyModule>) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(normalize_identifier, m)?)?;
    Ok(())
}

(The actual course kernel uses the current stable PyO3 API; see kernel/src/lib.rs.)

What you should understand

The domain function can be tested in Rust before any Python binding exists.
PyO3 is the bridge, not the place where domain rules live.
From the Python side the result looks like an ordinary function.

Exercise

Locate normalize_identifier in the kernel.
Call it from a small Python script after maturin develop.
Confirm the Rust unit test for the same logic still passes with cargo test.