PyString, &str and Python::allow_threads

[LukeMathWalker]

I'm adding a new concurrency chapter to our Rust-Python interoperability workshop and I ended up writing something equivalent to what follows:

use pyo3::{prelude::*, types::PyString};

#[pyfunction]
fn word_count(text: Bound<'_, PyString>) -> PyResult<()> {
    let py = text.py();

    // Get a Rust view (&str) over the Python string
    let text = text.to_str()?;

    py.allow_threads(|| {
        println!("Look ma, a GIL-related reference: {text}");
    });

    Ok(())
}

I want to check my understanding of what's happening (and if that's correct).
text.to_str() gives us a view over data that's still owned by the Python runtime, and thus GIL-constrained. Intuitively, that should translate into the view type being Ungil.
At the same time, though, strings are immutable in Python (are they?). Therefore, it's fine to read the string data as long as we are sure it hasn't been garbage collected.
Since the closure in py.allow_threads executes to completion before word_count returns, we know for sure that the reference counter for the string behind text is at least one, therefore it won't be garbage collected and we can sleep safe and sound.

Does that check out?

[ngoldbaum]

At the same time, though, strings are immutable in Python (are they?).

There are ways to modify strings via the C API. So much code assumes that strings are immutable after being created though that any C code actually mutating a string is certainly thread-unsafe in a way that should be considered a bug IMO.

Since the closure in py.allow_threads executes to completion before word_count returns, we know for sure that the reference counter for the string behind text is at least one, therefore it won't be garbage collected and we can sleep safe and sound.

Yup, that's exactly right. And you know that the reference count has to be at least one because of the Bound smart pointer.

[alex]

Python strings are immutable. It's UB to use the C APIs to mutate any string besides one you just allocated and have not given to other code.