py_any.downcast::<PyIterator>() behaviour is different on windows vs macos and linux

[samuelcolvin]

Bug Description

This is an extension of #2902

On Linux and MacOS, py_any.downcast::<PyIterator>() is only Ok for generators.

On windows it appears to be Ok for strings and anything which is a sequence.

Steps to Reproduce

Should be as simple as

let s = "foobar".to_object().into_ref(py);
let iter: PyIterator = s.downcast()?;

Should fail on linux and macos, but pass on windows.

Backtrace

No response

Your operating system and version

Ubuntu

Your Python version (python --version)

3.10

Your Rust version (rustc --version)

cargo 1.68.0-nightly (2381cbdb4 2022-12-23)

Your PyO3 version

v0.18

How did you install python? Did you use a virtualenv?

pyenv

Additional Info

No response

[davidhewitt]

So I've just run the following test locally on PyO3 main, both Windows and Unix, Python 3.10 and 3.11:

#[test]
fn test_iterator_downcast() {
    Python::with_gil(|py| {
        let s = "foobar".to_object(py).into_ref(py);
        let iter: &pyo3::types::PyIterator = s.downcast().unwrap();
    })
}

In all cases, I get the expected failure:

---- test_iterator_downcast stdout ----
thread 'test_iterator_downcast' panicked at 'called `Result::unwrap()` on an `Err` value: PyDowncastError { from: 'foobar', to: "Iterator" }', tests/test_downcast.rs:7:59
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

So I don't think this is a bug in PyO3. Instead let's go back to the discussion and get some more information about how you're setting up your test environment, so we can understand what's different about your Windows setup.

[davidhewitt]

Actually, with your example Python class in #2902 I can repro this, and will push a fix shortly.

[samuelcolvin]

My mistake, it was a subclass of str, sorry created this in a hurry.