Can't get async iterator to work

[aspcompiler]

Bug Description

I am running 0.17.1. I have a very simple async iterator:

#[pyclass]
struct PyAsyncIter {
    count: usize,
}

#[pymethods]
impl PyAsyncIter {
    #[new]
    pub fn new() -> Self {
        PyAsyncIter { count: 0 }
    }

    fn __aiter__(slf: PyRef<'_, Self>) -> PyRef<'_, Self> {
        slf
    }

    fn __anext__(&mut self) -> Option<usize> {
        if self.count < 5 {
            self.count += 1;
            Some(self.count)
        } else {
            None
        }
    }
}

#[pymodule]
fn pyo3_test(_py: Python, m: &PyModule) -> PyResult<()> {
    m.add_class::<PyAsyncIter>()?;
    Ok(())
}

I got "TypeError: 'async for' received an invalid object from anext: int" when I run the following Python program:

from pyo3_test import PyAsyncIter
import asyncio

async def task():
  async for i in PyAsyncIter():
    print(i)

asyncio.run(task())

Steps to Reproduce

See the Rust and Python programs in description

Backtrace

TypeError: object int can't be used in 'await' expression

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python@3.10/3.10.12/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/usr/local/Cellar/python@3.10/3.10.12/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
    return future.result()
  File "<stdin>", line 2, in task
TypeError: 'async for' received an invalid object from __anext__: int

Your operating system and version

macOS Darwin 22.5.0

Your Python version (python --version)

Python 3.10.12

Your Rust version (rustc --version)

1.70.0

Your PyO3 version

0.17.1

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

brew

Yes, I used virtualenv.

Additional Info

No response

[davidhewitt]

__anext__ should return a python awaitable. Here you have implemented a sync iterable, consider switching to use __iter__ and __next__.

[ShaneMurphy2]

What I didn't get was that __anext__ needs to return an option. If you forget that the error message is particularly incoherent.

[davidhewitt]

@ShaneMurphy2 it's worth pointing out that PyO3 0.21 will not require an option. However, we are busy finishing off #3684 before 0.21 will be ready for release.

[ShaneMurphy2]

@davidhewitt Hm, any interesting reason that the Rust signature had to be different than the Python one in 0.20?

[davidhewitt]

Historical errors in design. We learn! 😂

For some reading on the discussion where we eventually got it right:

#3190
#3661
https://pyo3.rs/main/migration#iteranextoutput-are-deprecated

[ShaneMurphy2]

Saw some of that, neat stuff. Love the crate, excited for 0.21!