Running in virtualenvs on Windows

[davidhewitt]

See also #1835

If I create a new virtualenv on Windows, install tomli into it, and run the following program:

use pyo3::prelude::*;

fn main() -> PyResult<()>{
    Python::with_gil(|py| {
        let m = PyModule::import(py, "tomli")?;
        Ok(())
    })
}

Then this fails to import tomli, despite building correctly with the virtualenv. On linux (at least Ubuntu 20.04) the equivalent works correctly.

I suspect we could investigate how pyo3::prepare_freethreaded_python sets up the interpreter - presumably on Windows it fails to detect the virtualenv, but on Linux it's fine.

[clouds56]

I suspect we could investigate how pyo3::prepare_freethreaded_python sets up the interpreter - presumably on Windows it fails to detect the virtualenv, but on Linux it's fine.

after some investigation: running this code in both linux and windows

#[test]
pub fn test_py() {
  pyo3::prepare_freethreaded_python();
  pyo3::Python::with_gil(|py| {
    let sys = py.import("sys").unwrap();
    let version: String = sys.getattr("version").unwrap().extract().unwrap();
    println!("Python version: {}", version);

    let prefix: String = sys.getattr("prefix").unwrap().extract().unwrap();
    println!("Python prefix: {}", prefix);

    let executable: String = sys.getattr("executable").unwrap().extract().unwrap();
    println!("Python executable: {}", executable);

    let python_path = sys.getattr("path").unwrap();
    let python_path: Vec<String> = python_path.extract().unwrap();
    println!("Python path: {:?}", python_path);

    let os = py.import("os").unwrap();
    let path: String = os.getattr("getcwd").unwrap().call0().unwrap().extract().unwrap();
    println!("Current directory: {}", path);
  });
}

I found that on linux the executable resolves to "python",

Python version: 3.13.2 (main, Mar 13 2025, 14:29:07) [GCC 14.2.0]
Python prefix: /home/clouds/Projects/Hello/pyo3/.venv
Python executable: /home/clouds/Projects/Hello/pyo3/.venv/bin/python3
Python path: ["/usr/lib/python313.zip", "/usr/lib/python3.13", "/usr/lib/python3.13/lib-dynload", "/home/clouds/Projects/Hello/pyo3/.venv/lib/python3.13/site-packages"]
Current directory: /home/clouds/Projects/Hello/pyo3

while on windows the executable resolves to "test.exe"

Python version: 3.13.2 (tags/v3.13.2:4f8bb39, Feb  4 2025, 15:23:48) [MSC v.1942 64 bit (AMD64)]
Python prefix: D:\Hello\pyo3
Python executable: D:\Hello\pyo3\target\debug\deps\pyo3-c13b9e0a445d0fc6.exe
Python path: ["D:\\Hello\\pyo3\\.venv\\Scripts\\python313.zip", "C:\\Users\\clouds\\AppData\\Local\\Programs\\Python\\Python313\\Lib", "C:\\Users\\clouds\\AppData\\Local\\Programs\\Python\\Python313\\DLLs", "D:\\Hello\\pyo3", "D:\\Hello\\pyo3\\Lib", "D:\\Hello\\pyo3\\target\\debug\\deps"]
Current directory: D:\Hello\pyo3

So they resolve to different prefix.

Change initial code to things like this would work

    let output = Command::new("python").env("PYTHONIOENCODING", "utf-8").arg("-c").arg("import sys; print(sys.executable)").output()?;
    let executable = String::from_utf8(output.stdout).unwrap();
    let mut config = unsafe { std::mem::zeroed() };
    unsafe {
      ffi::PyConfig_InitIsolatedConfig(&mut config);
      ffi::PyConfig_SetString(&mut config, &mut config.executable, executable.to_wchar().as_mut_ptr());
      ffi::Py_InitializeFromConfig(&config)
    }