Can not link to virtual environment on OS X

🐛 Bug Reports

I can not link to the correct virtual environment in OS X. Instead, the system Python is always used.

🌍 Environment

• Your operating system and version: OS X Big Sur 11.5
• Your python version: 3.8.11
• How did you install python (e.g. apt or pyenv)? Did you use a virtualenv?: Python was installed with homebrew via brew install python@3.8 A virtual environment was created with python3 -m venv redvox and is located at /Users/anthony/venvs/redvox
• Your Rust version (rustc --version): rustc 1.56.0-nightly (9c25eb7aa 2021-07-25)
• Your PyO3 version: 14.1
• Have you tried using latest PyO3 main (replace version = "0.x.y" with git = "https://github.com/PyO3/pyo3")?:

No

💥 Reproducing

1. Activate the virtual environment source /Users/anthony/venvs/redvox/bin/activate
2. Ensure matplotlib is installed pip install matplotlib
3. Run the rust code cargo run

use pyo3::prelude::*;
use std::io::Result;

fn main() -> Result<()> {
    let src = r#"
import sys
print(sys.executable, sys.path)
import matplotlib
            "#;

    Python::with_gil(|py| {
        py.run(src, None, None).unwrap();
    });

    Ok(())
}

The following shows how I activate the virtual environment and attempt to call the rust code. The error message I receive is also included.

anthony@REDVOX-00-MACBOOK-PRO rust-sandbox % source ~/venvs/redvox/bin/activate
(redvox) anthony@REDVOX-00-MACBOOK-PRO rust-sandbox % python3
Python 3.8.11 (default, Jun 29 2021, 03:08:07) 
[Clang 12.0.5 (clang-1205.0.22.9)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import matplotlib
>>> exit()
(redvox) anthony@REDVOX-00-MACBOOK-PRO rust-sandbox % cargo run 
   Compiling pyo3 v0.14.1
   Compiling rust-sandbox v0.1.0 (/Users/anthony/Development/rust-sandbox)
    Finished dev [unoptimized + debuginfo] target(s) in 4.63s
     Running `target/debug/rust-sandbox`
/Users/anthony/Development/rust-sandbox/target/debug/rust-sandbox ['/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python38.zip', '/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8', '/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/lib-dynload', '/usr/local/opt/python@3.8/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages']
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: PyErr { type: <class 'ModuleNotFoundError'>, value: ModuleNotFoundError("No module named 'matplotlib'"), traceback: Some(<traceback object at 0x100ddbbc0>) }', src/main.rs:12:33
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I've tried setting PYTHONHOME and PYTHONPATH. Neither of these have worked for me. The system Python is always used no matter what I try. How can I get pyo3 to work with my virtual environment on OS X?

Start with the reproducer in src/main.rs and run it with the stated macOS, Python, Rust, and PyO3 versions. Trace how Python::with_gil initializes the interpreter and selects its environment. Done means the embedded interpreter uses the activated virtual environment and the matplotlib import succeeds; verify with cargo run.