Python thread local variables are reset in callbacks from a rust thread

Bug Description

If a python function is called from a thread that was spawned by rust, python thread local variables are not persistent across calls.

Steps to Reproduce

The following module defines a python extension function that spawns a thread and repeatetly calls a python callback:

use pyo3::prelude::*;
use std::thread;

#[pyfunction]
fn call_callback(py: Python, callback: Py<PyAny>) -> PyResult<()> {
    py.detach(move || {
        let handle = thread::spawn(move || {
            for _ in 0..10 {
                Python::attach(|py| callback.call0(py)).unwrap();
            }
        });

        handle.join().unwrap();
    });
    Ok(())
}

#[pymodule]
fn pyo3_bug(m: &Bound<'_, PyModule>) -> PyResult<()> {
    m.add_function(wrap_pyfunction!(call_callback, m)?)?;
    Ok(())
}

In a corresponding python callback function we modify a thread-local variable:

import os
import threading

import pyo3_bug

tls = threading.local()


def func():
    thread_name = threading.current_thread().name
    thread_id = threading.get_ident()
    print(f"Called from PID {os.getpid()}, thread {thread_name} with ident {thread_id}")
    print(f"Value of thread local storage: {getattr(tls, 'seen', False)}")
    tls.seen = True


print("=== Calling from rust")
pyo3_bug.call_callback(func)


def call_from_py():
    for _ in range(10):
        func()


print("=== Calling from python")
thread = threading.Thread(target=call_from_py)
thread.start()

The output is

=== Calling from rust
Called from PID 649992, thread Dummy-1 with ident 140075138545344
Value of thread local storage: False
Called from PID 649992, thread Dummy-1 with ident 140075138545344
Value of thread local storage: False
Called from PID 649992, thread Dummy-1 with ident 140075138545344
Value of thread local storage: False
...
=== Calling from python
Called from PID 649992, thread Thread-2 (call_from_py) with ident 140075136444096
Value of thread local storage: False
Called from PID 649992, thread Thread-2 (call_from_py) with ident 140075136444096
Value of thread local storage: True
Called from PID 649992, thread Thread-2 (call_from_py) with ident 140075136444096
Value of thread local storage: True
...

If the callback is called from python, the thread-local variable correctly retains its state. If called from rust, the thread-local variables is reset every time.

Your operating system and version

arch linux

Your Python version (python --version)

3.12.9

Your Rust version (rustc --version)

1.90.0

Your PyO3 version

0.26.0

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

virtualenv with uv

Additional Info

No response

Start with the Rust extension example and the repeated Python callback using threading.local; inspect how Python::attach handles callbacks from the spawned Rust thread. The work is done when the callback retains its thread-local value across repeated calls, matching the behavior of the Python-created thread.