# Supporting Free-Threaded CPython

CPython 3.14 declared support for the "free-threaded" build of CPython that does not rely on the global interpreter lock (often referred to as the GIL) for thread safety.
Since version 0.23, PyO3 supports building Rust extension modules for the free-threaded Python build and calling into free-threaded Python from Rust.

If you want more background on free-threaded Python in general, see the [what's new](https://docs.python.org/3/whatsnew/3.13.html#whatsnew313-free-threaded-cpython) entry in the 3.13 release notes (when the "free-threaded" build was first added as an experimental mode), the [free-threading HOWTO guide](https://docs.python.org/3/howto/free-threading-extensions.html#freethreading-extensions-howto) in the CPython docs, the [extension module porting guide](https://py-free-threading.github.io/porting-extensions/) in the community-maintained Python free-threading guide, and [PEP 703](https://peps.python.org/pep-0703/), which provides the technical background for the free-threading implementation in CPython.

In the GIL-enabled build (the only choice before the "free-threaded" build was introduced), the global interpreter lock serializes access to the Python runtime.
The GIL is therefore a fundamental limitation to parallel scaling of multithreaded Python workflows, due to [Amdahl's law](https://en.wikipedia.org/wiki/Amdahl%27s_law), because any time spent executing a parallel processing task on only one execution context fundamentally cannot be sped up using parallelism.

The free-threaded build removes this limit on multithreaded Python scaling.
This means it's much more straightforward to achieve parallelism using the Python [`threading`] module.
If you have ever needed to use [`multiprocessing`](https://docs.python.org/3/library/multiprocessing.html) to achieve a parallel speedup for some Python code, free-threading will likely allow the use of Python threads instead for the same workflow.

PyO3's support for free-threaded Python will enable authoring native Python extension modules that are thread-safe by construction, with much stronger safety guarantees than C extension modules.
Our goal is to enable ["fearless concurrency"](https://doc.rust-lang.org/book/ch16-00-concurrency.html) in the native Python runtime by building on the Rust [`Send` and `Sync`](https://doc.rust-lang.org/nomicon/send-and-sync.html) traits.

This document provides advice for porting Rust code using PyO3 to run under free-threaded Python.

## Supporting free-threaded Python with PyO3

Since PyO3 0.28, PyO3 defaults to assuming Python modules created with it are thread-safe.
This will be the case except for Rust code which has used `unsafe` to assume thread-safety incorrectly.
An example of this is `unsafe` code which was written with the historical assumption that Python was single-threaded due to the GIL, and so the `Python<'py>` token used by PyO3 could be used to guarantee thread-safety.
A module can opt-out of supporting free-threaded Python until it has audited its `unsafe` code for correctness by declaring the module with `#[pymodule(gil_used = true)]` (see below).

Complicated `#[pyclass]` types may need to deal with thread-safety directly; there is [a dedicated section of the guide](./class/thread-safety.md) to discuss this.

At a low-level, annotating a module sets the `Py_MOD_GIL` slot on modules defined by an extension to `Py_MOD_GIL_NOT_USED`, which allows the interpreter to see at runtime that the author of the extension module attests it is thread-safe.

By opting-out of supporting free-threaded Python, the Python interpreter will re-enable the GIL at runtime while importing your module and print a `RuntimeWarning` with a message containing the name of the module causing it to re-enable the GIL.
You can force the GIL to remain disabled by setting the `PYTHON_GIL=0` as an environment variable or passing `-Xgil=0` when starting Python (`0` means the GIL is turned off).

If you are sure that all data structures exposed in a `PyModule` are thread-safe, then pass `gil_used = false` as a parameter to the `pymodule` procedural macro declaring the module or call `PyModule::gil_used` on a `PyModule` instance.
For example:

### Example opting-in

(Note: for PyO3 versions 0.23 through 0.27, the default was `gil_used = true` and so the opposite was needed; modules needed to opt-in to free-threaded Python support with `gil_used = false`.)

```rust,no_run
/// This module relies on the GIL for thread safety
#[pyo3::pymodule(gil_used = true)]
mod my_extension {
    use pyo3::prelude::*;

    // this type is not thread-safe
    #[pyclass]
    struct MyNotThreadSafeType {
        // insert not thread-safe code
    }
}
```

Or for a module that is set up without using the `pymodule` macro:

```rust,no_run
use pyo3::prelude::*;

# #[allow(dead_code)]
fn register_child_module(parent_module: &Bound<'_, PyModule>) -> PyResult<()> {
    let child_module = PyModule::new(parent_module.py(), "child_module")?;
    child_module.gil_used(true)?;
    parent_module.add_submodule(&child_module)
}

```

See the [`string-sum`](https://github.com/PyO3/pyo3/tree/main/pyo3-ffi/examples/string-sum) example for how to declare free-threaded support using raw FFI calls for modules using single-phase initialization and the [`sequential`](https://github.com/PyO3/pyo3/tree/main/pyo3-ffi/examples/sequential) example for modules using multi-phase initialization.

If you would like to use conditional compilation to trigger different code paths under the free-threaded build, you can use the `Py_GIL_DISABLED` attribute once you have configured your crate to generate the necessary build configuration data.
See [the guide section](./building-and-distribution/multiple-python-versions.md) for more details about supporting multiple different Python versions, including the free-threaded build.

## Special considerations for the free-threaded build

The free-threaded interpreter does not have a GIL.
Many existing extension modules providing mutable data structures relied on the GIL to lock Python objects and make interior mutability thread-safe.

Calling into the CPython C API is only legal when an OS thread is explicitly attached to the interpreter runtime.
In the GIL-enabled build, this happens when the GIL is acquired.
In the free-threaded build there is no GIL, but the same C macros that release or acquire the GIL in the GIL-enabled build instead ask the interpreter to attach the thread to the Python runtime, and there can be many threads simultaneously attached.
See [PEP 703](https://peps.python.org/pep-0703/#thread-states) for more background about how threads can be attached and detached from the interpreter runtime, in a manner analogous to releasing and acquiring the GIL in the GIL-enabled build.

In the GIL-enabled build, PyO3 uses the [`Python<'py>`] type and the `'py` lifetime to signify that the global interpreter lock is held.
In the freethreaded build, holding a `'py` lifetime means only that the thread is currently attached to the Python interpreter -- other threads can be simultaneously interacting with the interpreter.

### Attaching to the runtime

You still need to obtain a `'py` lifetime to interact with Python objects or call into the CPython C API.
If you are not yet attached to the Python runtime, you can register a thread using the [`Python::attach`] function.
Threads created via the Python [`threading`] module do not need to do this, and pyo3 will handle setting up the [`Python<'py>`] token when CPython calls into your extension module.

### Detaching to avoid hangs and deadlocks

The free-threaded build triggers global synchronization events in the following situations:

- During garbage collection in order to get a globally consistent view of
  reference counts and references between objects
- In Python 3.13, when the first background thread is started in
  order to mark certain objects as immortal
- When either `sys.settrace` or `sys.setprofile` are called in order to
  instrument running code objects and threads
- During a call to `os.fork()`, to ensure a process-wide consistent state.

This is a non-exhaustive list and there may be other situations in future Python versions that can trigger global synchronization events.

This means that you should detach from the interpreter runtime using [`Python::detach`] in exactly the same situations as you should detach from the runtime in the GIL-enabled build: when doing long-running tasks that do not require the CPython runtime or when doing any task that needs to re-attach to the runtime (see the [guide section](parallelism.md#sharing-python-objects-between-rust-threads) that covers this).
In the former case, you would observe a hang on threads that are waiting on the long-running task to complete, and in the latter case you would see a deadlock while a thread tries to attach after the runtime triggers a global synchronization event, but the spawning thread prevents the synchronization event from completing.

### Exceptions and panics for multithreaded access of mutable `pyclass` instances

Data attached to `pyclass` instances is protected from concurrent access by a `RefCell`-like pattern of runtime borrow checking.
Like a `RefCell`, PyO3 will raise exceptions (or in some cases panic) to enforce exclusive access for mutable borrows.
It was always possible to generate panics like this in PyO3 in code that releases the GIL with [`Python::detach`] or calling a python method accepting `&self` from a `&mut self` (see [the docs on interior mutability](./class.md#bound-and-interior-mutability),) but now in free-threaded Python there are more opportunities to trigger these panics from Python because there is no GIL to lock concurrent access to mutably borrowed data from Python.

The most straightforward way to trigger this problem is to use the Python [`threading`] module to simultaneously call a rust function that mutably borrows a [`pyclass`]({{#PYO3_DOCS_URL}}/pyo3/attr.pyclass.html) in multiple threads.
For example, consider the following implementation:

```rust,no_run
# use pyo3::prelude::*;
#[pyclass]
#[derive(Default)]
struct ThreadIter {
    count: usize,
}

#[pymethods]
impl ThreadIter {
    #[new]
    pub fn new() -> Self {
        Default::default()
    }

    fn __next__(&mut self, py: Python<'_>) -> usize {
        self.count += 1;
        self.count
    }
}
```

And then if we do something like this in Python:

```python
import concurrent.futures
from my_module import ThreadIter

i = ThreadIter()

def increment():
    next(i)

with concurrent.futures.ThreadPoolExecutor(max_workers=16) as tpe:
    futures = [tpe.submit(increment) for _ in range(100)]
    [f.result() for f in futures]
```

We will see an exception:

```text
Traceback (most recent call last)
  File "example.py", line 5, in <module>
    next(i)
RuntimeError: Already borrowed
```

We may allow user-selectable semantics for mutable pyclass definitions in a future version of PyO3, allowing some form of opt-in locking to emulate the GIL if that is needed.
For now you should explicitly add locking, possibly using conditional compilation or using the critical section API, to avoid creating deadlocks with the GIL.

### Cannot build extension modules using the limited API

The free-threaded build uses a completely new ABI and there is not yet an equivalent to the limited API for the free-threaded ABI.
That means if your crate depends on PyO3 using the `abi3` feature or an an `abi3-pyxx` feature, PyO3 will print a warning and ignore that setting when building extension modules using the free-threaded interpreter.

This means that if your package makes use of the ABI forward compatibility provided by the limited API to upload only one wheel for each release of your package, you will need to update your release procedure to also upload a version-specific free-threaded wheel.

See [the guide section](./building-and-distribution/multiple-python-versions.md) for more details about supporting multiple different Python versions, including the free-threaded build.

### Thread-safe single initialization

To initialize data exactly once, use the [`PyOnceLock`] type, which is a close equivalent to [`std::sync::OnceLock`][`OnceLock`] that also helps avoid deadlocks by detaching from the Python interpreter when threads are blocking waiting for another thread to complete initialization.
If already using [`OnceLock`] and it is impractical to replace with a [`PyOnceLock`], there is the [`OnceLockExt`] extension trait which adds [`OnceLockExt::get_or_init_py_attached`] to detach from the interpreter when blocking in the same fashion as [`PyOnceLock`].
Here is an example using [`PyOnceLock`] to single-initialize a runtime cache holding a `Py<PyDict>`:

```rust
# use pyo3::prelude::*;
use pyo3::sync::PyOnceLock;
use pyo3::types::PyDict;

let cache: PyOnceLock<Py<PyDict>> = PyOnceLock::new();

Python::attach(|py| {
    // guaranteed to be called once and only once
    cache.get_or_init(py, || PyDict::new(py).unbind())
});
```

In cases where a function must run exactly once, you can bring the [`OnceExt`] trait into scope.
The [`OnceExt`] trait adds [`OnceExt::call_once_py_attached`] and [`OnceExt::call_once_force_py_attached`] functions to the api of `std::sync::Once`, enabling use of [`Once`] in contexts where the thread is attached to the Python interpreter.
These functions are analogous to [`Once::call_once`], [`Once::call_once_force`] except they accept a [`Python<'py>`] token in addition to an `FnOnce`.
All of these functions detach from the interpreter before blocking and re-attach before executing the function, avoiding deadlocks that are possible without using the PyO3 extension traits.
Here the same example as above built using a [`Once`] instead of a
[`PyOnceLock`]:

```rust
# use pyo3::prelude::*;
use std::sync::Once;
use pyo3::sync::OnceExt;
use pyo3::types::PyDict;

struct RuntimeCache {
    once: Once,
    cache: Option<Py<PyDict>>
}

let mut cache = RuntimeCache {
    once: Once::new(),
    cache: None
};

Python::attach(|py| {
    // guaranteed to be called once and only once
    cache.once.call_once_py_attached(py, || {
        cache.cache = Some(PyDict::new(py).unbind());
    });
});
```

### `GILProtected` has been removed

[`GILProtected`] was a PyO3 type that allowed mutable access to static data by leveraging the GIL to lock concurrent access from other threads.
In free-threaded Python there is no GIL, so this type had to be replaced with alternative forms of locking.
In many cases, a type from [`std::sync::atomic`](https://doc.rust-lang.org/std/sync/atomic/) or a [`std::sync::Mutex`](https://doc.rust-lang.org/std/sync/struct.Mutex.html) was sufficient.

Before:

```rust,ignore
# fn main() {
# #[cfg(not(Py_GIL_DISABLED))] {
# use pyo3::prelude::*;
use pyo3::sync::GILProtected;
use pyo3::types::{PyDict, PyNone};
use std::cell::RefCell;

static OBJECTS: GILProtected<RefCell<Vec<Py<PyDict>>>> =
    GILProtected::new(RefCell::new(Vec::new()));

Python::attach(|py| {
    // stand-in for something that executes arbitrary Python code
    let d = PyDict::new(py);
    d.set_item(PyNone::get(py), PyNone::get(py)).unwrap();
    OBJECTS.get(py).borrow_mut().push(d.unbind());
});
# }}
```

After (using a `Mutex`):

```rust
# use pyo3::prelude::*;
# fn main() {
use pyo3::types::{PyDict, PyNone};
use std::sync::Mutex;

static OBJECTS: Mutex<Vec<Py<PyDict>>> = Mutex::new(Vec::new());

Python::attach(|py| {
    // stand-in for something that executes arbitrary Python code
    let d = PyDict::new(py);
    d.set_item(PyNone::get(py), PyNone::get(py)).unwrap();
    // as with any `Mutex` usage, lock the mutex for as little time as possible
    // in this case, we do it just while pushing into the `Vec`
    OBJECTS.lock().unwrap().push(d.unbind());
});
# }
```

If you are executing arbitrary Python code while holding the lock, then you should import the [`MutexExt`] trait and use the `lock_py_attached` method instead of `lock`.
This ensures that global synchronization events started by the Python runtime can proceed, avoiding possible deadlocks with the interpreter.

[`GILProtected`]: https://docs.rs/pyo3/0.22/pyo3/sync/struct.GILProtected.html
[`MutexExt`]: {{#PYO3_DOCS_URL}}/pyo3/sync/trait.MutexExt.html
[`Once`]: https://doc.rust-lang.org/stable/std/sync/struct.Once.html
[`Once::call_once`]: https://doc.rust-lang.org/stable/std/sync/struct.Once.html#method.call_once
[`Once::call_once_force`]: https://doc.rust-lang.org/stable/std/sync/struct.Once.html#method.call_once_force
[`OnceExt`]: {{#PYO3_DOCS_URL}}/pyo3/sync/trait.OnceExt.html
[`OnceExt::call_once_py_attached`]: {{#PYO3_DOCS_URL}}/pyo3/sync/trait.OnceExt.html#tymethod.call_once_py_attached
[`OnceExt::call_once_force_py_attached`]: {{#PYO3_DOCS_URL}}/pyo3/sync/trait.OnceExt.html#tymethod.call_once_force_py_attached
[`OnceLockExt`]: {{#PYO3_DOCS_URL}}/pyo3/sync/trait.OnceLockExt.html
[`OnceLockExt::get_or_init_py_attached`]: {{#PYO3_DOCS_URL}}/pyo3/sync/trait.OnceLockExt.html#tymethod.get_or_init_py_attached
[`OnceLock`]: https://doc.rust-lang.org/stable/std/sync/struct.OnceLock.html
[`Python::detach`]: {{#PYO3_DOCS_URL}}/pyo3/marker/struct.Python.html#method.detach
[`Python::attach`]: {{#PYO3_DOCS_URL}}/pyo3/marker/struct.Python.html#method.attach
[`Python<'py>`]: {{#PYO3_DOCS_URL}}/pyo3/marker/struct.Python.html
[`PyOnceLock`]: {{#PYO3_DOCS_URL}}/pyo3/sync/struct.PyOnceLock.html
[`threading`]: https://docs.python.org/3/library/threading.html
