Struct pyo3::once_cell::GILOnceCell

pub struct GILOnceCell<T>(_);

A write-once cell similar to once_cell::OnceCell.

Unlike once_cell::sync which blocks threads to achieve thread safety, this implementation uses the Python GIL to mediate concurrent access. This helps in cases where once_cell or lazy_static’s synchronization strategy can lead to deadlocks when interacting with the Python GIL. For an example, see the FAQ section of the guide.

Note that:

1. get_or_init and get_or_try_init do not protect against infinite recursion from reentrant initialization.
2. If the initialization function f provided to get_or_init (or get_or_try_init) temporarily releases the GIL (e.g. by calling Python::import) then it is possible for a second thread to also begin initializing the GITOnceCell. Even when this happens GILOnceCell guarantees that only one write to the cell ever occurs
   • this is treated as a race, other threads will discard the value they compute and return the result of the first complete computation.

Examples

The following example shows how to use GILOnceCell to share a reference to a Python list between threads:

use pyo3::once_cell::GILOnceCell;
use pyo3::prelude::*;
use pyo3::types::PyList;

static LIST_CELL: GILOnceCell<Py<PyList>> = GILOnceCell::new();

pub fn get_shared_list(py: Python<'_>) -> &PyList {
    LIST_CELL
        .get_or_init(py, || PyList::empty(py).into())
        .as_ref(py)
}

Implementations

impl<T> GILOnceCell<T>

pub const fn new() -> Self

Create a GILOnceCell which does not yet contain a value.

pub fn get(&self, _py: Python<'_>) -> Option<&T>

Get a reference to the contained value, or None if the cell has not yet been written.

pub fn get_or_init<F>(&self, py: Python<'_>, f: F) -> &Twhere
    F: FnOnce() -> T,

Get a reference to the contained value, initializing it if needed using the provided closure.

See the type-level documentation for detail on re-entrancy and concurrent initialization.

pub fn get_or_try_init<F, E>(&self, py: Python<'_>, f: F) -> Result<&T, E>where
    F: FnOnce() -> Result<T, E>,

Like get_or_init, but accepts a fallible initialization function. If it fails, the cell is left uninitialized.

See the type-level documentation for detail on re-entrancy and concurrent initialization.

pub fn get_mut(&mut self) -> Option<&mut T>

Get the contents of the cell mutably. This is only possible if the reference to the cell is unique.

pub fn set(&self, _py: Python<'_>, value: T) -> Result<(), T>

Set the value in the cell.

If the cell has already been written, Err(value) will be returned containing the new value which was not written.

Trait Implementations

impl<T: Send> Send for GILOnceCell<T>

impl<T: Send + Sync> Sync for GILOnceCell<T>