pub struct GILOnceCell<T>(_);
Expand description

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_sync 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.


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 {
        .get_or_init(py, || PyList::empty(py).into())


Create a GILOnceCell which does not yet contain a value.

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

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

Note that:

  1. reentrant initialization can cause a stack overflow.
  2. if f() temporarily releases the GIL (e.g. by calling Python::import) then it is possible (and well-defined) that a second thread may also call get_or_init and begin calling f(). Even when this happens GILOnceCell guarantees that only one write to the cell ever occurs - other threads will simply discard the value they compute and return the result of the first complete computation.
  3. if f() does not release the GIL and does not panic, it is guaranteed to be called exactly once, even if multiple threads attempt to call get_or_init
  4. if f() can panic but still does not release the GIL, it may be called multiple times, but it is guaranteed that f() will never be called concurrently

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

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

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Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.