Struct PyOnceLock 

pub struct PyOnceLock<T> { /* private fields */ }

An equivalent to std::sync::OnceLock for initializing objects while attached to the Python interpreter.

Unlike OnceLock<T>, this type will not deadlock with the interpreter. Before blocking calls the cell will detach from the runtime and then re-attach once the cell is unblocked.

Re-entrant initialization

Like OnceLock<T>, it is an error to re-entrantly initialize a PyOnceLock<T>. The exact behavior in this case is not guaranteed, it may either deadlock or panic.

Examples

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

use pyo3::sync::PyOnceLock;
use pyo3::prelude::*;
use pyo3::types::PyList;

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

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

Implementations

impl<T> PyOnceLock<T>

pub const fn new() -> Self

Create a PyOnceLock 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) -> &T

where 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.

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

Takes the value out of the cell, moving it back to an uninitialized state.

Has no effect and returns None if the cell has not yet been written.

pub fn into_inner(self) -> Option<T>

Consumes the cell, returning the wrapped value.

Returns None if the cell has not yet been written.

impl<T> PyOnceLock<Py<T>>

pub fn clone_ref(&self, py: Python<'_>) -> Self

Creates a new cell that contains a new Python reference to the same contained object.

Returns an uninitialized cell if self has not yet been initialized.

impl<T> PyOnceLock<Py<T>>

where T: PyTypeCheck,

pub fn import<'py>( &self, py: Python<'py>, module_name: &str, attr_name: &str, ) -> PyResult<&Bound<'py, T>>

This is a shorthand method for get_or_init which imports the type from Python on init.

Example: Using PyOnceLock to store a class in a static variable.

PyOnceLock can be used to avoid importing a class multiple times:

#[pyfunction]
fn create_ordered_dict<'py>(py: Python<'py>, dict: Bound<'py, PyDict>) -> PyResult<Bound<'py, PyAny>> {
    // Even if this function is called multiple times,
    // the `OrderedDict` class will be imported only once.
    static ORDERED_DICT: PyOnceLock<Py<PyType>> = PyOnceLock::new();
    ORDERED_DICT
        .import(py, "collections", "OrderedDict")?
        .call1((dict,))
}

Trait Implementations

impl<T: Default> Default for PyOnceLock<T>

fn default() -> PyOnceLock<T>

Returns the “default value” for a type.