On first glance, PyO3 provides a huge number of different types that can be used to wrap or refer to Python objects. This page delves into the details and gives an overview of their intended meaning, with examples when each type is best used.
Since Python has no concept of ownership, and works solely with boxed objects, any Python object can be referenced any number of times, and mutation is allowed from any reference.
The situation is helped a little by the Global Interpreter Lock (GIL), which ensures that only one thread can use the Python interpreter and its API at the same time, while non-Python operations (system calls and extension code) can unlock the GIL. (See the section on parallelism for how to do that in PyO3.)
In PyO3, holding the GIL is modeled by acquiring a token of the type
Python<'py>, which serves three purposes:
- It provides some global API for the Python interpreter, such as
eval. - It can be passed to functions that require a proof of holding the GIL,
such as
Py::clone_ref. - Its lifetime can be used to create Rust references that implicitly guarantee
holding the GIL, such as
&'py PyAny.
The latter two points are the reason why some APIs in PyO3 require the py: Python argument, while others don't.
The PyO3 API for Python objects is written such that instead of requiring a
mutable Rust reference for mutating operations such as
PyList::append, a shared reference (which, in turn, can only
be created through Python<'_> with a GIL lifetime) is sufficient.
However, Rust structs wrapped as Python objects (called pyclass types) usually
do need &mut access. Due to the GIL, PyO3 can guarantee thread-safe acces
to them, but it cannot statically guarantee uniqueness of &mut references once
an object's ownership has been passed to the Python interpreter, ensuring
references is done at runtime using PyCell, a scheme very similar to
std::cell::RefCell.
Represents: a Python object of unspecified type, restricted to a GIL
lifetime. Currently, PyAny can only ever occur as a reference, &PyAny.
Used: Whenever you want to refer to some Python object and will have the
GIL for the whole duration you need to access that object. For example,
intermediate values and arguments to pyfunctions or pymethods implemented
in Rust where any type is allowed.
Many general methods for interacting with Python objects are on the PyAny struct,
such as getattr, setattr, and .call.
Conversions:
# use pyo3::prelude::*;
# use pyo3::types::PyList;
# let gil = Python::acquire_gil();
# let py = gil.python();
let obj: &PyAny = PyList::empty(py);
// Convert to &ConcreteType using PyAny::downcast
let _: &PyList = obj.downcast().unwrap();
// Convert to Py<PyAny> (aka PyObject) using .into()
let _: Py<PyAny> = obj.into();
// Convert to Py<ConcreteType> using PyAny::extract
let _: Py<PyList> = obj.extract().unwrap();Represents: a native Python object of known type, restricted to a GIL
lifetime just like PyAny.
Used: Whenever you want to operate with native Python types while holding
the GIL. Like PyAny, this is the most convenient form to use for function
arguments and intermediate values.
These types all implement Deref<Target = PyAny>, so they all expose the same
methods which can be found on PyAny.
To see all Python types exposed by PyO3 you should consult the
pyo3::types module.
Conversions:
# use pyo3::prelude::*;
# use pyo3::types::PyList;
# let gil = Python::acquire_gil();
# let py = gil.python();
let list = PyList::empty(py);
// Can use methods from PyAny on all Python types due to Deref implementation
let _ = list.repr();
// Rust will convert &PyList etc. to &PyAny automatically due to Deref implementation
let _: &PyAny = list;
// For more explicit &PyAny conversion, use .as_ref()
let _: &PyAny = list.as_ref();
// To convert to Py<T> use .into() or Py::from()
let _: Py<PyList> = list.into();
// To convert to PyObject use .into() or .to_object(py)
let _: PyObject = list.into();Represents: a GIL independent reference to a Python object. This can be a Python native type
(like PyTuple), or a pyclass type implemented in Rust. The most commonly-used variant,
Py<PyAny>, is also known as PyObject.
Used: Whenever you want to carry around references to a Python object without caring about a GIL lifetime. For example, storing Python object references in a Rust struct that outlives the Python-Rust FFI boundary, or returning objects from functions implemented in Rust back to Python.
Can be cloned using Python reference counts with .clone().
Conversions:
# use pyo3::prelude::*;
# use pyo3::types::PyList;
# let gil = Python::acquire_gil();
# let py = gil.python();
let list: Py<PyList> = PyList::empty(py).into();
// Access to the native type using Py::as_ref(py) or Py::into_ref(py)
// (For #[pyclass] types T, these will return &PyCell<T>)
// Py::as_ref() borrows the object
let _: &PyList = list.as_ref(py);
# let list_clone = list.clone(); // Just so that the .into() example for PyObject compiles.
// Py::into_ref() moves the reference into pyo3's "object storage"; useful for making APIs
// which return gil-bound references.
let _: &PyList = list.into_ref(py);
# let list = list_clone;
// Convert to PyObject with .into()
let _: PyObject = list.into();Represents: a reference to a Rust object (instance of PyClass) which is
wrapped in a Python object. The cell part is an analog to stdlib's
RefCell to allow access to &mut references.
Used: for accessing pure-Rust API of the instance (members and functions
taking &SomeType or &mut SomeType) while maintaining the aliasing rules of
Rust references.
Like pyo3's Python native types, PyCell<T> implements Deref<Target = PyAny>,
so it also exposes all of the methods on PyAny.
Conversions:
# use pyo3::prelude::*;
# use pyo3::types::PyList;
# #[pyclass] struct MyClass { }
# let gil = Python::acquire_gil();
# let py = gil.python();
let cell: &PyCell<MyClass> = PyCell::new(py, MyClass { }).unwrap();
// Obtain PyRef<T> with .try_borrow()
let pr: PyRef<MyClass> = cell.try_borrow().unwrap();
# drop(pr);
// Obtain PyRefMut<T> with .try_borrow_mut()
let prm: PyRefMut<MyClass> = cell.try_borrow_mut().unwrap();
# drop(prm);
// Can use methods from PyAny on PyCell<T> due to Deref implementation
let _ = cell.repr();
// Rust will convert &PyCell<T> to &PyAny automatically due to Deref implementation
let _: &PyAny = cell;
// For more explicit &PyAny conversion, use .as_ref()
let any: &PyAny = cell.as_ref();
// To obtain a PyCell<T> from PyAny, use PyAny::downcast
let _: &PyCell<MyClass> = any.downcast().unwrap();Represents: reference wrapper types employed by PyCell to keep track of
borrows, analog to Ref and RefMut used by RefCell.
Used: while borrowing a PyCell. They can also be used with .extract()
on types like Py<T> and PyAny to get a reference quickly.
This trait marks structs defined in Rust that are also usable as Python classes,
usually defined using the #[pyclass] macro.
This trait marks structs that mirror native Python types, such as PyList.