Allow `#[new]` to return existing instances

fixes #2384

guide/src/class.md

@@ -114,6 +114,9 @@ impl Nonzero {
 }
 ```
 
+If you want to return an existing object (for example, because your `new`
+method caches the values it returns), `new` can return `pyo3::Py<Self>`.
+
 As you can see, the Rust method name is not important here; this way you can
 still, use `new()` for a Rust-level constructor.
 

newsfragments/3287.added.md

@@ -0,0 +1 @@
+`#[new]` methods may now return `Py<Self>` in order to return existing instances

src/pyclass_init.rs

@@ -1,7 +1,7 @@
 //! Contains initialization utilities for `#[pyclass]`.
 use crate::callback::IntoPyCallbackOutput;
 use crate::impl_::pyclass::{PyClassBaseType, PyClassDict, PyClassThreadChecker, PyClassWeakRef};
-use crate::{ffi, PyCell, PyClass, PyErr, PyResult, Python};
+use crate::{ffi, IntoPyPointer, Py, PyCell, PyClass, PyErr, PyResult, Python};
 use crate::{
     ffi::PyTypeObject,
     pycell::{
@@ -134,9 +134,14 @@ impl<T: PyTypeInfo> PyObjectInit<T> for PyNativeTypeInitializer<T> {
 ///     );
 /// });
 /// ```
-pub struct PyClassInitializer<T: PyClass> {
+pub struct PyClassInitializer<T: PyClass>(PyClassInitializerImpl<T>);
-    init: T,
+
-    super_init: <T::BaseType as PyClassBaseType>::Initializer,
+enum PyClassInitializerImpl<T: PyClass> {
+    Existing(Py<T>),
+    New {
+        init: T,
+        super_init: <T::BaseType as PyClassBaseType>::Initializer,
+    },
 }
 
 impl<T: PyClass> PyClassInitializer<T> {
@@ -144,7 +149,7 @@ impl<T: PyClass> PyClassInitializer<T> {
     ///
     /// It is recommended to use `add_subclass` instead of this method for most usage.
     pub fn new(init: T, super_init: <T::BaseType as PyClassBaseType>::Initializer) -> Self {
-        Self { init, super_init }
+        Self(PyClassInitializerImpl::New { init, super_init })
     }
 
     /// Constructs a new initializer from an initializer for the base class.
@@ -242,13 +247,18 @@ impl<T: PyClass> PyObjectInit<T> for PyClassInitializer<T> {
             contents: MaybeUninit<PyCellContents<T>>,
         }
 
-        let obj = self.super_init.into_new_object(py, subtype)?;
+        let (init, super_init) = match self.0 {
+            PyClassInitializerImpl::Existing(value) => return Ok(value.into_ptr()),
+            PyClassInitializerImpl::New { init, super_init } => (init, super_init),
+        };
+
+        let obj = super_init.into_new_object(py, subtype)?;
 
         let cell: *mut PartiallyInitializedPyCell<T> = obj as _;
         std::ptr::write(
             (*cell).contents.as_mut_ptr(),
             PyCellContents {
-                value: ManuallyDrop::new(UnsafeCell::new(self.init)),
+                value: ManuallyDrop::new(UnsafeCell::new(init)),
                 borrow_checker: <T::PyClassMutability as PyClassMutability>::Storage::new(),
                 thread_checker: T::ThreadChecker::new(),
                 dict: T::Dict::INIT,
@@ -284,6 +294,13 @@ where
     }
 }
 
+impl<T: PyClass> From<Py<T>> for PyClassInitializer<T> {
+    #[inline]
+    fn from(value: Py<T>) -> PyClassInitializer<T> {
+        PyClassInitializer(PyClassInitializerImpl::Existing(value))
+    }
+}
+
 // Implementation used by proc macros to allow anything convertible to PyClassInitializer<T> to be
 // the return value of pyclass #[new] method (optionally wrapped in `Result<U, E>`).
 impl<T, U> IntoPyCallbackOutput<PyClassInitializer<T>> for U

tests/test_class_new.rs

@@ -2,6 +2,7 @@
 
 use pyo3::exceptions::PyValueError;
 use pyo3::prelude::*;
+use pyo3::sync::GILOnceCell;
 use pyo3::types::IntoPyDict;
 
 #[pyclass]
@@ -204,3 +205,62 @@ fn new_with_custom_error() {
         assert_eq!(err.to_string(), "ValueError: custom error");
     });
 }
+
+#[pyclass]
+struct NewExisting {
+    #[pyo3(get)]
+    num: usize,
+}
+
+#[pymethods]
+impl NewExisting {
+    #[new]
+    fn new(py: pyo3::Python<'_>, val: usize) -> pyo3::Py<NewExisting> {
+        static PRE_BUILT: GILOnceCell<[pyo3::Py<NewExisting>; 2]> = GILOnceCell::new();
+        let existing = PRE_BUILT.get_or_init(py, || {
+            [
+                pyo3::PyCell::new(py, NewExisting { num: 0 })
+                    .unwrap()
+                    .into(),
+                pyo3::PyCell::new(py, NewExisting { num: 1 })
+                    .unwrap()
+                    .into(),
+            ]
+        });
+
+        if val < existing.len() {
+            return existing[val].clone_ref(py);
+        }
+
+        pyo3::PyCell::new(py, NewExisting { num: val })
+            .unwrap()
+            .into()
+    }
+}
+
+#[test]
+fn test_new_existing() {
+    Python::with_gil(|py| {
+        let typeobj = py.get_type::<NewExisting>();
+
+        let obj1 = typeobj.call1((0,)).unwrap();
+        let obj2 = typeobj.call1((0,)).unwrap();
+        let obj3 = typeobj.call1((1,)).unwrap();
+        let obj4 = typeobj.call1((1,)).unwrap();
+        let obj5 = typeobj.call1((2,)).unwrap();
+        let obj6 = typeobj.call1((2,)).unwrap();
+
+        assert!(obj1.getattr("num").unwrap().extract::<u32>().unwrap() == 0);
+        assert!(obj2.getattr("num").unwrap().extract::<u32>().unwrap() == 0);
+        assert!(obj3.getattr("num").unwrap().extract::<u32>().unwrap() == 1);
+        assert!(obj4.getattr("num").unwrap().extract::<u32>().unwrap() == 1);
+        assert!(obj5.getattr("num").unwrap().extract::<u32>().unwrap() == 2);
+        assert!(obj6.getattr("num").unwrap().extract::<u32>().unwrap() == 2);
+
+        assert!(obj1.is(obj2));
+        assert!(obj3.is(obj4));
+        assert!(!obj1.is(obj3));
+        assert!(!obj1.is(obj5));
+        assert!(!obj5.is(obj6));
+    });
+}