Allow use of `#[pyo3(get, set)]` with `Py<T>`

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 * `PyObject` and `Py<T>` reference counts are now decremented sooner after `drop()`. [#851](https://github.com/PyO3/pyo3/pull/851)
   * When the GIL is held, the refcount is now decreased immediately on drop. (Previously would wait until just before releasing the GIL.)
   * When the GIL is not held, the refcount is now decreased when the GIL is next acquired. (Previously would wait until next time the GIL was released.)
+* `FromPyObject` for `Py<T>` now works for a wider range of `T`, in particular for `T: PyClass`. [#880](https://github.com/PyO3/pyo3/pull/880)
 
 ### Added
 * `_PyDict_NewPresized`. [#849](https://github.com/PyO3/pyo3/pull/849)
@@ -21,6 +22,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 * `__radd__` and other `__r*__` methods now correctly work with operators. [#839](https://github.com/PyO3/pyo3/pull/839)
 * Garbage Collector causing random panics when traversing objects that were mutably borrowed. [#855](https://github.com/PyO3/pyo3/pull/855)
 * `&'static Py~` being allowed as arguments. [#869](https://github.com/PyO3/pyo3/pull/869)
+* `#[pyo3(get)]` for `Py<T>`. [#880](https://github.com/PyO3/pyo3/pull/880)
+
 
 ## [0.9.2]
 

src/derive_utils.rs

@@ -10,7 +10,7 @@ use crate::instance::PyNativeType;
 use crate::pyclass::PyClass;
 use crate::pyclass_init::PyClassInitializer;
 use crate::types::{PyAny, PyDict, PyModule, PyTuple};
-use crate::{ffi, GILPool, IntoPy, PyCell, PyObject, Python};
+use crate::{ffi, GILPool, IntoPy, Py, PyCell, PyObject, Python};
 use std::cell::UnsafeCell;
 
 /// Description of a python parameter; used for `parse_args()`.
@@ -215,6 +215,12 @@ impl GetPropertyValue for PyObject {
     }
 }
 
+impl<T> GetPropertyValue for Py<T> {
+    fn get_property_value(&self, py: Python) -> PyObject {
+        self.clone_ref(py).into()
+    }
+}
+
 /// Utilities for basetype
 #[doc(hidden)]
 pub trait PyBaseTypeUtils {

src/instance.rs

@@ -295,13 +295,14 @@ where
 
 impl<'a, T> FromPyObject<'a> for Py<T>
 where
-    T: AsPyPointer,
+    T: PyTypeInfo,
-    &'a T: 'a + FromPyObject<'a>,
+    &'a T::AsRefTarget: FromPyObject<'a>,
+    T::AsRefTarget: 'a + AsPyPointer,
 {
     /// Extracts `Self` from the source `PyObject`.
     fn extract(ob: &'a PyAny) -> PyResult<Self> {
         unsafe {
-            ob.extract::<&T>()
+            ob.extract::<&T::AsRefTarget>()
                 .map(|val| Py::from_borrowed_ptr(val.as_ptr()))
         }
     }

tests/test_class_conversion.rs

@@ -1,4 +1,8 @@
 use pyo3::prelude::*;
+use pyo3::{ObjectProtocol, ToPyObject};
+
+#[macro_use]
+mod common;
 
 #[pyclass]
 #[derive(Clone, Debug, PartialEq)]
@@ -25,3 +29,95 @@ fn test_cloneable_pyclass() {
     let mrc: PyRefMut<Cloneable> = py_c.extract(py).unwrap();
     assert_eq!(&c, &*mrc);
 }
+
+#[pyclass]
+struct BaseClass {}
+
+#[pymethods]
+impl BaseClass {
+    fn foo(&self) -> &'static str {
+        "BaseClass"
+    }
+}
+
+#[pyclass(extends=BaseClass)]
+struct SubClass {}
+
+#[pymethods]
+impl SubClass {
+    fn foo(&self) -> &'static str {
+        "SubClass"
+    }
+}
+
+#[pyclass]
+struct PolymorphicContainer {
+    #[pyo3(get, set)]
+    inner: Py<BaseClass>,
+}
+
+#[test]
+fn test_polymorphic_container_stores_base_class() {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+
+    let p = PyCell::new(
+        py,
+        PolymorphicContainer {
+            inner: Py::new(py, BaseClass {}).unwrap(),
+        },
+    )
+    .unwrap()
+    .to_object(py);
+
+    py_assert!(py, p, "p.inner.foo() == 'BaseClass'");
+}
+
+#[test]
+fn test_polymorphic_container_stores_sub_class() {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+
+    let p = PyCell::new(
+        py,
+        PolymorphicContainer {
+            inner: Py::new(py, BaseClass {}).unwrap(),
+        },
+    )
+    .unwrap()
+    .to_object(py);
+
+    p.as_ref(py)
+        .setattr(
+            "inner",
+            PyCell::new(
+                py,
+                PyClassInitializer::from(BaseClass {}).add_subclass(SubClass {}),
+            )
+            .unwrap(),
+        )
+        .unwrap();
+
+    py_assert!(py, p, "p.inner.foo() == 'SubClass'");
+}
+
+#[test]
+fn test_polymorphic_container_does_not_accept_other_types() {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+
+    let p = PyCell::new(
+        py,
+        PolymorphicContainer {
+            inner: Py::new(py, BaseClass {}).unwrap(),
+        },
+    )
+    .unwrap()
+    .to_object(py);
+
+    let setattr = |value: PyObject| p.as_ref(py).setattr("inner", value);
+
+    assert!(setattr(1i32.into_py(py)).is_err());
+    assert!(setattr(py.None()).is_err());
+    assert!(setattr((1i32, 2i32).into_py(py)).is_err());
+}