Update downcast documentation

src/instance.rs

@@ -991,10 +991,52 @@ impl<T> std::fmt::Debug for Py<T> {
 pub type PyObject = Py<PyAny>;
 
 impl PyObject {
-    /// Casts the PyObject to a concrete Python object type.
+    /// Downcast this `PyObject` to a concrete Python type or pyclass.
     ///
-    /// This can cast only to native Python types, not types implemented in Rust. For a more
-    /// flexible alternative, see [`Py::extract`](struct.Py.html#method.extract).
+    /// Note that you can often avoid downcasting yourself by just specifying
+    /// the desired type in function or method signatures.
+    /// However, manual downcasting is sometimes necessary.
+    ///
+    /// For extracting a Rust-only type, see [`Py::extract`](struct.Py.html#method.extract).
+    ///
+    /// # Example: Downcasting to a specific Python object
+    ///
+    /// ```rust
+    /// use pyo3::prelude::*;
+    /// use pyo3::types::{PyDict, PyList};
+    ///
+    /// Python::with_gil(|py| {
+    ///     let any: PyObject = PyDict::new(py).into();
+    ///
+    ///     assert!(any.downcast::<PyDict>(py).is_ok());
+    ///     assert!(any.downcast::<PyList>(py).is_err());
+    /// });
+    /// ```
+    ///
+    /// # Example: Getting a reference to a pyclass
+    ///
+    /// This is useful if you want to mutate a `PyObject` that
+    /// might actually be a pyclass.
+    ///
+    /// ```
+    /// # fn main() -> Result<(), pyo3::PyErr> {
+    /// use pyo3::prelude::*;
+    ///
+    /// #[pyclass]
+    /// struct Class {
+    ///     i: i32,
+    /// }
+    ///
+    /// Python::with_gil(|py| {
+    ///     let class: PyObject = Py::new(py, Class { i: 0 }).unwrap().into_py(py);
+    ///
+    ///     let class_cell: &PyCell<Class> = class.downcast(py)?;
+    ///
+    ///     class_cell.borrow_mut().i += 1;
+    ///     Ok(())
+    /// })
+    /// # }
+    /// ```
     #[inline]
     pub fn downcast<'p, T>(&'p self, py: Python<'p>) -> Result<&T, PyDowncastError<'_>>
     where
@@ -1005,9 +1047,6 @@ impl PyObject {
 
     /// Casts the PyObject to a concrete Python object type without checking validity.
     ///
-    /// This can cast only to native Python types, not types implemented in Rust. For a more
-    /// flexible alternative, see [`Py::extract`](struct.Py.html#method.extract).
-    ///
     /// # Safety
     ///
     /// Callers must ensure that the type is valid or risk type confusion.

src/types/any.rs

@@ -755,11 +755,15 @@ impl PyAny {
         self.downcast()
     }
 
-    /// Converts this `PyAny` to a concrete Python type.
+    /// Downcast this `PyAny` to a concrete Python type or pyclass.
     ///
-    /// This can cast only to native Python types, not types implemented in Rust.
+    /// Note that you can often avoid downcasting yourself by just specifying
+    /// the desired type in function or method signatures.
+    /// However, manual downcasting is sometimes necessary.
     ///
-    /// # Examples
+    /// For extracting a Rust-only type, see [`PyAny::extract`](struct.PyAny.html#method.extract).
+    ///
+    /// # Example: Downcasting to a specific Python object
     ///
     /// ```rust
     /// use pyo3::prelude::*;
@@ -769,10 +773,36 @@ impl PyAny {
     ///     let dict = PyDict::new(py);
     ///     assert!(dict.is_instance_of::<PyAny>().unwrap());
     ///     let any: &PyAny = dict.as_ref();
+    ///
     ///     assert!(any.downcast::<PyDict>().is_ok());
     ///     assert!(any.downcast::<PyList>().is_err());
     /// });
     /// ```
+    ///
+    /// # Example: Getting a reference to a pyclass
+    ///
+    /// This is useful if you want to mutate a `PyObject` that
+    /// might actually be a pyclass.
+    ///
+    /// ```
+    /// # fn main() -> Result<(), pyo3::PyErr> {
+    /// use pyo3::prelude::*;
+    ///
+    /// #[pyclass]
+    /// struct Class {
+    ///     i: i32,
+    /// }
+    ///
+    /// Python::with_gil(|py| {
+    ///     let class: &PyAny = Py::new(py, Class { i: 0 }).unwrap().into_ref(py);
+    ///
+    ///     let class_cell: &PyCell<Class> = class.downcast()?;
+    ///
+    ///     class_cell.borrow_mut().i += 1;
+    ///     Ok(())
+    /// })
+    /// # }
+    /// ```
     #[inline]
     pub fn downcast<'p, T>(&'p self) -> Result<&'p T, PyDowncastError<'_>>
     where