Doc: expose more prominently const class attributes

They are closer to the actual behavior.

State explicitly that function-based class attributes are also computed only once at initialization

Author: Tpt

guide/src/class.md

@@ -886,7 +886,7 @@ impl MyClass {
 
 ## Class attributes
 
-To create a class attribute (also called [class variable][classattr]), a method without any arguments can be annotated with the `#[classattr]` attribute.
+To create a class attribute (also called [class variable][classattr]), an associated constant can be annotated with the `#[classattr]` attribute.
 
 ```rust,no_run
 # use pyo3::prelude::*;
@@ -895,25 +895,19 @@ To create a class attribute (also called [class variable][classattr]), a method
 #[pymethods]
 impl MyClass {
     #[classattr]
-    fn my_attribute() -> String {
-        "hello".to_string()
-    }
+    const MY_ATTRIBUTE: &'static str = "foobar";
 }
-
-Python::attach(|py| {
-    let my_class = py.get_type::<MyClass>();
-    pyo3::py_run!(py, my_class, "assert my_class.my_attribute == 'hello'")
-});
+#
+#Python::attach(|py| {
+#    let my_class = py.get_type::<MyClass>();
+#    pyo3::py_run!(py, my_class, "assert my_class.MY_ATTRIBUTE == 'foobar'")
+#});
 ```
 
-> [!NOTE]
-> If the method has a `Result` return type and returns an `Err`, PyO3 will panic during
-class creation.
+If `const` code is too limiting, a method without any arguments can be annotated with the `#[classattr]` attribute.
 
 > [!NOTE]
-> `#[classattr]` does not work with [`#[pyo3(warn(...))]`](./function.md#warn) attribute.
-
-If the class attribute is defined with `const` code only, one can also annotate associated constants:
+> Here too, the class attribute value is computed once during the class creation and not each time the attribute is accessed.
 
 ```rust,no_run
 # use pyo3::prelude::*;
@@ -922,10 +916,24 @@ If the class attribute is defined with `const` code only, one can also annotate
 #[pymethods]
 impl MyClass {
     #[classattr]
-    const MY_CONST_ATTRIBUTE: &'static str = "foobar";
+    fn my_attribute() -> String {
+        "hello".to_string()
+    }
 }
+#
+#Python::attach(|py| {
+#    let my_class = py.get_type::<MyClass>();
+#    pyo3::py_run!(py, my_class, "assert my_class.my_attribute == 'hello'")
+#});
 ```
 
+> [!NOTE]
+> If the method has a `Result` return type and returns an `Err`, PyO3 will panic during
+class creation.
+
+> [!NOTE]
+> `#[classattr]` does not work with [`#[pyo3(warn(...))]`](./function.md#warn) attribute.
+
 ## Classes as function arguments
 
 Class objects can be used as arguments to `#[pyfunction]`s and `#[pymethods]` in the same way as the self parameters of instance methods, i.e. they can be passed as: