Add BaseException.add_note() and __notes__ attribute support (#6252)

Lib/test/test_exceptions.py

@@ -601,7 +601,6 @@ class ExceptionTests(unittest.TestCase):
         with self.assertRaisesRegex(TypeError, "state is not a dictionary"):
             e.__setstate__(42)
 
-    @unittest.expectedFailure # TODO: RUSTPYTHON
     def test_notes(self):
         for e in [BaseException(1), Exception(2), ValueError(3)]:
             with self.subTest(e=e):

vm/src/exceptions.rs

@@ -4,7 +4,7 @@ use crate::object::{Traverse, TraverseFn};
 use crate::{
     AsObject, Context, Py, PyObjectRef, PyPayload, PyRef, PyResult, TryFromObject, VirtualMachine,
     builtins::{
-        PyNone, PyStr, PyStrRef, PyTuple, PyTupleRef, PyType, PyTypeRef,
+        PyList, PyNone, PyStr, PyStrRef, PyTuple, PyTupleRef, PyType, PyTypeRef,
         traceback::{PyTraceback, PyTracebackRef},
     },
     class::{PyClassImpl, StaticType},
@@ -652,6 +652,29 @@ impl PyRef<PyBaseException> {
         Ok(self)
     }
 
+    #[pymethod]
+    fn add_note(self, note: PyStrRef, vm: &VirtualMachine) -> PyResult<()> {
+        let dict = self
+            .as_object()
+            .dict()
+            .ok_or_else(|| vm.new_attribute_error("Exception object has no __dict__"))?;
+
+        let notes = if let Ok(notes) = dict.get_item("__notes__", vm) {
+            notes
+        } else {
+            let new_notes = vm.ctx.new_list(vec![]);
+            dict.set_item("__notes__", new_notes.clone().into(), vm)?;
+            new_notes.into()
+        };
+
+        let notes = notes
+            .downcast::<PyList>()
+            .map_err(|_| vm.new_type_error("__notes__ must be a list"))?;
+
+        notes.borrow_vec_mut().push(note.into());
+        Ok(())
+    }
+
     #[pymethod]
     fn __reduce__(self, vm: &VirtualMachine) -> PyTupleRef {
         if let Some(dict) = self.as_object().dict().filter(|x| !x.is_empty()) {