BaseExceptionGroup.derive should not copy __notes__ (#112)

This makes the behaviour follow that of CPython more closely. Instead, copy `__notes__` (if present) in the *callers* of derive. The (modified) test passes now, and it passes on Python 3.11. It fails before the changes.

src/exceptiongroup/_exceptions.py

@@ -42,6 +42,17 @@ def get_condition_filter(
     raise TypeError("expected a function, exception type or tuple of exception types")
 
 
+def _derive_and_copy_attributes(self, excs):
+    eg = self.derive(excs)
+    eg.__cause__ = self.__cause__
+    eg.__context__ = self.__context__
+    eg.__traceback__ = self.__traceback__
+    if hasattr(self, "__notes__"):
+        # Create a new list so that add_note() only affects one exceptiongroup
+        eg.__notes__ = list(self.__notes__)
+    return eg
+
+
 class BaseExceptionGroup(BaseException, Generic[_BaseExceptionT_co]):
     """A combination of multiple unrelated exceptions."""
 
@@ -154,10 +165,7 @@ def subgroup(
         if not modified:
             return self
         elif exceptions:
-            group = self.derive(exceptions)
-            group.__cause__ = self.__cause__
-            group.__context__ = self.__context__
-            group.__traceback__ = self.__traceback__
+            group = _derive_and_copy_attributes(self, exceptions)
             return group
         else:
             return None
@@ -230,17 +238,13 @@ def split(
 
         matching_group: _BaseExceptionGroupSelf | None = None
         if matching_exceptions:
-            matching_group = self.derive(matching_exceptions)
-            matching_group.__cause__ = self.__cause__
-            matching_group.__context__ = self.__context__
-            matching_group.__traceback__ = self.__traceback__
+            matching_group = _derive_and_copy_attributes(self, matching_exceptions)
 
         nonmatching_group: _BaseExceptionGroupSelf | None = None
         if nonmatching_exceptions:
-            nonmatching_group = self.derive(nonmatching_exceptions)
-            nonmatching_group.__cause__ = self.__cause__
-            nonmatching_group.__context__ = self.__context__
-            nonmatching_group.__traceback__ = self.__traceback__
+            nonmatching_group = _derive_and_copy_attributes(
+                self, nonmatching_exceptions
+            )
 
         return matching_group, nonmatching_group
 
@@ -257,12 +261,7 @@ def derive(
     def derive(
         self, __excs: Sequence[_BaseExceptionT]
     ) -> BaseExceptionGroup[_BaseExceptionT]:
-        eg = BaseExceptionGroup(self.message, __excs)
-        if hasattr(self, "__notes__"):
-            # Create a new list so that add_note() only affects one exceptiongroup
-            eg.__notes__ = list(self.__notes__)
-
-        return eg
+        return BaseExceptionGroup(self.message, __excs)
 
     def __str__(self) -> str:
         suffix = "" if len(self._exceptions) == 1 else "s"

tests/test_exceptions.py

@@ -205,6 +205,13 @@ def test_notes_is_list_of_strings_if_it_exists(self):
         eg.add_note(note)
         self.assertEqual(eg.__notes__, [note])
 
+    def test_derive_doesn_copy_notes(self):
+        eg = create_simple_eg()
+        eg.add_note("hello")
+        assert eg.__notes__ == ["hello"]
+        eg2 = eg.derive([ValueError()])
+        assert not hasattr(eg2, "__notes__")
+
 
 class ExceptionGroupTestBase(unittest.TestCase):
     def assertMatchesTemplate(self, exc, exc_type, template):
@@ -786,6 +793,7 @@ def derive(self, excs):
                 except ValueError as ve:
                     raise EG("eg", [ve, nested], 42)
         except EG as e:
+            e.add_note("hello")
             eg = e
 
         self.assertMatchesTemplate(eg, EG, [ValueError(1), [TypeError(2)]])
@@ -796,29 +804,35 @@ def derive(self, excs):
         self.assertMatchesTemplate(rest, EG, [ValueError(1), [TypeError(2)]])
         self.assertEqual(rest.code, 42)
         self.assertEqual(rest.exceptions[1].code, 101)
+        self.assertEqual(rest.__notes__, ["hello"])
 
         # Match Everything
         match, rest = self.split_exception_group(eg, (ValueError, TypeError))
         self.assertMatchesTemplate(match, EG, [ValueError(1), [TypeError(2)]])
         self.assertEqual(match.code, 42)
         self.assertEqual(match.exceptions[1].code, 101)
+        self.assertEqual(match.__notes__, ["hello"])
         self.assertIsNone(rest)
 
         # Match ValueErrors
         match, rest = self.split_exception_group(eg, ValueError)
         self.assertMatchesTemplate(match, EG, [ValueError(1)])
         self.assertEqual(match.code, 42)
+        self.assertEqual(match.__notes__, ["hello"])
         self.assertMatchesTemplate(rest, EG, [[TypeError(2)]])
         self.assertEqual(rest.code, 42)
         self.assertEqual(rest.exceptions[0].code, 101)
+        self.assertEqual(rest.__notes__, ["hello"])
 
         # Match TypeErrors
         match, rest = self.split_exception_group(eg, TypeError)
         self.assertMatchesTemplate(match, EG, [[TypeError(2)]])
         self.assertEqual(match.code, 42)
         self.assertEqual(match.exceptions[0].code, 101)
+        self.assertEqual(match.__notes__, ["hello"])
         self.assertMatchesTemplate(rest, EG, [ValueError(1)])
         self.assertEqual(rest.code, 42)
+        self.assertEqual(rest.__notes__, ["hello"])
 
 
 def test_repr():