diff --git a/src/pandas_openscm/grouping.py b/src/pandas_openscm/grouping.py
index cdae034..4b8d034 100644
--- a/src/pandas_openscm/grouping.py
+++ b/src/pandas_openscm/grouping.py
@@ -10,7 +10,9 @@
 
 
 def groupby_except(
-    df: pd.DataFrame, non_groupers: str | list[str], observed: bool = True
+    df: pd.DataFrame,  # TODO: update to support Series too
+    non_groupers: str | list[str],
+    observed: bool = True,
 ) -> pd.core.groupby.generic.DataFrameGroupBy[Any]:
     """
     Group by all index levels except specified levels
diff --git a/src/pandas_openscm/index_manipulation.py b/src/pandas_openscm/index_manipulation.py
index 99c1d73..0d22c1c 100644
--- a/src/pandas_openscm/index_manipulation.py
+++ b/src/pandas_openscm/index_manipulation.py
@@ -309,7 +309,7 @@ def update_index_from_candidates(
 
 
 def update_index_levels_func(
-    df: pd.DataFrame,
+    df: pd.DataFrame,  # TODO: update to support Series too
     updates: dict[Any, Callable[[Any], Any]],
     copy: bool = True,
     remove_unused_levels: bool = True,