List are now formatted one line per element.

isinstance(obj, list) was not working because traitlets.List and derivatives are not considered list type.

config_files trait is also commented because it crashed with the empty list as default.

ctapipe/core/config_writer.py

@@ -79,8 +79,12 @@ def _trait_to_str(trait, help=True, indent_level=0):
         trait_value = f"'{trait_value}'"
 
     # Make sure that list of values end up on multiple lines (In particular quality criteria)
+    # Since traitlets.List derivative can't be compared with it, nor list/tuple, I have to check if "List" is in the
+    # type (converted as str)
     value_repr = ""
-    if isinstance(trait_value, list):
+    trait_value_type = str(type(trait_value))
+    trait_value_is_list = ("List" in trait_value_type) or isinstance(trait_value, list)
+    if trait_value_is_list:
         for v in trait_value:
             # tuples are not correctly handled by yaml, so we convert them to list here. They'll be converted to tuple
             # when reading again.
@@ -92,7 +96,9 @@ def _trait_to_str(trait, help=True, indent_level=0):
 
     # Automatically comment all parameters that are unvalid
     commented = ""
-    if trait_value == traitlets.Undefined:
+    if trait_value in (traitlets.Undefined, None):
+        commented = "#"
+    elif trait_type.startswith("List") and len(trait_value) == 0:
         commented = "#"
 
     trait_repr += f"{indent_str*indent_level}{commented}{trait.name}: {value_repr}\n"