UserList to tell apart user set empty list from undefined value

cylc/flow/parsec/util.py

@@ -20,7 +20,7 @@
 """
 
 from copy import copy
-from collections import deque
+from collections import deque, UserList
 import re
 import sys
 
@@ -178,7 +178,7 @@ def printcfg(cfg, level=0, indent=0, prefix='', none_str='',
                 key = "%s = " % key_i
             if cfg_i is None:
                 value = none_str
-            elif isinstance(cfg_i, list):
+            elif isinstance(cfg_i, (list, UserList)):
                 value = listjoin(cfg_i, none_str)
             elif "\n" in str(cfg_i) and key:
                 value = '"""\n'

cylc/flow/parsec/validate.py

@@ -24,7 +24,7 @@
 
 import re
 import shlex
-from collections import deque
+from collections import deque, UserList
 from textwrap import dedent
 from typing import List, Dict, Any, Optional, Tuple
 
@@ -597,7 +597,7 @@ def strip_and_unquote_list(cls, keys, value):
             # allow trailing commas
             if values[-1] == '':
                 values = values[0:-1]
-        return values
+        return UserList(values)
 
     @classmethod
     def _unquoted_list_parse(cls, keys, value):

cylc/flow/workflow_events.py

@@ -15,6 +15,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 """Workflow event handler."""
 
+from collections import UserList
 from enum import Enum
 import os
 from shlex import quote
@@ -235,7 +236,7 @@ def get_events_conf(
             glbl_cfg().get(['scheduler', 'mail'])
         ):
             value = getter.get(key)
-            if value is not None:
+            if value is not None and value != [] or type(value) == UserList:
                 return value
         return default
 

tests/unit/test_workflow_events.py

@@ -19,6 +19,7 @@
 
 import pytest
 
+from collections import UserList
 from types import SimpleNamespace
 
 from cylc.flow.workflow_events import (
@@ -32,11 +33,11 @@
     'key, workflow_cfg, glbl_cfg, expected',
     [
         ('handlers', True, True, ['stall']),
-        ('handlers', False, True, []),
-        ('handlers', False, False, []),
+        ('handlers', False, True, None),
+        ('handlers', False, False, None),
         ('mail events', True, True, []),
         ('mail events', False, True, ['abort']),
-        ('mail events', False, False, []),
+        ('mail events', False, False, None),
         ('from', True, True, 'docklands@railway'),
         ('from', False, True, 'highway@mixture'),
         ('from', False, False, None),
@@ -65,7 +66,7 @@ def test_get_events_handler(
     config = SimpleNamespace()
     config.cfg = {
         'scheduler': {
-            'events': {'handlers': ['stall'], 'mail events': []},
+            'events': {'handlers': ['stall'], 'mail events': UserList()},
             'mail': {'from': 'docklands@railway'},
         } if workflow_cfg else {'events': {}}
     }