UserList to tell apart user set empty list from undefined value

cylc/flow/parsec/util.py

@@ -212,7 +212,7 @@ def replicate(target, source):
                 target[key].defaults_ = pdeepcopy(val.defaults_)
             replicate(target[key], val)
         elif isinstance(val, list):
-            target[key] = val[:]
+            target[key] = type(val)(val)
         else:
             target[key] = val
 
@@ -247,7 +247,7 @@ def poverride(target, sparse, prepend=False):
                 # Override in-place in the target ordered dict.
                 setitem = target.__setitem__
             if isinstance(val, list):
-                setitem(key, val[:])
+                setitem(key, type(val)(val))
             else:
                 setitem(key, val)
 
@@ -290,25 +290,21 @@ def m_override(target, sparse):
                 stack.append(
                     (val, dest[key], keylist + [key], child_many_defaults))
             else:
-                if key not in dest:
-                    if not (
-                        '__MANY__' in dest
-                        or key in many_defaults
-                        or '__MANY__' in many_defaults
-                    ):
-                        # TODO - validation prevents this, but handle properly
-                        # for completeness.
-                        raise Exception(
-                            "parsec dict override: no __MANY__ placeholder" +
-                            "%s" % (keylist + [key])
-                        )
-                    if isinstance(val, list):
-                        dest[key] = val[:]
-                    else:
-                        dest[key] = val
+                if not (
+                    key in dest
+                    or '__MANY__' in dest
+                    or key in many_defaults
+                    or '__MANY__' in many_defaults
+                ):
+                    # TODO - validation prevents this, but handle properly
+                    # for completeness.
+                    raise Exception(
+                        "parsec dict override: no __MANY__ placeholder" +
+                        "%s" % (keylist + [key])
+                    )
 
                 if isinstance(val, list):
-                    dest[key] = val[:]
+                    dest[key] = type(val)(val)
                 else:
                     dest[key] = val
     for dest_dict, defaults in defaults_list:

cylc/flow/parsec/validate.py

@@ -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):
@@ -647,6 +647,11 @@ def __str__(self):
         return f'{self[0]} .. {self[1]}'
 
 
+class UserList(list):
+    # distinguish user defined, possibly empty list from automatic defaults
+    pass
+
+
 class CylcConfigValidator(ParsecValidator):
     """Type validator and coercer for Cylc configurations.
 

cylc/flow/workflow_events.py

@@ -24,6 +24,7 @@
 from cylc.flow.cfgspec.glbl_cfg import glbl_cfg
 from cylc.flow.hostuserutil import get_host, get_user
 from cylc.flow.log_diagnosis import run_reftest
+from cylc.flow.parsec.validate import UserList
 from cylc.flow.subprocctx import SubProcContext
 
 if TYPE_CHECKING:
@@ -235,7 +236,7 @@ def get_events_conf(
             glbl_cfg().get(['scheduler', 'mail'])
         ):
             value = getter.get(key)
-            if value is not None:
+            if value not in (None, []) or isinstance(value, UserList):
                 return value
         return default
 

tests/unit/test_workflow_events.py

@@ -21,6 +21,7 @@
 
 from types import SimpleNamespace
 
+from cylc.flow.parsec.validate import UserList
 from cylc.flow.workflow_events import (
     WorkflowEventHandler,
     get_template_variables,
@@ -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': {}}
     }