list subclass to indicate unassigned list values in expanded config

cylc/flow/parsec/config.py

@@ -35,6 +35,10 @@
     from optparse import Values
 
 
+class DefaultList(list):
+    """List subclass to indicate unassigned list values in expanded config."""
+
+
 class ParsecConfig:
     """Object wrapper for parsec functions."""
 
@@ -112,7 +116,7 @@ def expand(self) -> None:
                     else:
                         if node.default == ConfigNode.UNSET:
                             if node.vdr and node.vdr.endswith('_LIST'):
-                                defs[node.name] = []
+                                defs[node.name] = DefaultList()
                             else:
                                 defs[node.name] = None
                         else:

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] = type(val)(val)
+            target[key] = 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, type(val)(val))
+                setitem(key, val[:])
             else:
                 setitem(key, val)
 
@@ -290,21 +290,25 @@ def m_override(target, sparse):
                 stack.append(
                     (val, dest[key], keylist + [key], child_many_defaults))
             else:
-                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 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 isinstance(val, list):
-                    dest[key] = type(val)(val)
+                    dest[key] = 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 UserList(values)
+        return values
 
     @classmethod
     def _unquoted_list_parse(cls, keys, value):
@@ -647,11 +647,6 @@ 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

@@ -24,7 +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.parsec.config import DefaultList
 from cylc.flow.subprocctx import SubProcContext
 
 if TYPE_CHECKING:
@@ -236,7 +236,7 @@ def get_events_conf(
             glbl_cfg().get(['scheduler', 'mail'])
         ):
             value = getter.get(key)
-            if value not in (None, []) or isinstance(value, UserList):
+            if value is not None and not isinstance(value, DefaultList):
                 return value
         return default
 

tests/unit/test_workflow_events.py

@@ -21,7 +21,6 @@
 
 from types import SimpleNamespace
 
-from cylc.flow.parsec.validate import UserList
 from cylc.flow.workflow_events import (
     WorkflowEventHandler,
     get_template_variables,
@@ -66,7 +65,7 @@ def test_get_events_handler(
     config = SimpleNamespace()
     config.cfg = {
         'scheduler': {
-            'events': {'handlers': ['stall'], 'mail events': UserList()},
+            'events': {'handlers': ['stall'], 'mail events': []},
             'mail': {'from': 'docklands@railway'},
         } if workflow_cfg else {'events': {}}
     }