More gracefully override JPS ExtensionApp.config_file_paths

Author: MetRonnie

cylc/uiserver/app.py

@@ -59,7 +59,7 @@
 from pathlib import Path, PurePath
 import sys
 from textwrap import dedent
-from typing import List
+from typing import List, Optional
 
 from pkg_resources import parse_version
 from tornado import ioloop
@@ -145,15 +145,6 @@ class CylcUIServer(ExtensionApp):
     cylc gui --no-browser     # Start the server but don't open the browser
 
     ''')  # type: ignore[assignment]
-    config_file_paths = get_conf_dir_hierarchy(
-        [
-            SITE_CONF_ROOT,  # site configuration
-            USER_CONF_ROOT,  # user configuration
-        ], filename=False
-    )
-    # Next include currently needed for directory making
-    config_file_paths.insert(0, str(Path(uis_pkg).parent))  # packaged config
-    config_file_paths.reverse()
     # TODO: Add a link to the access group table mappings in cylc documentation
     # https://github.com/cylc/cylc-uiserver/issues/466
     AUTH_DESCRIPTION = '''
@@ -405,6 +396,7 @@ def _get_ui_path(self):
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
+        self._config_file_paths: Optional[List[str]] = None
         self.executor = ProcessPoolExecutor(max_workers=self.max_workers)
         self.workflows_mgr = WorkflowsManager(self, log=self.log)
         self.data_store_mgr = DataStoreMgr(
@@ -421,6 +413,21 @@ def __init__(self, *args, **kwargs):
             workflows_mgr=self.workflows_mgr,
         )
 
+    @property
+    def config_file_paths(self) -> List[str]:
+        if self._config_file_paths is None:
+            ret = get_conf_dir_hierarchy(
+                [
+                    SITE_CONF_ROOT,  # site configuration
+                    USER_CONF_ROOT,  # user configuration
+                ], filename=False
+            )
+            # Next include currently needed for directory making
+            ret.insert(0, str(Path(uis_pkg).parent))  # packaged config
+            ret.reverse()
+            self._config_file_paths = ret
+        return self._config_file_paths
+
     def initialize_settings(self):
         """Update extension settings.