Fix the --expose-app-in-browser and --custom-css CLI flags (#8060)

Author: jtpio

notebook/app.py

@@ -14,7 +14,7 @@
     ExtensionHandlerJinjaMixin,
     ExtensionHandlerMixin,
 )
-from jupyter_server.serverapp import flags
+from jupyter_server.serverapp import ServerApp, flags
 from jupyter_server.utils import url_escape, url_is_absolute
 from jupyter_server.utils import url_path_join as ujoin
 from jupyterlab.commands import (
@@ -355,6 +355,16 @@ def initialize_handlers(self) -> None:
         self.handlers.append(("/custom/custom.css", CustomCssHandler))
         super().initialize_handlers()
 
+    @classmethod
+    def make_serverapp(cls, **kwargs: t.Any) -> ServerApp:
+        """
+        Add the notebook flags to the server that parses the command line,
+        because jupyter_server merges only the extension aliases.
+        """
+        serverapp = super().make_serverapp(**kwargs)
+        serverapp.flags.update(cls.flags)  # type:ignore[arg-type]
+        return serverapp
+
     def initialize(self, argv: list[str] | None = None) -> None:  # noqa: ARG002
         """Subclass because the ExtensionApp.initialize() method does not take arguments"""
         super().initialize()

tests/test_app.py

@@ -1,6 +1,7 @@
 import os
 
 import pytest
+from jupyter_server.serverapp import ServerApp
 from jupyter_server.serverapp import flags as serverapp_flags
 from tornado.httpclient import HTTPClientError
 
@@ -27,6 +28,17 @@ def test_notebook_app_flags_are_isolated():
     assert notebook_flags.isdisjoint(serverapp_flags)
 
 
+def test_notebook_app_flags_reach_serverapp():
+    ServerApp.clear_instance()
+    try:
+        serverapp = JupyterNotebookApp.make_serverapp()
+        serverapp.parse_command_line(["--expose-app-in-browser", "--custom-css"])
+        assert serverapp.config.JupyterNotebookApp.expose_app_in_browser is True
+        assert serverapp.config.JupyterNotebookApp.custom_css is True
+    finally:
+        ServerApp.clear_instance()
+
+
 async def test_notebook_handler(notebooks, jp_fetch):
     for nbpath in notebooks:
         r = await jp_fetch("/", nbpath)