Silence EMPTY_LAYOUT warnings (#4056)

* silence EMPTY_LAYOUT bokeh warnings

* add a test

panel/command/serve.py

@@ -19,6 +19,8 @@
 from bokeh.application.handlers.function import FunctionHandler
 from bokeh.command.subcommands.serve import Serve as _BkServe
 from bokeh.command.util import build_single_handler_applications
+from bokeh.core.validation import silence
+from bokeh.core.validation.warnings import EMPTY_LAYOUT
 from bokeh.server.contexts import ApplicationContext
 from tornado.ioloop import PeriodicCallback
 from tornado.web import StaticFileHandler
@@ -490,3 +492,9 @@ def customize_kwargs(self, args, server_kwargs):
             kwargs['cookie_secret'] = config.cookie_secret
 
         return kwargs
+
+    def invoke(self, args):
+        # Empty layout are valid and the Bokeh warning is silenced as usually
+        # not relevant to Panel users.
+        silence(EMPTY_LAYOUT, True)
+        super().invoke(args)

panel/io/server.py

@@ -41,6 +41,8 @@
 from bokeh.application.handlers.function import FunctionHandler
 from bokeh.command.util import build_single_handler_application
 from bokeh.core.templates import AUTOLOAD_JS
+from bokeh.core.validation import silence
+from bokeh.core.validation.warnings import EMPTY_LAYOUT
 from bokeh.embed.bundle import Script
 from bokeh.embed.elements import (
     html_page_for_render_items, script_for_render_items,
@@ -682,6 +684,9 @@ def serve(
     kwargs: dict
       Additional keyword arguments to pass to Server instance
     """
+    # Empty layout are valid and the Bokeh warning is silenced as usually
+    # not relevant to Panel users.
+    silence(EMPTY_LAYOUT, True)
     kwargs = dict(kwargs, **dict(
         port=port, address=address, websocket_origin=websocket_origin,
         loop=loop, show=show, start=start, title=title, verbose=verbose,

panel/tests/test_server.py

@@ -1,5 +1,6 @@
 import asyncio
 import datetime as dt
+import logging
 import os
 import pathlib
 import time
@@ -911,3 +912,30 @@ def app():
     time.sleep(0.5)
 
     assert len(exceptions) == 1
+
+
+def test_server_no_warning_empty_layout(port, caplog):
+
+    bk_logger = logging.getLogger('bokeh')
+    old_level = bk_logger.level
+    old_propagate = bk_logger.propagate
+    try:
+        # Test pretty dependent on how Bokeh sets up its logging system
+        bk_logger.propagate = True
+        bk_logger.setLevel(logging.WARNING)
+
+        app = Row()
+
+        serve(app, port=port, threaded=True, show=False)
+
+        # Wait for server to start
+        time.sleep(1)
+        requests.get(f"http://localhost:{port}")
+        time.sleep(1)
+
+        for rec in caplog.records:
+            if rec.levelname == 'WARNING':
+                assert 'EMPTY_LAYOUT' not in rec.message
+    finally:
+        bk_logger.setLevel(old_level)
+        bk_logger.propagate = old_propagate