support inherited --show-config args from base traitlets.config.Application

inherits flags & aliases from base classes

jupyterhub/app.py

@@ -118,6 +118,8 @@ common_aliases = {
     'config': 'JupyterHub.config_file',
     'db': 'JupyterHub.db_url',
 }
+if isinstance(Application.aliases, dict):
+    common_aliases.update(Application.aliases)
 
 aliases = {
     'base-url': 'JupyterHub.base_url',
@@ -134,7 +136,10 @@ token_aliases = {}
 token_aliases.update(common_aliases)
 aliases.update(common_aliases)
 
-flags = {
+flags = {}
+if isinstance(Application.flags, dict):
+    flags.update(Application.flags)
+hub_flags = {
     'debug': (
         {'Application': {'log_level': logging.DEBUG}},
         "set log level to logging.DEBUG (maximize logging output)",
@@ -164,6 +169,7 @@ flags = {
         "[DEPRECATED in 0.7: does nothing]",
     ),
 }
+flags.update(hub_flags)
 
 COOKIE_SECRET_BYTES = (
     32  # the number of bytes to use when generating new cookie secrets
@@ -3191,6 +3197,18 @@ class JupyterHub(Application):
             self.http_server.stop()
         self.io_loop.add_callback(self.io_loop.stop)
 
+    async def start_show_config(self):
+        """Async wrapper around base start_show_config method"""
+        # We need this because of our custom launch_instance_async,
+        # where `start` isn't a blocking call,
+        # it only gets async things going
+        # and `--show-config` replaces `start` with a blocking function.
+        # so our version:
+        # 1. calls the original blocking method
+        # 2. stops the event loop when we are done, so the process exits
+        super().start_show_config()
+        self.exit(0)
+
     async def launch_instance_async(self, argv=None):
         try:
             await self.initialize(argv)

jupyterhub/tests/test_app.py

@@ -1,10 +1,12 @@
 """Test the JupyterHub entry point"""
 import binascii
+import json
 import logging
 import os
 import re
 import sys
 import time
+from distutils.version import LooseVersion as V
 from subprocess import check_output
 from subprocess import PIPE
 from subprocess import Popen
@@ -13,6 +15,7 @@ from tempfile import TemporaryDirectory
 from unittest.mock import patch
 
 import pytest
+import traitlets
 from traitlets.config import Config
 
 from .. import orm
@@ -30,6 +33,27 @@ def test_help_all():
     assert '--JupyterHub.ip' in out
 
 
+@pytest.mark.skipif(V(traitlets.__version__) < V('5'), reason="requires traitlets 5")
+def test_show_config(tmpdir):
+    tmpdir.chdir()
+    p = Popen(
+        [sys.executable, '-m', 'jupyterhub', '--show-config', '--debug'], stdout=PIPE
+    )
+    p.wait(timeout=10)
+    out = p.stdout.read().decode('utf8', 'replace')
+    assert 'log_level' in out
+
+    p = Popen(
+        [sys.executable, '-m', 'jupyterhub', '--show-config-json', '--debug'],
+        stdout=PIPE,
+    )
+    p.wait(timeout=10)
+    out = p.stdout.read().decode('utf8', 'replace')
+    config = json.loads(out)
+    assert 'JupyterHub' in config
+    assert config["JupyterHub"]["log_level"] == 10
+
+
 def test_token_app():
     cmd = [sys.executable, '-m', 'jupyterhub', 'token']
     out = check_output(cmd + ['--help-all']).decode('utf8', 'replace')