Fix undesirable raw Python error

Using the novaclient without a subcommand while
passing an optional argument triggers the raw Python
error `ERROR: 'Namespace' object has no attribute
'func'`. This bug can be reproduced by issuing the
command `nova --os-compute-api-version 2.87`.
Added a default value to `func` and an empty value to
`command` as placeholders so that a help message is
shown instead of the Python error.

Change-Id: Ic3e87b67f6d27d40b03d7d8e520fa0f79a4d09e5
Closes-Bug: #1903727
Signed-off-by: Takashi Natsume <takanattie@gmail.com>

novaclient/shell.py

@@ -365,6 +365,9 @@ class OpenStackComputeShell(object):
             help=_("Use this API endpoint instead of the Service Catalog. "
                    "Defaults to env[OS_ENDPOINT_OVERRIDE]."))
 
+        parser.set_defaults(func=self.do_help)
+        parser.set_defaults(command='')
+
         if osprofiler_profiler:
             parser.add_argument('--profile',
                                 metavar='HMAC_KEY',

novaclient/tests/unit/test_shell.py

@@ -451,6 +451,9 @@ class ShellTest(utils.TestCase):
     def test_help_no_options(self):
         self._test_help('')
 
+    def test_help_no_subcommand(self):
+        self._test_help('--os-compute-api-version 2.87')
+
     def test_help_on_subcommand(self):
         required = [
             '.*?^usage: nova set-password',

releasenotes/notes/fix-raw-python-error-debd3edb17c2f675.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    `Bug #1903727 <https://bugs.launchpad.net/python-novaclient/+bug/1903727>`_:
+    Fixed raw Python error message when using ``nova`` without
+    a subcommand while passing an optional argument, such as
+    ``--os-compute-api-version 2.87``.