Fix undesirable raw Python error

Using the cinderclient 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 `cinder --os-volume-api-version 3.40`.
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: Idb51e8635b97f0da2976f3268d5e19100ec77203
Closes-Bug: #1867061

cinderclient/shell.py

@@ -225,6 +225,9 @@ class OpenStackCinderShell(object):
                             default=0,
                             help=_('Number of retries.'))
 
+        parser.set_defaults(func=self.do_help)
+        parser.set_defaults(command='')
+
         if osprofiler_profiler:
             parser.add_argument('--profile',
                                 metavar='HMAC_KEY',

cinderclient/tests/unit/test_shell.py

@@ -148,6 +148,18 @@ class ShellTest(utils.TestCase):
             self.assertThat(help_text,
                             matchers.MatchesRegex(r, re.DOTALL | re.MULTILINE))
 
+    def test_help_arg_no_subcommand(self):
+        required = [
+            r'.*?^usage: ',
+            r'.*?(?m)^\s+create\s+Creates a volume.',
+            r'.*?(?m)^\s+summary\s+Get volumes summary.',
+            r'.*?(?m)^Run "cinder help SUBCOMMAND" for help on a subcommand.',
+        ]
+        help_text = self.shell('--os-volume-api-version 3.40')
+        for r in required:
+            self.assertThat(help_text,
+                            matchers.MatchesRegex(r, re.DOTALL | re.MULTILINE))
+
     @ddt.data('backup-create --help', '--help backup-create')
     def test_dash_dash_help_on_subcommand(self, cmd):
         required = ['.*?^Creates a volume backup.']

releasenotes/notes/bug-1867061-fix-py-raw-error-msg-ff3c6da0b01d5d6c.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    `Bug #1867061 <https://bugs.launchpad.net/python-cinderclient/+bug/1867061>`_:
+    Fixed raw Python error message when using ``cinder`` without
+    a subcommand while passing an optional argument, such as
+    ``--os-volume-api-version``.