print usage when no argument is specified for python3

When running just 'glance' under python3, we will get the error:
ERROR: 'Namespace' object has no attribute 'func'

This is because map() is used to decode sys.argv, but under Python3
it returns a map object which is an iterable. Some code later tries
to use this in a boolean context and it will always return True,
even if it's empty.

Change-Id: I2f03e462cb813833b75b9f2de7badd10b10cddff
Closes-Bug: #1295356

glanceclient/shell.py

@@ -759,7 +759,8 @@ class HelpFormatter(argparse.HelpFormatter):
 
 def main():
     try:
-        OpenStackImagesShell().main(map(encodeutils.safe_decode, sys.argv[1:]))
+        argv = [encodeutils.safe_decode(a) for a in sys.argv[1:]]
+        OpenStackImagesShell().main(argv)
     except KeyboardInterrupt:
         utils.exit('... terminating glance client', exit_code=130)
     except Exception as e:

glanceclient/tests/unit/test_shell.py

@@ -504,6 +504,20 @@ class ShellTest(testutils.TestCase):
         glance_shell = openstack_shell.OpenStackImagesShell()
         self.assertRaises(exc.CommandError, glance_shell.main, args.split())
 
+    @mock.patch('sys.argv', ['glance'])
+    @mock.patch('sys.stdout', six.StringIO())
+    @mock.patch('sys.stderr', six.StringIO())
+    def test_main_noargs(self):
+        # Ensure that main works with no command-line arguments
+        try:
+            openstack_shell.main()
+        except SystemExit:
+            self.fail('Unexpected SystemExit')
+
+        # We expect the normal usage as a result
+        self.assertIn('Command-line interface to the OpenStack Images API',
+                      sys.stdout.getvalue())
+
 
 class ShellTestWithKeystoneV3Auth(ShellTest):
     # auth environment to use