change help action to use its own exception for exit

Provide a new exception class for the help action to use to indicate
that the app should exit, instead of calling sys.exit(). This allows
argument parsing errors in interactive mode to print help without
exiting the entire application, while still being treated as a
short-cut to avoid every command plugin having to process argument
errors itself.

Change-Id: If882b305ff9186f97ece6c77ef8c1b888c24a72d
Story: 2008071
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

cliff/app.py

@@ -400,6 +400,8 @@ class App(object):
             cmd_parser = cmd.get_parser(full_name)
             parsed_args = cmd_parser.parse_args(sub_argv)
             result = cmd.run(parsed_args)
+        except help.HelpExit:
+            result = 0
         except SystemExit as ex:
             raise cmd2.exceptions.Cmd2ArgparseError from ex
         except Exception as err:

cliff/help.py

@@ -12,14 +12,24 @@
 
 import argparse
 import inspect
-import sys
 import traceback
 
 from . import command
 from . import utils
 
 
+class HelpExit(SystemExit):
+    """Special exception type to trigger quick exit from the application
+
+    We subclass from SystemExit to preserve API compatibility for
+    anything that used to catch SystemExit, but use a different class
+    so that cliff's Application can tell the difference between
+    something trying to hard-exit and help saying it's done.
+    """
+
+
 class HelpAction(argparse.Action):
+
     """Provide a custom action so the -h and --help options
     to the main app will print a list of the commands.
 
@@ -65,7 +75,7 @@ class HelpAction(argparse.Action):
             else:
                 dist_info = ''
             app.stdout.write('  %-13s  %s%s\n' % (name, one_liner, dist_info))
-        sys.exit(0)
+        raise HelpExit()
 
 
 class HelpCommand(command.Command):

cliff/tests/test_help.py

@@ -42,7 +42,7 @@ class TestHelp(base.TestBase):
         parsed_args = parser.parse_args(['one'])
         try:
             help_cmd.run(parsed_args)
-        except SystemExit:
+        except help.HelpExit:
             pass
         self.assertEqual('TestParser', stdout.getvalue())
 
@@ -60,7 +60,7 @@ class TestHelp(base.TestBase):
         parsed_args = parser.parse_args(['t'])
         try:
             help_cmd.run(parsed_args)
-        except SystemExit:
+        except help.HelpExit:
             pass
         help_output = stdout.getvalue()
         self.assertIn('Command "t" matches:', help_output)
@@ -99,7 +99,7 @@ class TestHelp(base.TestBase):
         parsed_args = parser.parse_args([])
         try:
             help_cmd.run(parsed_args)
-        except SystemExit:
+        except help.HelpExit:
             pass
         help_text = stdout.getvalue()
         basecommand = os.path.split(sys.argv[0])[1]
@@ -122,7 +122,7 @@ class TestHelp(base.TestBase):
         app.NAME = 'test'
         try:
             app.run(['--help'])
-        except SystemExit:
+        except help.HelpExit:
             pass
         help_output = stdout.getvalue()
         self.assertIn('two words', help_output)
@@ -144,7 +144,7 @@ class TestHelp(base.TestBase):
         parsed_args = parser.parse_args([])
         try:
             help_cmd.run(parsed_args)
-        except SystemExit:
+        except help.HelpExit:
             pass
         help_output = stdout.getvalue()
         self.assertIn('Commands:', help_output)
@@ -166,7 +166,7 @@ class TestHelp(base.TestBase):
         parsed_args = parser.parse_args([])
         try:
             help_cmd.run(parsed_args)
-        except SystemExit:
+        except help.HelpExit:
             pass
         help_output = stdout.getvalue()
         self.assertIn('Commands:', help_output)