Patch python3.7's argparse.py to support overriding the strings that are shown as default values for flags

argparse_help_default.diff

diff --git a/Lib/argparse.py b/Lib/argparse.py
index 209b4e9..254b803 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -665,11 +665,11 @@ class ArgumentDefaultsHelpFormatter(HelpFormatter):
 
     def _get_help_string(self, action):
         help = action.help
-        if '%(default)' not in action.help:
+        if '%(default)' not in help and '%(help_default)' not in help:
             if action.default is not SUPPRESS:
                 defaulting_nargs = [OPTIONAL, ZERO_OR_MORE]
                 if action.option_strings or action.nargs in defaulting_nargs:
-                    help += ' (default: %(default)s)'
+                    help += ' (default: %(help_default)s)'
         return help
 
 
@@ -782,6 +782,13 @@ class Action(_AttributeHolder):
 
         - help -- The help string describing the argument.
 
+        - help_default -- A string describing the default value of the
+            argument to the user, if the internal Pythonic default value would
+            be confusing. For instance some flags to interactive programs need
+            help_default="Ask"; another common pattern is
+            '--disable-thing', dest='thing', action='store_false', default=True, 
+            help_default='False' or help_default='Enabled'.
+            
         - metavar -- The name to be used for the option's argument with the
             help string. If None, the 'dest' value will be used as the name.
     """
@@ -796,6 +803,7 @@ class Action(_AttributeHolder):
                  choices=None,
                  required=False,
                  help=None,
+                 help_default=None,
                  metavar=None):
         self.option_strings = option_strings
         self.dest = dest
@@ -806,6 +814,7 @@ class Action(_AttributeHolder):
         self.choices = choices
         self.required = required
         self.help = help
+        self.help_default = default if help_default is None else help_default
         self.metavar = metavar
 
     def _get_kwargs(self):
@@ -818,6 +827,7 @@ class Action(_AttributeHolder):
             'type',
             'choices',
             'help',
+            'help_default',
             'metavar',
         ]
         return [(name, getattr(self, name)) for name in names]
@@ -838,6 +848,7 @@ class _StoreAction(Action):
                  choices=None,
                  required=False,
                  help=None,
+                 help_default=None,
                  metavar=None):
         if nargs == 0:
             raise ValueError('nargs for store actions must be > 0; if you '
@@ -855,6 +866,7 @@ class _StoreAction(Action):
             choices=choices,
             required=required,
             help=help,
+            help_default=help_default,
             metavar=metavar)
 
     def __call__(self, parser, namespace, values, option_string=None):
@@ -870,6 +882,7 @@ class _StoreConstAction(Action):
                  default=None,
                  required=False,
                  help=None,
+                 help_default=None,
                  metavar=None):
         super(_StoreConstAction, self).__init__(
             option_strings=option_strings,
@@ -878,7 +891,8 @@ class _StoreConstAction(Action):
             const=const,
             default=default,
             required=required,
-            help=help)
+            help=help,
+            help_default=help_default)
 
     def __call__(self, parser, namespace, values, option_string=None):
         setattr(namespace, self.dest, self.const)
@@ -891,14 +905,16 @@ class _StoreTrueAction(_StoreConstAction):
                  dest,
                  default=False,
                  required=False,
-                 help=None):
+                 help=None,
+                 help_default=None):
         super(_StoreTrueAction, self).__init__(
             option_strings=option_strings,
             dest=dest,
             const=True,
             default=default,
             required=required,
-            help=help)
+            help=help,
+            help_default=help_default)
 
 
 class _StoreFalseAction(_StoreConstAction):
@@ -908,14 +924,16 @@ class _StoreFalseAction(_StoreConstAction):
                  dest,
                  default=True,
                  required=False,
-                 help=None):
+                 help=None,
+                 help_default=None):
         super(_StoreFalseAction, self).__init__(
             option_strings=option_strings,
             dest=dest,
             const=False,
             default=default,
             required=required,
-            help=help)
+            help=help,
+            help_default=help_default)
 
 
 class _AppendAction(Action):
@@ -930,6 +948,7 @@ class _AppendAction(Action):
                  choices=None,
                  required=False,
                  help=None,
+                 help_default=None,
                  metavar=None):
         if nargs == 0:
             raise ValueError('nargs for append actions must be > 0; if arg '
@@ -947,6 +966,7 @@ class _AppendAction(Action):
             choices=choices,
             required=required,
             help=help,
+            help_default=help_default,
             metavar=metavar)
 
     def __call__(self, parser, namespace, values, option_string=None):
@@ -964,6 +984,7 @@ class _AppendConstAction(Action):
                  default=None,
                  required=False,
                  help=None,
+                 help_default=None,
                  metavar=None):
         super(_AppendConstAction, self).__init__(
             option_strings=option_strings,
@@ -973,6 +994,7 @@ class _AppendConstAction(Action):
             default=default,
             required=required,
             help=help,
+            help_default=help_default,
             metavar=metavar)
 
     def __call__(self, parser, namespace, values, option_string=None):
@@ -988,14 +1010,16 @@ class _CountAction(Action):
                  dest,
                  default=None,
                  required=False,
-                 help=None):
+                 help=None,
+                 help_default=None):
         super(_CountAction, self).__init__(
             option_strings=option_strings,
             dest=dest,
             nargs=0,
             default=default,
             required=required,
-            help=help)
+            help=help,
+            help_default=help_default)
 
     def __call__(self, parser, namespace, values, option_string=None):
         new_count = _ensure_value(namespace, self.dest, 0) + 1
@@ -1008,13 +1032,15 @@ class _HelpAction(Action):
                  option_strings,
                  dest=SUPPRESS,
                  default=SUPPRESS,
-                 help=None):
+                 help=None,
+                 help_default=None):
         super(_HelpAction, self).__init__(
             option_strings=option_strings,
             dest=dest,
             default=default,
             nargs=0,
-            help=help)
+            help=help,
+            help_default=help_default)
 
     def __call__(self, parser, namespace, values, option_string=None):
         parser.print_help()
@@ -1065,6 +1091,7 @@ class _SubParsersAction(Action):
                  parser_class,
                  dest=SUPPRESS,
                  help=None,
+                 help_default=None,
                  metavar=None):
 
         self._prog_prefix = prog
@@ -1078,6 +1105,7 @@ class _SubParsersAction(Action):
             nargs=PARSER,
             choices=self._name_parser_map,
             help=help,
+            help_default=help_default,
             metavar=metavar)
 
     def add_parser(self, name, **kwargs):
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index bc83161..d7f95b9 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -4010,6 +4010,8 @@ class TestHelpArgumentDefaults(HelpTestCase):
     argument_signatures = [
         Sig('--foo', help='foo help - oh and by the way, %(default)s'),
         Sig('--bar', action='store_true', help='bar help'),
+        Sig('--disable-floopery', action='store_false', help='disable floopery',
+            dest='floopery', default=True, help_default='False'),
         Sig('spam', help='spam help'),
         Sig('badger', nargs='?', default='wooden', help='badger help'),
     ]
@@ -4018,25 +4020,27 @@ class TestHelpArgumentDefaults(HelpTestCase):
          [Sig('--baz', type=int, default=42, help='baz help')]),
     ]
     usage = '''\
-        usage: PROG [-h] [--foo FOO] [--bar] [--baz BAZ] spam [badger]
+        usage: PROG [-h] [--foo FOO] [--bar] [--disable-floopery] [--baz BAZ]
+                    spam [badger]
         '''
     help = usage + '''\
 
         description
 
         positional arguments:
-          spam        spam help
-          badger      badger help (default: wooden)
+          spam                spam help
+          badger              badger help (default: wooden)
 
         optional arguments:
-          -h, --help  show this help message and exit
-          --foo FOO   foo help - oh and by the way, None
-          --bar       bar help (default: False)
+          -h, --help          show this help message and exit
+          --foo FOO           foo help - oh and by the way, None
+          --bar               bar help (default: False)
+          --disable-floopery  disable floopery (default: False)
 
         title:
           description
 
-          --baz BAZ   baz help (default: 42)
+          --baz BAZ           baz help (default: 42)
         '''
     version = ''
 
@@ -4488,7 +4492,7 @@ class TestStrings(TestCase):
         string = (
             "Action(option_strings=['--foo', '-a', '-b'], dest='b', "
             "nargs='+', const=None, default=42, type='int', "
-            "choices=[1, 2, 3], help='HELP', metavar='METAVAR')")
+            "choices=[1, 2, 3], help='HELP', help_default=42, metavar='METAVAR')")
         self.assertStringEqual(option, string)
 
     def test_argument(self):
@@ -4504,7 +4508,7 @@ class TestStrings(TestCase):
         string = (
             "Action(option_strings=[], dest='x', nargs='?', "
             "const=None, default=2.5, type=%r, choices=[0.5, 1.5, 2.5], "
-            "help='H HH H', metavar='MV MV MV')" % float)
+            "help='H HH H', help_default=2.5, metavar='MV MV MV')" % float)
         self.assertStringEqual(argument, string)
 
     def test_namespace(self):