allow overriding of argparse.ArgumentDefaultsHelpFormatter default values

argparse_default_override.diff

diff --git a/Doc/library/argparse.rst b/Doc/library/argparse.rst
index c6b2bf6..15f2fe8 100644
--- a/Doc/library/argparse.rst
+++ b/Doc/library/argparse.rst
@@ -429,13 +429,15 @@ should not be line-wrapped::
 including argument descriptions.
 
 :class:`ArgumentDefaultsHelpFormatter` automatically adds information about
-default values to each of the argument help messages::
+default values to each of the argument help messages, unless they already
+contain a ``(default:`` substring or a ``%(default)s`` format specifier::
 
    >>> parser = argparse.ArgumentParser(
    ...     prog='PROG',
    ...     formatter_class=argparse.ArgumentDefaultsHelpFormatter)
    >>> parser.add_argument('--foo', type=int, default=42, help='FOO!')
    >>> parser.add_argument('bar', nargs='*', default=[1, 2, 3], help='BAR!')
+   >>> parser.add_argument('--bleep', help='bleep (default: override)')
    >>> parser.print_help()
    usage: PROG [-h] [--foo FOO] [bar [bar ...]]
 
diff --git a/Lib/argparse.py b/Lib/argparse.py
index 209b4e9..c5b2755 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -665,7 +665,7 @@ class ArgumentDefaultsHelpFormatter(HelpFormatter):
 
     def _get_help_string(self, action):
         help = action.help
-        if '%(default)' not in action.help:
+        if '%(default)' not in action.help and '(default:' not in action.help:
             if action.default is not SUPPRESS:
                 defaulting_nargs = [OPTIONAL, ZERO_OR_MORE]
                 if action.option_strings or action.nargs in defaulting_nargs:
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index bc83161..f0432be 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -4010,6 +4010,9 @@ 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('--quiz', action='store_true', help='should we have a quiz (default: Ask)'),
+        Sig('--disable-floopery', action='store_false', help='disable floopery (default: Enabled)',
+            dest='floopery', default=True),
         Sig('spam', help='spam help'),
         Sig('badger', nargs='?', default='wooden', help='badger help'),
     ]
@@ -4018,25 +4021,28 @@ 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] [--quiz] [--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)
+          --quiz              should we have a quiz (default: Ask)
+          --disable-floopery  disable floopery (default: Enabled)
 
         title:
           description
 
-          --baz BAZ   baz help (default: 42)
+          --baz BAZ           baz help (default: 42)
         '''
     version = ''