bpo-46080: fix argparse help generation exception in edge case (GH-30111)

Fix an uncaught exception during help text generation when
argparse.BooleanOptionalAction is used with default=argparse.SUPPRESS
and help is specified.
(cherry picked from commit 9e87c0e)

Co-authored-by: Felix Fontein <[email protected]>

Author: miss-islington

Lib/argparse.py

@@ -878,7 +878,7 @@ def __init__(self,
                 option_string = '--no-' + option_string[2:]
                 _option_strings.append(option_string)
 
-        if help is not None and default is not None:
+        if help is not None and default is not None and default is not SUPPRESS:
             help += " (default: %(default)s)"
 
         super().__init__(

Lib/test/test_argparse.py

@@ -3597,6 +3597,8 @@ class TestHelpUsage(HelpTestCase):
         Sig('--bar', help='Whether to bar', default=True,
                      action=argparse.BooleanOptionalAction),
         Sig('-f', '--foobar', '--barfoo', action=argparse.BooleanOptionalAction),
+        Sig('--bazz', action=argparse.BooleanOptionalAction,
+                      default=argparse.SUPPRESS, help='Bazz!'),
     ]
     argument_group_signatures = [
         (Sig('group'), [
@@ -3609,8 +3611,8 @@ class TestHelpUsage(HelpTestCase):
     usage = '''\
         usage: PROG [-h] [-w W [W ...]] [-x [X ...]] [--foo | --no-foo]
                     [--bar | --no-bar]
-                    [-f | --foobar | --no-foobar | --barfoo | --no-barfoo] [-y [Y]]
-                    [-z Z Z Z]
+                    [-f | --foobar | --no-foobar | --barfoo | --no-barfoo]
+                    [--bazz | --no-bazz] [-y [Y]] [-z Z Z Z]
                     a b b [c] [d ...] e [e ...]
         '''
     help = usage + '''\
@@ -3627,6 +3629,7 @@ class TestHelpUsage(HelpTestCase):
           --foo, --no-foo       Whether to foo
           --bar, --no-bar       Whether to bar (default: True)
           -f, --foobar, --no-foobar, --barfoo, --no-barfoo
+          --bazz, --no-bazz     Bazz!
 
         group:
           -y [Y]                y

Misc/NEWS.d/next/Library/2021-12-15-06-29-00.bpo-46080.AuQpLt.rst

@@ -0,0 +1,3 @@
+Fix exception in argparse help text generation if a
+:class:`argparse.BooleanOptionalAction` argument's default is
+``argparse.SUPPRESS`` and it has ``help`` specified.  Patch by Felix Fontein.