From 79805d228440814c0674ab5190ef17f235503d2e Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Mon, 11 Nov 2024 18:28:30 +0200
Subject: [PATCH] gh-117941: Reject option names starting with "--no-" in
 argparse.BooleanOptionalAction (GH-125894)

They never worked correctly.
---
 Lib/argparse.py                                            | 3 +++
 Lib/test/test_argparse.py                                  | 7 +++++++
 .../Library/2024-10-23-20-44-30.gh-issue-117941.Y9jdlW.rst | 2 ++
 3 files changed, 12 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2024-10-23-20-44-30.gh-issue-117941.Y9jdlW.rst

diff --git a/Lib/argparse.py b/Lib/argparse.py
index 072cd5e7dc0..5ecfdca1717 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -863,6 +863,9 @@ class BooleanOptionalAction(Action):
             _option_strings.append(option_string)
 
             if option_string.startswith('--'):
+                if option_string.startswith('--no-'):
+                    raise ValueError(f'invalid option name {option_string!r} '
+                                     f'for BooleanOptionalAction')
                 option_string = '--no-' + option_string[2:]
                 _option_strings.append(option_string)
 
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index ba987657038..cbf119ed2da 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -789,6 +789,13 @@ class TestBooleanOptionalAction(ParserTestCase):
 
         self.assertIn("got an unexpected keyword argument 'const'", str(cm.exception))
 
+    def test_invalid_name(self):
+        parser = argparse.ArgumentParser()
+        with self.assertRaises(ValueError) as cm:
+            parser.add_argument('--no-foo', action=argparse.BooleanOptionalAction)
+        self.assertEqual(str(cm.exception),
+                         "invalid option name '--no-foo' for BooleanOptionalAction")
+
 class TestBooleanOptionalActionRequired(ParserTestCase):
     """Tests BooleanOptionalAction required"""
 
diff --git a/Misc/NEWS.d/next/Library/2024-10-23-20-44-30.gh-issue-117941.Y9jdlW.rst b/Misc/NEWS.d/next/Library/2024-10-23-20-44-30.gh-issue-117941.Y9jdlW.rst
new file mode 100644
index 00000000000..9c2553f0f0e
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-10-23-20-44-30.gh-issue-117941.Y9jdlW.rst
@@ -0,0 +1,2 @@
+:class:`!argparse.BooleanOptionalAction` now rejects option names starting
+with ``--no-``.