From 2dbeeb739bdadf6099f56340f890812efe480be5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20G=C3=B3rny?= <mgorny@gentoo.org>
Date: Sun, 1 Jun 2025 07:56:56 +0200
Subject: [PATCH] gh-134970: Fix exception message in argparse module
 (GH-134971)
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Fix the "unknown action" exception in argparse.ArgumentParser.add_argument_group()
to correctly replace the action class.
(cherry picked from commit 965c48056633d3f4b41520c8cd07f0275f00fb4c)

Co-authored-by: Michał Górny <mgorny@gentoo.org>
---
 Lib/argparse.py                                                | 2 +-
 .../Library/2025-05-31-12-08-12.gh-issue-134970.lgSaxq.rst     | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-05-31-12-08-12.gh-issue-134970.lgSaxq.rst

diff --git a/Lib/argparse.py b/Lib/argparse.py
index d1a6350c3fda6d..83258cf3e0f37d 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1534,7 +1534,7 @@ def add_argument(self, *args, **kwargs):
         action_name = kwargs.get('action')
         action_class = self._pop_action_class(kwargs)
         if not callable(action_class):
-            raise ValueError('unknown action {action_class!r}')
+            raise ValueError(f'unknown action {action_class!r}')
         action = action_class(**kwargs)
 
         # raise an error if action for positional argument does not
diff --git a/Misc/NEWS.d/next/Library/2025-05-31-12-08-12.gh-issue-134970.lgSaxq.rst b/Misc/NEWS.d/next/Library/2025-05-31-12-08-12.gh-issue-134970.lgSaxq.rst
new file mode 100644
index 00000000000000..20f53569ef4566
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-05-31-12-08-12.gh-issue-134970.lgSaxq.rst
@@ -0,0 +1,3 @@
+Fix the "unknown action" exception in
+:meth:`argparse.ArgumentParser.add_argument_group` to correctly replace the
+action class.