From 8e4c96295bd78ae5f70b908e5dbac0da7c4c21bd Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Wed, 15 Dec 2021 04:20:04 -0800
Subject: =?UTF-8?q?bpo-26952:=20[argparse]=20clearer=20error=20when=20form?=
 =?UTF-8?q?atting=20an=20empty=20mutually=E2=80=A6=20(GH-30099)=20(GH-3011?=
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(cherry picked from commit 86de99588db3beff964137f4fe27dd1077a09b35)

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>
---
 Lib/argparse.py                                                   | 3 +++
 Lib/test/test_argparse.py                                         | 7 +++++++
 Misc/NEWS.d/next/Library/2021-12-14-13-18-45.bpo-26952.hjhISq.rst | 1 +
 3 files changed, 11 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2021-12-14-13-18-45.bpo-26952.hjhISq.rst

diff --git a/Lib/argparse.py b/Lib/argparse.py
index 9eed24c..b2db312 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -392,6 +392,9 @@ class HelpFormatter(object):
         group_actions = set()
         inserts = {}
         for group in groups:
+            if not group._group_actions:
+                raise ValueError(f'empty group {group}')
+
             try:
                 start = actions.index(group._group_actions[0])
             except ValueError:
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index ec49b2a..c96a540 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -2582,6 +2582,13 @@ class TestMutuallyExclusiveGroupErrors(TestCase):
               '''
         self.assertEqual(parser.format_help(), textwrap.dedent(expected))
 
+    def test_empty_group(self):
+        # See issue 26952
+        parser = argparse.ArgumentParser()
+        group = parser.add_mutually_exclusive_group()
+        with self.assertRaises(ValueError):
+            parser.parse_args(['-h'])
+
 class MEMixin(object):
 
     def test_failures_when_not_required(self):
diff --git a/Misc/NEWS.d/next/Library/2021-12-14-13-18-45.bpo-26952.hjhISq.rst b/Misc/NEWS.d/next/Library/2021-12-14-13-18-45.bpo-26952.hjhISq.rst
new file mode 100644
index 0000000..379dbb5
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-12-14-13-18-45.bpo-26952.hjhISq.rst
@@ -0,0 +1 @@
+:mod:`argparse` raises :exc:`ValueError` with clear message when trying to render usage for an empty mutually-exclusive group. Previously it raised a cryptic :exc:`IndexError`.
\ No newline at end of file
-- 
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