commit 8e4c96295bd78ae5f70b908e5dbac0da7c4c21bd
author Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com> Wed Dec 15 04:20:04 2021 -0800
committer GitHub <noreply@github.com> Wed Dec 15 12:20:04 2021 +0000
tree ebbc320a1cfd9fad4c2c2a7fbdb3241d816216e0
parent 908fd691f96403a3c30d85c17dd74ed1f26a60fd

bpo-26952: [argparse] clearer error when formatting an empty mutually… (GH-30099) (GH-30114)

(cherry picked from commit 86de99588db3beff964137f4fe27dd1077a09b35)

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>

Lib/argparse.py

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 @@ def _format_actions_usage(self, actions, groups):
         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:

Lib/test/test_argparse.py

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 @@ def test_help(self):
               '''
         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):