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

2 files changed, 10 insertions, 0 deletions

diff --git a/Lib/argparse.py b/Lib/argparse.py
index b44fa4f0f6..8a81801ba9 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 f3edde3de8..eb37d4d365 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -2601,6 +2601,13 @@ 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):