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

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:

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):

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`.