From 92b531b8589b733c4e44e291f08271fa34947400 Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Fri, 11 Nov 2022 18:30:38 -0800
Subject: [PATCH] gh-80448: argparse: Fix IndexError on store_true action
 (GH-15656)
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(cherry picked from commit e02cc6d42aee1f0a9ee629f76576eee478224d9d)

Co-authored-by: Hai Shi <shihai1992@gmail.com>
Co-authored-by: Rémi Lapeyre <remi.lapeyre@henki.fr>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>
---
 Lib/argparse.py                                             | 6 +++++-
 Lib/test/test_argparse.py                                   | 2 +-
 .../next/Library/2019-09-03-15-45-19.bpo-36267.z42Ex7.rst   | 1 +
 3 files changed, 7 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2019-09-03-15-45-19.bpo-36267.z42Ex7.rst

diff --git a/Lib/argparse.py b/Lib/argparse.py
index 1c5520c4b41..77619088614 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1997,7 +1997,11 @@ def consume_optional(start_index):
                     # arguments, try to parse more single-dash options out
                     # of the tail of the option string
                     chars = self.prefix_chars
-                    if arg_count == 0 and option_string[1] not in chars:
+                    if (
+                        arg_count == 0
+                        and option_string[1] not in chars
+                        and explicit_arg != ''
+                    ):
                         action_tuples.append((action, [], option_string))
                         char = option_string[0]
                         option_string = char + explicit_arg[0]
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index 25ac03cf148..1acecbb8aba 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -296,7 +296,7 @@ class TestOptionalsSingleDashCombined(ParserTestCase):
         Sig('-z'),
     ]
     failures = ['a', '--foo', '-xa', '-x --foo', '-x -z', '-z -x',
-                '-yx', '-yz a', '-yyyx', '-yyyza', '-xyza']
+                '-yx', '-yz a', '-yyyx', '-yyyza', '-xyza', '-x=']
     successes = [
         ('', NS(x=False, yyy=None, z=None)),
         ('-x', NS(x=True, yyy=None, z=None)),
diff --git a/Misc/NEWS.d/next/Library/2019-09-03-15-45-19.bpo-36267.z42Ex7.rst b/Misc/NEWS.d/next/Library/2019-09-03-15-45-19.bpo-36267.z42Ex7.rst
new file mode 100644
index 00000000000..7c9b592d6ec
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-09-03-15-45-19.bpo-36267.z42Ex7.rst
@@ -0,0 +1 @@
+Fix IndexError in :class:`argparse.ArgumentParser` when a ``store_true`` action is given an explicit argument.