[3.12] gh-109475: Fix support of explicit option value "--" in argparse (GH-114814) (GH-115036)

For example "--option=--".
(cherry picked from commit 4aa4f0906df9fc9c6c6f6657f2c521468c6b1688)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

3 files changed, 19 insertions, 1 deletions

diff --git a/Lib/argparse.py b/Lib/argparse.py
index 543d994..484a1ef 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -2488,7 +2488,7 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
     # ========================
     def _get_values(self, action, arg_strings):
         # for everything but PARSER, REMAINDER args, strip out first '--'
-        if action.nargs not in [PARSER, REMAINDER]:
+        if not action.option_strings and action.nargs not in [PARSER, REMAINDER]:
             try:
                 arg_strings.remove('--')
             except ValueError:
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index 3a62a16..88cc62a 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -5332,6 +5332,22 @@ class TestParseKnownArgs(TestCase):
         args = parser.parse_args([])
         self.assertEqual(NS(x=[]), args)
 
+    def test_double_dash(self):
+        parser = argparse.ArgumentParser()
+        parser.add_argument('-f', '--foo', nargs='*')
+        parser.add_argument('bar', nargs='*')
+
+        args = parser.parse_args(['--foo=--'])
+        self.assertEqual(NS(foo=['--'], bar=[]), args)
+        args = parser.parse_args(['--foo', '--'])
+        self.assertEqual(NS(foo=[], bar=[]), args)
+        args = parser.parse_args(['-f--'])
+        self.assertEqual(NS(foo=['--'], bar=[]), args)
+        args = parser.parse_args(['-f', '--'])
+        self.assertEqual(NS(foo=[], bar=[]), args)
+        args = parser.parse_args(['--foo', 'a', 'b', '--', 'c', 'd'])
+        self.assertEqual(NS(foo=['a', 'b'], bar=['c', 'd']), args)
+
 
 # ===========================
 # parse_intermixed_args tests
diff --git a/Misc/NEWS.d/next/Library/2024-01-31-20-07-11.gh-issue-109475.lmTb9S.rst b/Misc/NEWS.d/next/Library/2024-01-31-20-07-11.gh-issue-109475.lmTb9S.rst
new file mode 100644
index 0000000..7582cb2
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-01-31-20-07-11.gh-issue-109475.lmTb9S.rst
@@ -0,0 +1,2 @@
+Fix support of explicit option value "--" in :mod:`argparse` (e.g.
+``--option=--``).