pythongh-109475: Fix support of explicit option value "--" in argparse

For example "--option=--".

Lib/argparse.py

@@ -2489,7 +2489,7 @@ def parse_known_intermixed_args(self, args=None, namespace=None):
     # ========================
     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:

Lib/test/test_argparse.py

@@ -5405,6 +5405,22 @@ def test_zero_or_more_optional(self):
         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

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=--``).