gh-95468: Fix handling of multiple "--" (double dashes) in argparse

Only the first one has now been removed, all subsequent ones are now
taken literally.

Author: serhiy-storchaka

Lib/argparse.py

@@ -1916,12 +1916,14 @@ def _parse_known_args(self, arg_strings, namespace):
         # which has an 'O' if there is an option at an index,
         # an 'A' if there is an argument, or a '-' if there is a '--'
         option_string_indices = {}
+        double_dash_index = len(arg_strings)
         arg_string_pattern_parts = []
         arg_strings_iter = iter(arg_strings)
         for i, arg_string in enumerate(arg_strings_iter):
 
             # all args after -- are non-options
             if arg_string == '--':
+                double_dash_index = i
                 arg_string_pattern_parts.append('-')
                 for arg_string in arg_strings_iter:
                     arg_string_pattern_parts.append('A')
@@ -2070,6 +2072,12 @@ def consume_positionals(start_index):
             # and add the Positional and its args to the list
             for action, arg_count in zip(positionals, arg_counts):
                 args = arg_strings[start_index: start_index + arg_count]
+                # Strip out the first '--' if it is not in PARSER or REMAINDER arg.
+                if (arg_count
+                    and start_index <= double_dash_index < start_index + arg_count
+                    and action.nargs not in [PARSER, REMAINDER]):
+                    assert args.index('--') == double_dash_index - start_index
+                    args.remove('--')
                 start_index += arg_count
                 if args and action.deprecated and action.dest not in warned:
                     self._warning(_("argument '%(argument_name)s' is deprecated") %
@@ -2470,13 +2478,6 @@ def parse_known_intermixed_args(self, args=None, namespace=None):
     # Value conversion methods
     # ========================
     def _get_values(self, action, arg_strings):
-        # for everything but PARSER, REMAINDER args, strip out first '--'
-        if not action.option_strings and action.nargs not in [PARSER, REMAINDER]:
-            try:
-                arg_strings.remove('--')
-            except ValueError:
-                pass
-
         # optional argument produces a default when not present
         if not arg_strings and action.nargs == OPTIONAL:
             if action.option_strings:

Lib/test/test_argparse.py

@@ -5721,7 +5721,30 @@ def test_zero_or_more_optional(self):
         self.assertEqual(NS(x=[]), args)
 
     def test_double_dash(self):
-        parser = argparse.ArgumentParser()
+        parser = argparse.ArgumentParser(exit_on_error=False)
+        parser.add_argument('-f', '--foo')
+        parser.add_argument('bar', nargs='*')
+
+        args = parser.parse_args(['--foo=--'])
+        self.assertEqual(NS(foo='--', bar=[]), args)
+        self.assertRaisesRegex(argparse.ArgumentError,
+            'argument -f/--foo: expected one argument',
+            parser.parse_args, ['--foo', '--'])
+        args = parser.parse_args(['-f--'])
+        self.assertEqual(NS(foo='--', bar=[]), args)
+        self.assertRaisesRegex(argparse.ArgumentError,
+            'argument -f/--foo: expected one argument',
+            parser.parse_args, ['-f', '--'])
+        args = parser.parse_args(['--foo', 'a', '--', 'b', 'c'])
+        self.assertEqual(NS(foo='a', bar=['b', 'c']), args)
+        args = parser.parse_args(['a', 'b', '--foo', 'c'])
+        self.assertEqual(NS(foo='c', bar=['a', 'b']), args)
+        args = parser.parse_args(['a', '--', 'b', '--foo', 'c'])
+        self.assertEqual(NS(foo=None, bar=['a', 'b', '--foo', 'c']), args)
+        args = parser.parse_args(['a', '--', 'b', '--', 'c', '--foo', 'd'])
+        self.assertEqual(NS(foo=None, bar=['a', 'b', '--', 'c', '--foo', 'd']), args)
+
+        parser = argparse.ArgumentParser(exit_on_error=False)
         parser.add_argument('-f', '--foo', nargs='*')
         parser.add_argument('bar', nargs='*')
 
@@ -5735,6 +5758,41 @@ def test_double_dash(self):
         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)
+        args = parser.parse_args(['a', 'b', '--foo', 'c', 'd'])
+        self.assertEqual(NS(foo=['c', 'd'], bar=['a', 'b']), args)
+        args = parser.parse_args(['a', '--', 'b', '--foo', 'c', 'd'])
+        self.assertEqual(NS(foo=None, bar=['a', 'b', '--foo', 'c', 'd']), args)
+        args, argv = parser.parse_known_args(['a', 'b', '--foo', 'c', '--', 'd'])
+        self.assertEqual(NS(foo=['c'], bar=['a', 'b']), args)
+        self.assertEqual(argv, ['--', 'd'])
+
+        parser = argparse.ArgumentParser(exit_on_error=False)
+        parser.add_argument('foo')
+        parser.add_argument('bar', nargs='*')
+
+        args = parser.parse_args(['--', 'a', 'b', 'c'])
+        self.assertEqual(NS(foo='a', bar=['b', 'c']), args)
+        args = parser.parse_args(['a', '--', 'b', 'c'])
+        self.assertEqual(NS(foo='a', bar=['b', 'c']), args)
+        args = parser.parse_args(['a', 'b', '--', 'c'])
+        self.assertEqual(NS(foo='a', bar=['b', 'c']), args)
+        args = parser.parse_args(['a', '--', 'b', '--', 'c'])
+        self.assertEqual(NS(foo='a', bar=['b', '--', 'c']), args)
+        args = parser.parse_args(['--', '--', 'a', '--', 'b', 'c'])
+        self.assertEqual(NS(foo='--', bar=['a', '--', 'b', 'c']), args)
+
+        parser = argparse.ArgumentParser(exit_on_error=False)
+        parser.add_argument('foo')
+        parser.add_argument('bar', nargs=argparse.REMAINDER)
+
+        args = parser.parse_args(['--', 'a', 'b', 'c'])
+        self.assertEqual(NS(foo='a', bar=['b', 'c']), args)
+        args = parser.parse_args(['a', '--', 'b', 'c'])
+        self.assertEqual(NS(foo='a', bar=['b', 'c']), args)
+        args = parser.parse_args(['a', 'b', '--', 'c'])
+        self.assertEqual(NS(foo='a', bar=['b', '--', 'c']), args)
+        args = parser.parse_args(['a', '--', 'b', '--', 'c'])
+        self.assertEqual(NS(foo='a', bar=['b', '--', 'c']), args)
 
 
 # ===========================

Misc/NEWS.d/next/Library/2024-09-19-10-36-18.gh-issue-95468.Hyhp_U.rst

@@ -0,0 +1,3 @@
+Fix handling of multiple ``"--"`` (double dashes) in :mod:`argparse`. Only
+the first one has now been removed, all subsequent ones are now taken
+literally.