Fix dropped pipes when mutex args wrap

Author: savannahostrowski

Lib/argparse.py

@@ -355,8 +355,14 @@ def _format_usage(self, usage, actions, groups, prefix):
             if len(prefix) + len(self._decolor(usage)) > text_width:
 
                 # break usage into wrappable parts
-                opt_parts = self._get_actions_usage_parts(optionals, groups)
-                pos_parts = self._get_actions_usage_parts(positionals, groups)
+                # keep optionals and positionals together to preserve
+                # mutually exclusive group formatting (gh-75949)
+                all_actions = optionals + positionals
+                parts, pos_start = self._get_actions_usage_parts_split(
+                    all_actions, groups, len(optionals)
+                )
+                opt_parts = parts[:pos_start]
+                pos_parts = parts[pos_start:]
 
                 # helper for wrapping lines
                 def get_lines(parts, indent, prefix=None):
@@ -420,6 +426,17 @@ def _is_long_option(self, string):
         return len(string) > 2
 
     def _get_actions_usage_parts(self, actions, groups):
+        parts, _ = self._get_actions_usage_parts_split(actions, groups, None)
+        return parts
+
+    def _get_actions_usage_parts_split(self, actions, groups, opt_count):
+        """Get usage parts with split index for optionals/positionals.
+
+        Returns (parts, pos_start) where pos_start is the index in parts
+        where positionals begin. When opt_count is None, pos_start is None.
+        This preserves mutually exclusive group formatting across the
+        optionals/positionals boundary (gh-75949).
+        """
         # find group indices and identify actions in groups
         group_actions = set()
         inserts = {}
@@ -515,8 +532,16 @@ def _get_actions_usage_parts(self, actions, groups):
             for i in range(start + group_size, end):
                 parts[i] = None
 
-        # return the usage parts
-        return [item for item in parts if item is not None]
+        # calculate the split point for optionals/positionals
+        # before filtering out None entries
+        if opt_count is not None:
+            # Count non-None parts in the optionals section
+            pos_start = sum(1 for p in parts[:opt_count] if p is not None)
+        else:
+            pos_start = None
+
+        # return the usage parts and split point
+        return [item for item in parts if item is not None], pos_start
 
     def _format_text(self, text):
         if '%(prog)' in text:

Lib/test/test_argparse.py

@@ -4966,6 +4966,28 @@ def test_long_mutex_groups_wrap(self):
         ''')
         self.assertEqual(parser.format_usage(), usage)
 
+    def test_mutex_groups_with_mixed_optionals_positionals_wrap(self):
+        # https://github.com/python/cpython/issues/75949
+        # Mutually exclusive groups containing both optionals and positionals
+        # should preserve pipe separators when the usage line wraps.
+        parser = argparse.ArgumentParser(prog='PROG')
+        g = parser.add_mutually_exclusive_group()
+        g.add_argument('-v', '--verbose', action='store_true')
+        g.add_argument('-q', '--quiet', action='store_true')
+        g.add_argument('-x', '--extra-long-option-name', nargs='?')
+        g.add_argument('-y', '--yet-another-long-option', nargs='?')
+        g.add_argument('positional1', nargs='?')
+        g.add_argument('positional2', nargs='?')
+        g.add_argument('positional3', nargs='?')
+        g.add_argument('positional4', nargs='?')
+
+        usage = textwrap.dedent('''\
+        usage: PROG [-h] [-v | -q | -x [EXTRA_LONG_OPTION_NAME] |
+                    -y [YET_ANOTHER_LONG_OPTION] |
+                    positional1 | positional2 | positional3 | positional4]
+        ''')
+        self.assertEqual(parser.format_usage(), usage)
+
 
 class TestHelpVariableExpansion(HelpTestCase):
     """Test that variables are expanded properly in help messages"""