GH-75949: Fix argparse dropping '|' in mutually exclusive groups on line wrap (#142312)

Lib/argparse.py

@@ -353,8 +353,14 @@ 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_with_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):
@@ -418,6 +424,17 @@ def _is_long_option(self, string):
         return len(string) > 2
 
     def _get_actions_usage_parts(self, actions, groups):
+        parts, _ = self._get_actions_usage_parts_with_split(actions, groups)
+        return parts
+
+    def _get_actions_usage_parts_with_split(self, actions, groups, opt_count=None):
+        """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 = {}
@@ -513,8 +530,16 @@ 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]
+        # if opt_count is provided, calculate where positionals start in
+        # the final parts list (for wrapping onto separate lines).
+        # Count before filtering None entries since indices shift after.
+        if opt_count is not None:
+            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 (gh-75949)
+        return [item for item in parts if item is not None], pos_start
 
     def _format_text(self, text):
         if '%(prog)' in text: