[Bugfix] Support append/extend/count actions in TrackingArgumentParser (#4639)

Signed-off-by: Junhee Yoo <junhee.yoo@navercorp.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

vllm_omni/utils/tracking_parser.py

@@ -9,6 +9,29 @@ from vllm.utils.argparse_utils import FlexibleArgumentParser
 UNSET = object()
 
 
+def build_shadow_kwargs(kwargs: dict[str, Any]) -> dict[str, Any]:
+    """Build kwargs for the shadow argument with an ``UNSET`` default.
+
+    Actions that mutate their default in place (append/extend/append_const/
+    count) would crash on the bare ``UNSET`` sentinel, so they are remapped to
+    an equivalent store-style action; the shadow value only needs to flip away
+    from ``UNSET`` when the arg is passed explicitly.
+    """
+    shadow_kwargs = {**kwargs, "default": UNSET}
+    action = kwargs.get("action")
+
+    if action in ("append", "extend"):
+        shadow_kwargs["action"] = "store"
+
+    elif action in ("append_const", "count"):
+        shadow_kwargs["action"] = "store_const"
+
+        if action == "count":
+            shadow_kwargs["const"] = True
+
+    return shadow_kwargs
+
+
 class TrackingNamespace(argparse.Namespace):
     """Proxy that wraps an argparse namespace with explicit keys, which
     can be filtered down to a dict containing only explicitly passed values.
@@ -58,7 +81,7 @@ class TrackingGroup:
     def add_argument(self, *args: Any, **kwargs: Any) -> argparse.Action:
         """Add an argument to the real group and to the shadow group."""
         action = self._real.add_argument(*args, **kwargs)
-        default_kwargs = {**kwargs, "default": UNSET}
+        default_kwargs = build_shadow_kwargs(kwargs)
         self._shadow.add_argument(*args, **default_kwargs)
         return action
 
@@ -115,7 +138,7 @@ class TrackingArgumentParser(FlexibleArgumentParser):
     def add_argument(self, *args: Any, **kwargs: Any) -> argparse.Action:
         """Add an arg to the parser & the shadow, where the latter has UNSET for the default."""
         action = super().add_argument(*args, **kwargs)
-        shadow_kwargs = {**kwargs, "default": UNSET}
+        shadow_kwargs = build_shadow_kwargs(kwargs)
         self._shadow.add_argument(*args, **shadow_kwargs)
         return action