Allow tasks to be used with placement groups (#9738)

python/ray/experimental/placement_group.py

@@ -4,7 +4,7 @@ from typing import (List, Dict)
 
 def placement_group(bundles: List[Dict[str, float]],
                     strategy: str = "PACK",
-                    name: str = None):
+                    name: str = "unnamed_group"):
     """
     Create a placement group.
 
@@ -21,6 +21,13 @@ def placement_group(bundles: List[Dict[str, float]],
         name: The name of the placement group.
     """
     worker = ray.worker.global_worker
+    worker.check_connected()
+
+    if not isinstance(bundles, list):
+        raise ValueError(
+            "The type of bundles must be list, got {}".format(bundles))
+
     placement_group_id = worker.core_worker.create_placement_group(
         name, bundles, strategy)
+
     return placement_group_id