Disallow calling expand with no arguments

airflow/decorators/base.py

@@ -312,6 +312,9 @@ def _validate_arg_names(self, func: ValidationSource, kwargs: Dict[str, Any]):
             raise TypeError(f"{func}() got unexpected keyword arguments {names}")
 
     def expand(self, **map_kwargs: "Mappable") -> XComArg:
+        if not map_kwargs:
+            raise TypeError("no arguments to expand against")
+
         self._validate_arg_names("expand", map_kwargs)
         prevent_duplicates(self.kwargs, map_kwargs, fail_reason="mapping already partial")
         ensure_xcomarg_return_value(map_kwargs)

airflow/models/mappedoperator.py

@@ -191,6 +191,11 @@ def __del__(self):
             warnings.warn(f"Task {task_id} was never mapped!")
 
     def expand(self, **mapped_kwargs: "Mappable") -> "MappedOperator":
+        if not mapped_kwargs:
+            raise TypeError("no arguments to expand against")
+        return self._expand(**mapped_kwargs)
+
+    def _expand(self, **mapped_kwargs: "Mappable") -> "MappedOperator":
         self._expand_called = True
 
         from airflow.operators.empty import EmptyOperator

airflow/serialization/serialized_objects.py

@@ -96,7 +96,8 @@ def _get_default_mapped_partial() -> Dict[str, Any]:
     are defaults, they are automatically supplied on de-serialization, so we
     don't need to store them.
     """
-    default_partial_kwargs = BaseOperator.partial(task_id="_").expand().partial_kwargs
+    # Use the private _expand() method to avoid the empty kwargs check.
+    default_partial_kwargs = BaseOperator.partial(task_id="_")._expand().partial_kwargs
     return BaseSerialization._serialize(default_partial_kwargs)[Encoding.VAR]
 
 

tests/api_connexion/endpoints/test_task_endpoint.py

@@ -67,8 +67,10 @@ def setup_dag(self, configured_app):
             task2 = EmptyOperator(task_id=self.task_id2, start_date=self.task2_start_date)
 
         with DAG(self.mapped_dag_id, start_date=self.task1_start_date) as mapped_dag:
-            task3 = EmptyOperator(task_id=self.task_id3)  # noqa
-            mapped_task = EmptyOperator.partial(task_id=self.mapped_task_id).expand()  # noqa
+            EmptyOperator(task_id=self.task_id3)
+            # Use the private _expand() method to avoid the empty kwargs check.
+            # We don't care about how the operator runs here, only its presence.
+            EmptyOperator.partial(task_id=self.mapped_task_id)._expand()
 
         task1 >> task2
         dag_bag = DagBag(os.devnull, include_examples=False)

tests/models/test_taskinstance.py

@@ -1053,7 +1053,9 @@ def test_xcom_pull(self, dag_maker):
 
     def test_xcom_pull_mapped(self, dag_maker, session):
         with dag_maker(dag_id="test_xcom", session=session):
-            task_1 = EmptyOperator.partial(task_id="task_1").expand()
+            # Use the private _expand() method to avoid the empty kwargs check.
+            # We don't care about how the operator runs here, only its presence.
+            task_1 = EmptyOperator.partial(task_id="task_1")._expand()
             EmptyOperator(task_id="task_2")
 
         dagrun = dag_maker.create_dagrun(start_date=timezone.datetime(2016, 6, 1, 0, 0, 0))
@@ -2763,7 +2765,9 @@ def cmds():
 def test_ti_xcom_pull_on_mapped_operator_return_lazy_iterable(mock_deserialize_value, dag_maker, session):
     """Ensure we access XCom lazily when pulling from a mapped operator."""
     with dag_maker(dag_id="test_xcom", session=session):
-        task_1 = EmptyOperator.partial(task_id="task_1").expand()
+        # Use the private _expand() method to avoid the empty kwargs check.
+        # We don't care about how the operator runs here, only its presence.
+        task_1 = EmptyOperator.partial(task_id="task_1")._expand()
         EmptyOperator(task_id="task_2")
 
     dagrun = dag_maker.create_dagrun()