XD-DENG commented on a change in pull request #3688: [AIRFLOW-2843] 
ExternalTaskSensor-check if external task exists
URL: https://github.com/apache/incubator-airflow/pull/3688#discussion_r207610690
 
 

 ##########
 File path: airflow/sensors/external_task_sensor.py
 ##########
 @@ -70,9 +76,24 @@ def __init__(self,
         self.execution_date_fn = execution_date_fn
         self.external_dag_id = external_dag_id
         self.external_task_id = external_task_id
+        self.check_existence = check_existence
 
     @provide_session
     def poke(self, context, session=None):
+        TI = TaskInstance
+
+        if self.check_existence:
+            existence = session.query(TI).filter(
+                TI.dag_id == self.external_dag_id,
+                TI.task_id == self.external_task_id,
+            ).count()
+            session.commit()
+            if existence == 0:
+                raise AirflowException('The external task "' +
 
 Review comment:
   But I do agree this feature may be too specific and only applicable to 
limited use cases (like some I'm dealing with).
   
   Please share your thoughts, and feel free to close this PR should you find 
pro < con. Thanks.

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