Oops, fixing patch

airflow/operators/sensors.py

@@ -123,7 +123,7 @@ class ExternalTaskSensor(BaseSensorOperator):
         TI = TaskInstance
         session = settings.Session()
         import dateutil.parser
-        dttm = dateutil.parser.parse(dttm)
+        self.execution_date = dateutil.parser.parse(self.execution_date)
         count = session.query(TI).filter(
             TI.dag_id == self.external_dag_id,
             TI.task_id == self.external_task_id,