[GitHub] [airflow] zhongjiajie commented on a change in pull request #4779: [AIRFLOW-3958] List tasks upstream work in chain

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zhongjiajie commented on a change in pull request #4779: [AIRFLOW-3958] List tasks upstream work in chain
URL: https://github.com/apache/airflow/pull/4779#discussion_r283672166
 
 

 ##########
 File path: airflow/utils/helpers.py
 ##########
 @@ -150,19 +150,40 @@ def as_flattened_list(iterable):
 
 
 def chain(*tasks):
-    """
+    r"""
     Given a number of tasks, builds a dependency chain.
+    Support mix airflow.models.BaseOperator and List[airflow.models.BaseOperator].
+    If you want to chain between two List[airflow.models.BaseOperator], have to
+    make sure they have same length.
 
-    chain(task_1, task_2, task_3, task_4)
+    chain(t1, [t2, t3], [t4, t5], t6)
 
     is equivalent to
 
-    task_1.set_downstream(task_2)
-    task_2.set_downstream(task_3)
-    task_3.set_downstream(task_4)
+      / -> t2 -> t4 \
+    t1               -> t6
+      \ -> t3 -> t5 /
+
+    t1.set_downstream(t2)
+    t1.set_downstream(t3)
+    t2.set_downstream(t4)
+    t3.set_downstream(t5)
+    t4.set_downstream(t6)
+    t5.set_downstream(t6)
+
+    :param tasks: List of tasks or List[airflow.models.BaseOperator] to set dependencies
+    :type tasks: List[airflow.models.BaseOperator] or airflow.models.BaseOperator
     """
-    for up_task, down_task in zip(tasks[:-1], tasks[1:]):
-        up_task.set_downstream(down_task)
+    from airflow.models import BaseOperator
+    for index, up_task in enumerate(tasks[:-1]):
+        down_task = tasks[index + 1]
+        if isinstance(up_task, BaseOperator):
+            up_task.set_downstream(down_task)
+        elif isinstance(down_task, BaseOperator):
+            down_task.set_upstream(up_task)
+        else:
 
 Review comment:
   We discuss in https://github.com/apache/airflow/pull/4779#issuecomment-480687872 but some of contributors think we should not check list size, WDYT

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