[GitHub] [airflow] zhongjiajie commented on a change in pull request #4779: [WIP][AIRFLOW-3958] Support list tasks as upstream in chain

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zhongjiajie commented on a change in pull request #4779: [WIP][AIRFLOW-3958] Support list tasks as upstream in chain
URL: https://github.com/apache/airflow/pull/4779#discussion_r272986625
 
 

 ##########
 File path: airflow/utils/helpers.py
 ##########
 @@ -154,19 +156,46 @@ def as_flattened_list(iterable):
 
 
 def chain(*tasks):
-    """
-    Given a number of tasks, builds a dependency chain.
+    r"""
+    Given a number of tasks or list of tasks, builds a dependency chain.
 
-    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: tasks or list[tasks] to set dependencies
+    :type tasks: airflow.models.BaseOperator, List[airflow.models.BaseOperator]
     """
-    for up_task, down_task in zip(tasks[:-1], tasks[1:]):
-        up_task.set_downstream(down_task)
+    for index, task in enumerate(tasks[:-1]):
+        down_task = tasks[index + 1]
+        if isinstance(task, BaseOperator):
+            task.set_downstream(down_task)
+        elif isinstance(down_task, BaseOperator):
+            down_task.set_upstream(task)
+        elif isinstance(task, list) and isinstance(down_task, list):
+            # Parallel pipelines
+            if len(task) == len(down_task):
+                for a, b in zip(task, down_task):
+                    a.set_downstream(b)
+            else:
+                raise AirflowException(
 
 Review comment:
   Not really, that the `cross` mean, we already have `cross_downstream` in `airflow.utils.helper`.I am inspired in https://apache-airflow.slack.com/archives/CCPRP7943/p1554633832061000?thread_ts=1554374078.016800&cid=CCPRP7943 , thinking we should have a function to make parallel pipelines work
   
   ```
        /  -> t1 -> t3
   t0
        \  -> t2 -> t4
   ```
   If we do that, `chain` could do `chain(t0, [t1, t2, t3], [t4, t5, t6])` like
   
   ```
        /  -> t1 -> t4
   t0    --> t2 -> t5
        \  -> t3 -> t6
   ```
   and could `cross_downstream([t1, t2, t3], [t4, t5, t6])`
   
   ```
   t1   \     /  -> t4
           \ /
   t2 -> X   -> t5
           / \
   t3   /      \ -> t6
   ```
   
   make our dependent more possibilities.

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