ashb 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_r273448445
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File path: airflow/utils/helpers.py
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@@ -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:
Oh yeah, `product` is totally different behaviour.
I'm not so sure that it would be obvious - the "extra" tasks from the long
list would instead end up with no deps set. That sounds like it would be more
confusing than the behaviour in the PR: an error would show up in the UI.
But I am not a user of this fn so I can't say for certain.
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