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https://issues.apache.org/jira/browse/AIRFLOW-3556?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16727313#comment-16727313
 ] 

ASF GitHub Bot commented on AIRFLOW-3556:
-----------------------------------------

BasPH opened a new pull request #4356: [AIRFLOW-3556] Add cross join set 
downstream function
URL: https://github.com/apache/incubator-airflow/pull/4356
 
 
   Make sure you have checked _all_ steps below.
   
   ### Jira
   
   - [x] My PR addresses the following [Airflow 
Jira](https://issues.apache.org/jira/browse/AIRFLOW/) issues and references 
them in the PR title. For example, "\[AIRFLOW-XXX\] My Airflow PR"
     - https://issues.apache.org/jira/browse/AIRFLOW-3556
     - In case you are fixing a typo in the documentation you can prepend your 
commit with \[AIRFLOW-XXX\], code changes always need a Jira issue.
   
   ### Description
   
   - [x] Here are some details about my PR, including screenshots of any UI 
changes:
   
   Add function to set "cross join style" downstream dependencies between two 
list of tasks. For example:
   
   ```
   cross_downstream(from_tasks=[t1, t2, t3], to_tasks=[t4, t5, t6])
   
   Sets dependencies:
       t1 --> t4
          \ /
       t2 -X> t5
          / \
       t3 --> t6
   
   Equivalent to:
   t1.set_downstream(t4)
   t1.set_downstream(t5)
   t1.set_downstream(t6)
   t2.set_downstream(t4)
   t2.set_downstream(t5)
   t2.set_downstream(t6)
   t3.set_downstream(t4)
   t3.set_downstream(t5)
   t3.set_downstream(t6)
   ```
   
   ### Tests
   
   - [x] My PR adds the following unit tests __OR__ does not need testing for 
this extremely good reason:
   
   HelpersTest.test_cross_downstream()
   
   ### Commits
   
   - [x] My commits all reference Jira issues in their subject lines, and I 
have squashed multiple commits if they address the same issue. In addition, my 
commits follow the guidelines from "[How to write a good git commit 
message](http://chris.beams.io/posts/git-commit/)":
     1. Subject is separated from body by a blank line
     1. Subject is limited to 50 characters (not including Jira issue reference)
     1. Subject does not end with a period
     1. Subject uses the imperative mood ("add", not "adding")
     1. Body wraps at 72 characters
     1. Body explains "what" and "why", not "how"
   
   ### Documentation
   
   - [x] In case of new functionality, my PR adds documentation that describes 
how to use it.
     - When adding new operators/hooks/sensors, the autoclass documentation 
generation needs to be added.
     - All the public functions and the classes in the PR contain docstrings 
that explain what it does
   
   ### Code Quality
   
   - [x] Passes `flake8`
   

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> Add a "cross join" function for setting dependencies between two lists of 
> tasks
> -------------------------------------------------------------------------------
>
>                 Key: AIRFLOW-3556
>                 URL: https://issues.apache.org/jira/browse/AIRFLOW-3556
>             Project: Apache Airflow
>          Issue Type: New Feature
>            Reporter: Bas Harenslak
>            Priority: Major
>
> Similar to airflow.utils.helpers.chain(), it would be useful to have a helper 
> function that sets downstream dependencies in a cross join fashion between 
> two lists of tasks.
> For example:
> {code}
> cross_downstream(from_tasks=[t1, t2, t3], to_tasks=[t4, t5, t6])
> Sets dependencies:
>     t1 --> t4
>        \ /
>     t2 -X> t5
>        / \
>     t3 --> t6
> Equivalent to:
> t1.set_downstream(t4)
> t1.set_downstream(t5)
> t1.set_downstream(t6)
> t2.set_downstream(t4)
> t2.set_downstream(t5)
> t2.set_downstream(t6)
> t3.set_downstream(t4)
> t3.set_downstream(t5)
> t3.set_downstream(t6){code}



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