baolsen commented on a change in pull request #6999: [AIRFLOW-XXXX] Clarify 
wait_for_downstream and execution_date
URL: https://github.com/apache/airflow/pull/6999#discussion_r362704684
 
 

 ##########
 File path: docs/tutorial.rst
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 @@ -313,8 +315,16 @@ to track the progress. ``airflow webserver`` will start a 
web server if you
 are interested in tracking the progress visually as your backfill progresses.
 
 Note that if you use ``depends_on_past=True``, individual task instances
-will depend on the success of the preceding task instance, except for the
-start_date specified itself, for which this dependency is disregarded.
+will depend on the success of their previous task instance (that is, previous
 
 Review comment:
   Thanks for the feedback. 
   
   So I wasn't 100% decided between "preceding" vs "previous", but decided to 
change it to previous because "previous" is the word used elsewhere in the 
documentation and code to refer to "past instances". 
   I learned yesterday that the two words have very similar definition and can 
be used interchangeably :). 
   
   It can be confusing to a developer. "previous + next" usually go together as 
concepts and refer to objects that are linked somehow, yet in this context the 
tasks that are linked are actually called "upstream/downstream". 
   
   However, across the code we have used "previous" to refer to "past 
instances" of tasks and dag runs, meaning perhaps it is more natural to refer 
to "previous" things rather than "past"/"preceding"/"prior" things.
   
   I've just found the Concepts section and I think it would be good to add 
some definitions here. As a new user is can be really confusing to understand 
upstream/downstream vs previous/next execution. Execution_date isn't even 
mentioned in Concepts at all - I'll see if I can add something.
   

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