[GitHub] [airflow] uranusjr commented on a change in pull request #21715: Fix incorrect data_interval_start due to scheduling time change

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uranusjr commented on a change in pull request #21715:
URL: https://github.com/apache/airflow/pull/21715#discussion_r812567103



##########
File path: airflow/timetables/interval.py
##########
@@ -211,6 +217,17 @@ def _align(self, current: DateTime) -> DateTime:
             return next_time
         return current
 
+    def _prev_align(self, current: DateTime) -> DateTime:
+        """Get the prev scheduled time.
+
+        This is ``current - interval``, unless ``current`` falls right on the
+        interval boundary, when ``current`` is returned.
+        """
+        prev_time = self._get_prev(current)
+        if self._get_next(prev_time) != current:
+            return prev_time
+        return current

Review comment:
       This seems to be always the same as `self._get_prev(self._get_next(current))`?
   
   Say a schedule runs one every midnight. For this implementation:
   
   * If `current` is at midnight, the return value is at midnight
   * If `current` is on 1am, the return value is at midnight
   
   For `self._get_prev(self._get_next(current))`:
   
   * If `current` is at midnight, `_get_next` returns the next midnight, and `_get_prev` returns _this_ midnight
   * If `current` is at 1am, `_get_next` returns the next midnight, and `_get_prev` returns _this_ midnight
   
   And for the timedelta interval, `self._get_prev(self._get_next(current))` is always `current`.
   
   So if I’m not mistaken, this function is not needed.




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