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Fix reversed direction examples in FanOutMapper docs #69313

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Fix reversed direction examples in FanOutMapper docs #69313

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12 changes: 7 additions & 5 deletions airflow-core/src/airflow/partition_mappers/temporal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -469,9 +469,11 @@ class FanOutMapper(PartitionMapper):
is N→1 (downstream waits until all members arrive), fan-out is 1→N (one
upstream event creates one downstream Dag run per member).

For forward fan-out (emit the trailing period ending at the upstream key,
instead of the period it represents), pass ``direction=Window.Direction.FORWARD``
to the window:
``window``'s direction controls which period each upstream key fans out to.
The default, ``Window.Direction.FORWARD``, yields the period *starting at*
the upstream key (the period it represents); ``Window.Direction.BACKWARD``
yields the trailing period *ending at* the upstream key. Pass
``direction=Window.Direction.BACKWARD`` to the window for the latter:

.. code-block:: python

Expand All @@ -482,8 +484,8 @@ class FanOutMapper(PartitionMapper):
FanOutMapper(upstream_mapper=StartOfWeekMapper(), window=WeekWindow())

# Weekly upstream → the 7 days ending at the upstream Monday (trailing period)
forward_window = WeekWindow(direction=Window.Direction.FORWARD)
FanOutMapper(upstream_mapper=StartOfWeekMapper(), window=forward_window)
backward_window = WeekWindow(direction=Window.Direction.BACKWARD)
FanOutMapper(upstream_mapper=StartOfWeekMapper(), window=backward_window)
"""

# Keep ``FanOutMapper.default_downstream_mapper_by_window_name`` in sync with
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15 changes: 9 additions & 6 deletions task-sdk/src/airflow/sdk/definitions/partition_mappers/temporal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -137,20 +137,23 @@ class (e.g. ``WeekWindow`` → ``StartOfDayMapper``).
waits for all members), fan-out is 1→N (one upstream event creates many
downstream Dag runs).

For forward fan-out (emit the *next* period's members instead of the current
one), pass ``direction=Window.Direction.FORWARD`` to the window:
``window``'s direction controls which period each upstream key fans out to.
The default, ``Window.Direction.FORWARD``, yields the period *starting at*
the upstream key (the period it represents); ``Window.Direction.BACKWARD``
yields the trailing period *ending at* the upstream key. Pass
``direction=Window.Direction.BACKWARD`` to the window for the latter:

.. code-block:: python

from airflow.sdk import WeekWindow, Window
from airflow.sdk.definitions.partition_mappers.temporal import FanOutMapper, StartOfWeekMapper

# Weekly upstream → 7 daily downstream Dag runs (current week)
# Weekly upstream → 7 daily downstream Dag runs (the 7 days the upstream Monday represents)
FanOutMapper(upstream_mapper=StartOfWeekMapper(), window=WeekWindow())

# Weekly upstream → 7 daily keys for the *following* week
forward_window = WeekWindow(direction=Window.Direction.FORWARD)
FanOutMapper(upstream_mapper=StartOfWeekMapper(), window=forward_window)
# Weekly upstream → the 7 days ending at the upstream Monday (trailing period)
backward_window = WeekWindow(direction=Window.Direction.BACKWARD)
FanOutMapper(upstream_mapper=StartOfWeekMapper(), window=backward_window)
"""

# Keep ``FanOutMapper.default_downstream_mapper_by_window_name`` in sync with
Expand Down
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