Fix mapped task group XCom collapsing to a bare value for a single expansion#69075
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…pansion A task consuming the output of a mapped task group from outside that group must always receive a list with one element per expansion. Since the task-start upstream map-index computation moved into the Task SDK, an unmapped downstream resolving a task inside a mapped task group fell through to pulling all map indexes via xcom_pull, which collapses a single value to a bare scalar and an empty set of values to None. A group that expanded only once therefore handed the raw value to the downstream task instead of a one-element list, and a group whose expansions all returned None handed None instead of an empty list. The pre-3.2 server returned list(range(mapped_ti_count)) for this case, so the behaviour regressed. Resolve this case by eagerly materialising the group's per-expansion XComs into a list (a single slice request), so the result is always a list regardless of how many times the group expanded or whether every value was None, and stays serializable when a task returns it unchanged.
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closes: #69036
related: #48005
Human Summary
Currently, when
map_indexesis left unset, a single mapped result collapses to its bare value instead of staying one-element list.It started to happen after #60803 was merged, when the upstream map-index computation was relocated to the task-sdk.
This PR fixes it by returning an eagerly-materialized
LazyXComSequenceobject.AI Summary
Click here
A task consuming the output of a mapped task group from **outside** that group must always receive a list with one element per expansion. When the task-start upstream map-index computation moved into the Task SDK (#60803, released in 3.2.0), an unmapped downstream resolving a task inside a mapped task group started falling through to pulling all map indexes via `xcom_pull`, which collapses a single value to a bare scalar and an empty set of values to `None`.As a result:
'14'and iterated'1','4'instead of receiving['14']).Nonewould handNoneinstead of an empty list (the Result of expanded task is None instead of empty list when all mapped tasks return None #48005 class of bug, via the task-group resolution path).This worked correctly in 3.1.5: the pre-3.2 server returned
list(range(mapped_ti_count))for this case, guaranteeing a list. The fix resolves this case through aLazyXComSequence, mirroring how a directly mapped task already resolves, so the result is always a list regardless of how many times the group expanded or whether every value wasNone.Verified end to end with
airflow dags teston the reproduction DAGs from both issues:['14'](was'14'),Nonegroup expansions yield[],Nonecase from Result of expanded task is None instead of empty list when all mapped tasks return None #48005 continues to yield[].Was generative AI tooling used to co-author this PR?
Generated-by: Claude Code (Opus 4.8) following the guidelines