What is the long-term plan for this? If an operation would normally return a list of items but only happens to find one, I'd much rather have it be a list of one item, for the sake of consistency. On the other hand, if the operation intrinsically returns a single item, e.g. butler.get("calexp", dataId) then it should not be a list.

this actually has to do with the introduction of the Repository class, and if it has parent repositories AND the parent search behavior is to join all the possible results. Russell Owen's question is a good one:how should the results be returned (list or no list). And what if the search behavior is to find the first one found?

I don't have a good answer off the top of my head, and I think it's appropriate to take time for the users to discuss it. It actually doesn't have any impact right now, since everyone is still using the 'old' butler API, and the default behavior of the under-the-hood implementation is to return what was found without putting it in a list. So you'll get what was returned without modification.

I'll make a note to bring it up in an RFC.

KT reviewed. The Jira case didn't go through the 'in review' phase because the Jira server was inaccessible.

I confirm that this ticket fix (alone) solved my previous issue of running CoaddDriverTask.

Alternatively, changing the comparison here from

if self._parentJoin is 'left':

to

if self._parentJoin == 'left':

also solves the issue alone. This seems to explain why the bug affects some but not all cases, and this happens while all my cases have _parentJoin of 'left'. May I suggest this change as well, per our coding style guide?

Hsin-Fang Chiang thank you for catching the comparison operator. I'm surprised I used 'is' there. I'll file a ticket for this sprint to check and correct comparisons in butler code.

Thank you for filing DM-5607 and your help solving the problems.

For future reference, below was the old error messages from running CoaddDriverTask with a DECam data repo:

 File "/opt/sw/lsstsw/build/pipe_drivers/python/lsst/pipe/drivers/coaddDriver.py", line 164, in readSelection

    ref = getDataRef(cache.butler, selectId, "calexp")

  File "/opt/sw/lsstsw/build/pipe_drivers/python/lsst/pipe/drivers/utils.py", line 18, in getDataRef

    butler.subset(datasetType, **dataId)

  File "/opt/sw/lsstsw/stack/DarwinX86/daf_persistence/2016_01.0-6-g27ca0e9/python/lsst/daf/persistence/butler.py", line 425, in subset

    return ButlerSubset(self, datasetType, level, dataId)

  File "/opt/sw/lsstsw/stack/DarwinX86/daf_persistence/2016_01.0-6-g27ca0e9/python/lsst/daf/persistence/butlerSubset.py", line 80, in __init__

    fmt = list(keys.iterkeys())

AttributeError: 'list' object has no attribute 'iterkeys'