I've updated the daf_butler branch with __repr__ implementations for the query results objects, and added a note to the pipelines.lsst.io branch about the fact that these objects are lazy iterables, along with an example.

One thing I forgot to comment on earlier:

I suspect most people will hit the empty quantum graph from the command line. Could you format the helpful debugging prompt to use butler query-dataids instead of Registry.queryDataIds, and format the arguments for the command line API?

While I'd concede that this would probably make the first step in debugging easier, I'd actually advise users to go to IPython or a Jupyter notebook for debugging because it'll make the second through Nth steps easier; the command-line is just too clunky as a way to pass around and modify even moderately complicated data like the lists and dictionaries here. It's also harder to write out a command-line version that's exactly equivalent, so I may be rationalizing a bit, but I can certainly say that whenever I help people debug these I do it in IPython. And, of course, I'd have a much stronger case if there was a pipetask Python interface one could use from IPython or Jupyter, too, but that's a different problem, and one we will definitely fix at some point.

Thanks, .  This problem is a persistent one that I don't know how to fix, so I've started a new conversation on Slack about it, and I may punt the fix to another ticket unless that converges quickly to something simple.

Looks good overall; just a couple of very minor comments.

Trying this out on a pipetask qgraph command that gives an empty QuantumGraph:
With weekly 45 I ran:
pipetask qgraph -d "instrument='DECam' AND skymap='hsc_rings_v1' AND band IN ('r') AND exposure!=413585 AND detector=28 AND tract=9813" -b /repo/main/butler.yaml -i DECam/raw/hits2014,DECam/calib,u/mrawls/DM-30703-hits-crosstalk-sources,refcats,skymaps -p /project/sullivan/hits_dcr_fall_2021/yaml/generate_dcr_template.yaml -q /project/sullivan/hits_dcr_fall_2021/qgraph_test.qgraph

Which gives the error:

After setting up daf_butler and pipe_base, I now get:

This looks very helpful and looks formatted for python, so I launch ipython

I'm almost certainly doing things wrong, but I'm not sure what I should do with this output.

Two requests:

• I suspect most people will hit the empty quantum graph from the command line. Could you format the helpful debugging prompt to use butler query-dataids instead of Registry.queryDataIds, and format the arguments for the command line API?

• Please provide an example in the FAQ of how to use butler.registry.queryDataIds

No need to rush on that timescale; my only goal is to make the weekly by merging by the end of the day on Wednesday.