Multiple cycle numbers break single TTree detection

[masonproffitt]

Functions like uproot.lazy normally work without specifying the tree name if there is only one TTree in the file, but this doesn't work if there are multiple cycle numbers:

>>> import uproot
>>> uproot.__version__
'4.1.7'
>>> uproot.lazy('root://eospublic.cern.ch//eos/opendata/cms/derived-data/AOD2NanoAODOutreachTool/Run2012BC_DoubleMuParked_Muons.root')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/miniconda3/envs/iris-hep/lib/python3.9/site-packages/uproot/behaviors/TBranch.py", line 551, in lazy
    obj = _regularize_object_path(
  File "/home/user/miniconda3/envs/iris-hep/lib/python3.9/site-packages/uproot/behaviors/TBranch.py", line 2982, in _regularize_object_path
    raise ValueError(
ValueError: TTree object paths must be specified in the 'files' as {"filenames*.root": "path"} if any files have more than one TTree

    TTrees: 'Events;75', 'Events;74'

in file root://eospublic.cern.ch//eos/opendata/cms/derived-data/AOD2NanoAODOutreachTool/Run2012BC_DoubleMuParked_Muons.root

This is confusing since you otherwise never have to worry about cycle numbers. For example: uproot.open('root://eospublic.cern.ch//eos/opendata/cms/derived-data/AOD2NanoAODOutreachTool/Run2012BC_DoubleMuParked_Muons.root:Events') doesn't require a cycle number.

[jpivarski]

Should it just assume the highest one? Or do so with a default option?

[masonproffitt]

I would think it should do whatever uproot.open('file.root').key('Events') does if there are multiple cycle numbers present for Events, which seems to be taking the highest one:

>>> uproot.open('root://eospublic.cern.ch//eos/opendata/cms/derived-data/AOD2NanoAODOutreachTool/Run2012BC_DoubleMuParked_Muons.root').key('Events')
<ReadOnlyKey Events;75: TTree (seek pos 2244422054) at 0x7f6d397692e0>

[jpivarski]

The actual selection of TTree follows the same code path in lazy and non-lazy cases; what's different is the preparation of key names to ask for. So the substantial part of PR #500 (!) is

@@ -2965,7 +2965,7 @@ def _regularize_object_path(
             **options  # NOTE: a comma after **options breaks Python 2
         ).root_directory
         if object_path is None:
-            trees = [k for k, v in file.classnames().items() if v == "TTree"]
+            trees = file.keys(filter_classname="TTree", cycle=False)
             if len(trees) == 0:
                 if allow_missing:
                     return None

I also had to fix ReadOnlyFile.keys (and values, items, classnames) to not return duplicates when the disambiguating cycle number is suppressed. Actually, that part could be considered a bug, but no one ever brought it up before. (Mapping.keys should return unique strings...)