Saving to a ROOT file with "/" in tree name

[graipher]

In normal ROOT, trees may be within a TDirectory. This is transparent during reading (e.g., one can do f->Get("DirectoryName/TreeName" in C++).

However, when saving a DataFrame one cannot generate this directory structure. When saving a DataFrame with key="DirectoryName/TreeName", this will just become the name of the tree. This object cannot be read back, because f->Get("DirectoryName/TreeName" will look for a tree with the name TreeName within the TDirectory DirectoryName, instead of a tree with that full name.

[graipher]

This is similar to a (closed) issue with TDataFrame, which I reported two years ago here: https://root-forum.cern.ch/t/tdirectory-in-tdataframe-snapshot/25134

[chrisburr]

Thanks for the report @graipher!

The key option in root_pandas.to_root is passed directly to root_numpy so your pull request there should fix it. We can then add a warning (or error) in root_pandas to check if someone tries to create a directory with a buggy version of root_numpy.

[graipher]

@chrisburr Yes, I noticed that afterwards as well (which is why I started creating a PR there). The warning sounds like a good idea.