SaveGraph producing unclear results, when the TChain has no name

[ikabadzhov]

If the underlying TChain has no name (""), then the output of SaveGraph is far from helpful.
The following script:

#include<ROOT/RDataFrame.hxx>
#include<ROOT/RDFHelpers.hxx>
#include<TSystem.h>

using namespace ROOT;

int main(){
  ROOT::RDataFrame(1).Define("x", "42").Snapshot("t", "f.root");

  TChain ch("");
  ch.Add("f.root?#t");
  RDataFrame df(ch);
  auto df2 = df.Count();
  ROOT::RDF::SaveGraph(df, "res.dot");
  gSystem->Exec("dot -Tpng res.dot -o img_res.png");

  //df.Describe().Print();
}

Produces:

Example solution: if the chain has no name => call the source node TChain with no name.

(Remark, same comment is applicable for the Describe() action. The commented line would produce Dataframe from TChain in file f.root [...]. Notice the 2 spaces after "TChain".)

CC: @eguiraud @vepadulano

[eguiraud]

:)

[vepadulano]

Blessed TChains with no name :D I vote for Unnamed: it's one word, can be easily written in the yellow circle above and used in the sentence Dataframe from TChain "Unnamed"

[eguiraud]

Just "Unnamed" is not very descriptive. If the chain has no name SaveGraph can show e.g. "TChain (5 files)" instead.

[ikabadzhov]

FYI: Added in Milestone 6.28. since this is the default behaviour in the RDatasetSpec when having a chain with different tree names.

I should solve it if noone comes before the release of 6.28.

[eguiraud]

Removed the 6.28 milestone, this is not severe enough to block the release.

[bernhardmgruber]

This might be affected/solved by #13088.

[eguiraud]

Thanks @bernhardmgruber , unfortunately this is independent of RNTuple. TChains with no name are a thing, in general.

[bernhardmgruber]

Ah, I didn't check the implementation and assumed #13088 was implemented in TChain. I just had a look now. Sorry about the noise then.

[dpiparo]

Nice change. I proposed a simplification. The tests seem to fail because the change does what it is supposed to do and the reference would need to be adapted.

[martamaja10]

Nice change. I proposed a simplification. The tests seem to fail because the change does what it is supposed to do and the reference would need to be adapted.

Thanks! We're on it