Cannot use custom class with RDataFrame Fill method

ROOT
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3

Hi Simone,
welcome to the ROOT forum!

Indeed there are several things going on. The main problem is that you just re-discovered a bug that was supposed to be fixed, ROOT-9737 ("[DF] Cannot use fill if the object is not an histogram"). I re-opened it, and it will be fixed as soon as possible (but note that there are a bunch of critical RDF bugs that need attention first, so it might take a little while).

Then for python usage specifically, another bug that might bite us is ROOT-10396 (“Failure when instantiating RDataFrame::Fill”).

Depending on what you actually need to accomplish, one or the other workaround might appy.
Do you really need a custom Fill even if it is afflicted by the bugs above?

The simplest way to use the custom Fill is to make your Boo class inherit from TH1 which is (wrongly) a requirement for Fill to work. You will also have to override the definitions of TH1’s Fill, Add and Merge and add a copy-constructor for things to work in multi-thread runs (i.e. when ROOT::EnableImplicitMT has been called; each thread will have its copy of Boo).

In a single-thread, in C++, this works:

#include <ROOT/RDataFrame.hxx>
#include <iostream>

class Boo : public TH1 {
   TH1F *hh;

public:
   Boo() { hh = new TH1F("Boo", "Boo", 100, -1000, 1000); }
   Boo(const Boo &b2) { hh = b2.hh; }
   Boo(const char *name, const char *title, int nBins, float xLow, float xHigh)
   {
      hh = new TH1F(name, title, nBins, xLow, xHigh);
   }
   void Fill(float x, float w) { hh->Fill(x, w); }
   TH1F *GetHisto() { return hh; }
};

int main()
{
   ROOT::RDataFrame df_(10);
   auto df = df_.Define("mjj", "10").Define("weight", "1");
   auto filled = df.Fill<int, int>(Boo(), {"mjj", "weight"});
   std::cout << filled->GetHisto()->GetEntries() << std::endl;
   std::cout << filled->GetHisto()->GetMean() << std::endl;

   return 0;
}

Now for python, note that in general you need -fPIC when compiling a shared library that you need to link with ROOT. And to get around ROOT-10396 that I linked above, we can use a C++ helper function.
This works in v6.18, but fails in v6.14 and v6.16 due to PyROOT not playing nice with templates in those older versions

import ROOT
ROOT.gInterpreter.Declare('#include "Boo.h"')
ROOT.gSystem.Load("libBoo.so")

ROOT.gInterpreter.Declare("""
template <typename DF>
ROOT::RDF::RResultPtr<Boo> FillBoo(DF df) {
    return df.Fill(Boo(), {"mjj", "weight"});
}
""")


df = ROOT.RDataFrame(10).Define("mjj", "10").Define("weight", "1");
b = ROOT.FillBoo(df)
print(b.GetHisto().GetEntries())

To summarize, we can work around RDF’s bug by inheriting from TH1, and we can work around pyROOT’s bug by using a C++ helper function (at least since v6.18). Or, depending on your usecase, you can change the rules of the game and use something else than a custom Fill. I’ll try to address at least the RDF bug as soon as possible, but the fix will most probably not be backported to ROOT v6.14, maybe v6.18, definitely v6.20.

Sorry you hit such a nasty corner of ugly things.
Cheers,
Enrico

P.S.
@etejedor or @swunsch might have an idea about how to use something like that C++ helper function also in v6.14.