tree.GetEntry(x) triggers a Traceback in PyRoot

ROOT
2

In C++:

    //Reset the data
    leptondata->Clear();
    metdata->Clear();
    jetdata->Clear();

yet in Python:

  #Reset the data
  leptondata=TClonesArray( "TLorentzVector", nentries)
  metdata=TClonesArray( "TLorentzVector", nentries)
  jetdata=TClonesArray( "TLorentzVector", nentries)

and you’re surprised that the behavior differs, why?

To be specific, that “problem in C++; program state has been reset” means you got a segfault on the C++ side. The reason is that by overwriting the XYZdata references, the old values are garbage collected even as you had passed pointers to them to C++. Then the GetEntry touches that deleted memory.

Just call Clear() on the clones arrays in Python like you do in C++ …