Q: When I try accessing the output D3PD with Python (PyROOT), my script crashes with an error like this: "SystemError: problem in C++; program state has been reset", at the point when I get the entries from the TTree with "TTree.GetEntry()" command.

The problem:

I get a crash with the error message above running a script like this:

import ROOT
#open the .root file
f = ROOT.TFile( "OutputFile.root" )
#open the TTree
tree = f.Get( "MyAnalysisTree" )
tree.GetEntry( 0 )

The script does not crash if we load !MyAnalysisInfoTree instead of "MyAnalysisTree", with tree = f.Get( "!MyAnalysisInfoTree" ).

This because the !InfoTree stores only simple plain ROOT objects, while the Tree stores physics data, and in order to store them in an effective way, it uses data structures as vector< TLorentzVector > and vector< vector < int > >.

Unfortunately current ROOT version (at time I'm writing) does not support natively those objects, thus we need to load the Python dictionary module from ATLASWatchMan, !ATLASWatchMan.Bindings, after the !ROOT module.

The Solution:

import ROOT
import ATLASWatchMan.Bindings

#open the .root file
f = ROOT.TFile( "OutputD3PD.root" )

#open the TTree
tree = f.Get( "GeneralSearchTree" ); tree.GetEntry( 0 )

-- RiccardoMariaBianchi - 23-Oct-2009