Reading large number of data from file.

gistfile1.py

import ROOT
import numpy

ROOT.gROOT.SetBatch()
ROOT.gSystem.Load("libEXOUtilities")
ROOT.gROOT.ProcessLine(".L $EXOLIB/htmldoc/exo_tree_alias.C")
chain = ROOT.TChain("tree")

chain.Add("/nfs/slac/g/exo/exo_data/data/MC/ATeamBackgrounds/MC_Background_0000*.root")
ROOT.SetEXOAlias(chain)

entries = chain.GetEntries()

chain.SetBranchStatus("*", 0)
chain.SetBranchStatus("fChargeClusters.fCollectionTime", 1)
chain.SetBranchStatus("fChargeClusters.fX", 1)
chain.SetBranchStatus("fChargeClusters.fY", 1)
chain.SetBranchStatus("fChargeClusters.fZ", 1)
#c1 = ROOT.TCanvas()
print "Getting initial estimate"
chain.Draw("tcl:xcl:ycl:zcl", "", "goff")
#c1.Print("temp.eps")
num = chain.GetSelectedRows()
chain.SetEstimate(num)
print "Getting data"
chain.Draw("tcl:xcl:ycl:zcl", "", "goff")
tcl_arr = numpy.array([chain.GetV1()[i] for i in range(num)])
xcl_arr = numpy.array([chain.GetV2()[i] for i in range(num)])
ycl_arr = numpy.array([chain.GetV3()[i] for i in range(num)])
zcl_arr = numpy.array([chain.GetV4()[i] for i in range(num)])

print tcl_arr