import ROOT
import tempfile
import os
import array

treename = 'mytree'
branchname = 'mybranch'
aliasname = 'myalias'

tmpdir = tempfile.mkdtemp()

files = []
files.append(os.path.join(tmpdir, 'f1.root'))
files.append(os.path.join(tmpdir, 'f2.root'))

for i, fname in enumerate(files):
  f = ROOT.TFile.Open(fname, 'NEW')
  t = ROOT.TTree(treename, treename)
  t.SetDirectory(f)
  # add alias
  t.SetAlias(aliasname, '{0:d} * {1:s}'.format(i+1, branchname))
  # create branch
  branchvals = array.array('f', [3.1415926])
  branch = t.Branch(branchname, branchvals, "{0:s}/F".format(branchname))
  #branch.Fill()
  t.Fill()
  t.Write("", ROOT.TObject.kWriteDelete)
  f.Close()


scanExpression = ":".join([branchname, aliasname])
print 'Scan expression:',scanExpression
for fname in files:
  print 'Scanning',os.path.basename(fname)
  f = ROOT.TFile.Open(fname)
  getattr(f, treename).Scan(scanExpression)

print 'Using TChain'
tc = ROOT.TChain(treename)
for fname in files: tc.Add(fname)
tc.Scan(scanExpression)

print 'Using TChain (add files in reverse)'
tc = ROOT.TChain(treename)
for fname in files[::-1]: tc.Add(fname)
tc.Scan(scanExpression)