import ROOT
import numpy

f = ROOT.TFile('alias.root', 'recreate')
t = ROOT.TTree('t', 't')

a = numpy.zeros(1, dtype=float)
t.Branch('a', a, 'a/D')

for i in range(10):
    a[0] = i*i
    t.Fill()

t.Write()
f.Close()

f = ROOT.TFile('alias.root', 'update')
t = f.Get('t')
t.SetAlias('b', 'a/10.0')
t.Write()
f.Close()