Make a simple TTree using PyROOT

make_tree.py

#!/usr/bin/env python

import ROOT
from array import array
import random

ROOT.PyConfig.IgnoreCommandLineOptions = True
ROOT.gROOT.SetBatch(1)

f = ROOT.TFile("my_tree.root", "RECREATE")
tree = ROOT.TTree("valid", "An Example Tree")

pt = array('f', [0.])
ptRef = array('f', [0.])
rsp = array('f', [0.])

tree.Branch("pt", pt, 'pt/F')
tree.Branch("ptRef", ptRef,'ptRef/F')
tree.Branch("rsp", rsp, 'rsp/F')

for i in range(1000000):
    pt[0] = (i+1) * 1.0
    ptRef[0] = (i+1) * 1.0
    rsp[0] = 1. * pt[0] / ptRef[0]
    tree.Fill()

tree.Write("", ROOT.TObject.kOverwrite);
f.Close()