from ROOT import TFile, TTree, gRandom
from array import array

# make new root file with new tree
file = TFile("tree.root", 'recreate')
tree = TTree("tree_name", "tree title")

# create 1 dimensional float arrays as fill variables, in this way the float
# array serves as a pointer which can be passed to the branch
px  = array('d',[0])
phi = array('d',[0])

# create the branches and assign the fill-variables to them as doubles (D)
tree.Branch("px",  px,  'normal/D')
tree.Branch("phi", phi, 'uniform/D')

# create some random numbers, assign them into the fill variables and call Fill()
for i in xrange(10000):
    px[0]  = gRandom.Gaus(20,2)
    phi[0] = gRandom.Uniform(2*3.1416)
    tree.Fill()
    
# write the tree into the output file and close the file
file.Write()
file.Close()