Problem when filling tree from Pythia

ROOT
, ,
8 
import ROOT
from array import array

# Creating file
file = ROOT.TFile.Open("aa.root", "recreate")

# Tree
tree = ROOT.TTree("tree", "tree")
phi_i = array('f', [0])
tree.Branch("phi", phi_i, "phi/F")

for i in range(0, len(pythia.event.size())):
    phi_i[0] = pythia.event[i].phi()
    # Filling
    tree.Fill()    

# Writing file
file.Write()

file.Close()