file为什么最后一个write 得不到数据

CODE:

import MySQLdb
from xml.dom.minidom import parse, parseString

connstr = MySQLdb.connect("localhost", "root", "12345", "xoops")
conn = connstr.cursor()
conn.execute("select * from my_art_article")
conn.scroll(0)
filename = "3.xml"
f = file(filename, "a")
f.write("<?xml version=\"1.0\" encoding=\"utf-8\"?>")
f.write("<root>")
while(1):
    irows = conn.fetchone()
    if irows == None:
        break
    f = file(filename, "a")
    f.write("<title>"+irows[5]+"</title>")
f = file(filename, "a")
f.write("</root>")
f.close
print "xml create succeed."