def writedata(fd, msg):
    fd.write(msg)
   
f = open('aaa.txt', 'w')

writedata(f, "xxxxxxxxxxxx")
f.close()

s = StringIO.StringIO()
writedata(s, "xxxxxxxxxxxxxx")

s = StringIO.StringIO()
s.write("aaaa")
lines = ['xxxxx', 'bbbbbbb']
s.writelines(lines)

s.seek(0)
print s.read()

print s.getvalue()
s.write(" ttttttttt ")
s.seek(0)
print s.readlines()
print s.len