python xml中写入文件如果要带xml头该怎么办

python xml中写入文件如果要带

<?xml version="1.0" encoding="utf-8"?> 

字样应该怎么办？ 

doc = minidom.Document()  
f = open(filename, "w")
doc.writexml(f, indent="  ", addindent='\t', newl="\n",encoding="utf-8")
f.close()