python将utf8無bom格式轉化為utf8 bom格式

#coding=utf8


filename = input("input filename: ")
f = open(filename,'rb')
content = f.read()
f.close()
temp = content.decode('utf-8')
result = temp.encode('utf-8-sig')
f = open(filename,'wb')
f.write(result)
f.close()