python解析ip header

class IpHeader(object):
    def __init__(self):
        self.versionAndLen=None   # 4bit version 4bit header length
        self.tos=None             # 8bit type of service
        self.tolalLen = None      # 16bit total length
        self.identification =None # 16bit header identification
        self.fragment = None      # 16bit others and fragment offset
        self.ttl = None           # 8bit time to live
        self.protocal = None      # 8bit type of protocal
        self.checksum = None    # 16bit header checksum
        self.srcIP = None       # 32bit src IP address
        self.dstIP = None       # 32bit dst IP address 
    def decodeIP(self,code_stream):
        (self.versionAndLen,self.tos,self.tolalLen,self.identification,self.fragment,
         self.ttl,self.protocal,self.checksum,self.srcIP,self.dstIP) = struct.unpack('>BBHHHBBHII',code_stream)
    def IntToIP(self,ipInt):
        return socket.inet_ntoa(struct.pack('I',socket.htonl(ipInt)))
    def __str__(self):
        return('IP header:\n'
               'Version and Length:{}\n'
        'Type of service:{}\n'
        'Total length:{}\n'
        'Header identification:{}\n'
        'Fragment offset:{}\n'
        'Time to live(TTL):{}\n'
        'Type of protocal:{}\n'
        'Header checksum:{}\n'
        'Source IP address:{}\n'
        'Destination IP address:{}\n'.format(self.versionAndLen,self.tos,self.tolalLen,self.identification,
                                self.fragment,self.ttl,self.protocal,self.checksum,self.IntToIP(self.srcIP),self.IntToIP(self.dstIP)))