Invert the order of the first four bytes of a CAN packet

        This method is meant to be used specifically to convert a CAN packet
        between the pcap format and the socketCAN format

        :param pkt: str of the CAN packet
        :return: packet str with the first four bytes swapped

def inv_endianness(pkt):
        """ Invert the order of the first four bytes of a CAN packet

        This method is meant to be used specifically to convert a CAN packet
        between the pcap format and the socketCAN format

        :param pkt: str of the CAN packet
        :return: packet str with the first four bytes swapped
        """
        len_partial = len(pkt) - 4  # len of the packet, CAN ID excluded
        return struct.pack('<I{}s'.format(len_partial),
                           *struct.unpack('>I{}s'.format(len_partial), pkt))