classes helping update some VHD structures checksum

vhd.py

import os
import sys
import struct

class vhdStruct(object):    
    def __init__(self):
        self._structSize = None
        self._buffer = None
        self._checsumOffset = None
        self._offset = None

    def calculateChecksum(self):
        checksum = 0
        for i in range(0,self._structSize):
            if i >= self._checksumOffset and i < self._checksumOffset + 4:
                continue
            checksum += ord(self._buffer[i])
        checksum = (~checksum) & 0xffffffff
        return checksum

    def getChecksumOffset(self):
        return self._offset + self._checksumOffset

class vhdFooter(vhdStruct):
    DISK_TYPE_OFFSET = 0x3c
    CHECKSUM_OFFSET  = 0x40
    def __init__(self,buffer,offset):
        self._buffer = buffer
        self._offset = offset
        self._checksumOffset = vhdFooter.CHECKSUM_OFFSET
        self._structSize = len(buffer)        

    def getDiskType(self):
        return struct.unpack(">I",self._buffer[vhdFooter.DISK_TYPE_OFFSET:vhdFooter.DISK_TYPE_OFFSET+4])[0]

    def getDiskTypeByte(self):
        return ord(self._buffer[vhdFooter.DISK_TYPE_OFFSET+3])
    
class vhdDynamicHeader(vhdStruct):
    CHECKSUM_OFFSET  = 0x24    
    
    def __init__(self,buffer,offset):
        self._buffer = buffer    
        self._checksumOffset = vhdDynamicHeader.CHECKSUM_OFFSET
        self._structSize = len(buffer)
        self._offset = offset

class vhd(object):

    FOOTER_SIZE  = 512
    VHD_FIXED    = 2
    VHD_DYNAMIC  = 3

    def __init__(self,filePath):
        self.__filePath = filePath
        self.__fw = None
        stats = os.stat(filePath)
        if stats.st_size < vhd.FOOTER_SIZE:
            print "File size is less than 512 bytes"
            return
        try:
            self.__fw = file(filePath,'rb+')
            self.__fw.seek(-vhd.FOOTER_SIZE,os.SEEK_END)
            offset = self.__fw.tell()
            self.footer = vhdFooter(self.__fw.read(vhd.FOOTER_SIZE), offset)
            if self.footer.getDiskType() == vhd.VHD_DYNAMIC:
                self.__fw.seek(0,os.SEEK_SET)
                self.footerCopy     = vhdFooter( self.__fw.read(vhd.FOOTER_SIZE),0)
                self.dynamicHeader  = vhdDynamicHeader( self.__fw.read(1024), vhd.FOOTER_SIZE )
        except Exception as e:
            print e.message

    def updateFile(self,updates):
        for update in updates:
            offset,value = update
            self.__fw.seek(offset,os.SEEK_SET)
            self.__fw.write( struct.pack(">I",value) )
            
    def close(self):
        if not self.__fw:
            self.__fw.close()

if __name__ == "__main__":
    v = vhd(r't:\vdisk_dynamic_test.vhd')    
    updates = []
    if v.footer.getDiskType() == vhd.VHD_DYNAMIC:
        updates.append( (v.footerCopy.getChecksumOffset(),v.footerCopy.calculateChecksum()) )
        updates.append( (v.dynamicHeader.getChecksumOffset(),v.dynamicHeader.calculateChecksum()) )

    updates.append( ( v.footer.getChecksumOffset(), v.footer.calculateChecksum() ) )
    v.updateFile(updates)
    v.close()