Python binary patcher for .dif files

dif2bin.py

#!/usr/bin/env python
# Small .dif patcher for Python 3+

# File format of .dif file (description and filename should be on the 1st and 3rd lines exactly):

#        File description
#
#        filename.ext
#        0000000a: 00 01
#        0000000b: 00 02

import re,ntpath,sys
from sys import argv,exit
from ctypes import c_uint8

def path_leaf(path):
    head, tail = ntpath.split(path)
    return tail or ntpath.basename(head)

def patch(file, dif, revert=False):
    code = open(file,'rb').read()
    open(file+'.bak','wb').write(code)
    dif = open(dif,'r').read()
    m = re.findall('([0-9a-fA-F]+): ([0-9a-fA-F]+) ([0-9a-fA-F]+)', dif)
    for offset,orig,new in m:
        o, orig, new = int(offset,16), int(orig, 16), int(new, 16)
        print("%08X: %02X %02X" % (o,orig,new))
        if revert:
            if code[o]==new:
                code = code[:o]+bytes(c_uint8(orig))+code[o+1:]
            else:
                raise Exception("patched byte at %s is not %02X" % (offset, new))
        else:
            if code[o]==orig:
                code = code[:o]+bytes(c_uint8(new))+code[o+1:]
            else:
                raise Exception("original byte at %s is not %02X" % (offset, orig))
    open(file,'wb').write(code)

def main():
    if sys.version_info[0] < 3:
        print("Python 3.0 or higher required.")
        sys.exit(1) 

    if len(argv)<2:
        print(" ")
        print("Usage: %s <.dif file> [-revert]" % path_leaf(argv[0]))
        print("Applies given .dif file to a binary; use -revert to revert patch.")
        exit(0)
    
    file, dif, revert = "", argv[1], False
    diff = open(dif,'r')
    for i, line in enumerate(diff):
        if i == 2:
            file = line.replace('\n', '').replace('\r', '')
            break
    diff.close()
    if len(argv)>2:
        revert = True
        print("Reverting patch %r on file %r" % (dif, file))
    else:
        print("Patching file %r with %r" % (file, dif))
    
    try:
        patch(file, dif, revert)
        print("Done")
    except Exception as e:
        print("Error: %s" % str(e))
        exit(1)

if __name__ == "__main__":
    main()