Nayuta .arb Inserter v1.1

#1.1: Updates data.lst
import os
import struct

def get_data(filename):
    totalbytes = os.path.getsize(filename)
    infile = open(filename, 'rb')
    totalfiledata = infile.read(totalbytes)
    infile.close()
    return totalfiledata

def replacestr(origstr,replacestr,startpos,replacelen):
#Returns a string with a replaced sub-string
#origstr - the original string, replacestr = the string to replace
#startpos - where the replacement string should go
#replacelen - how many characters of the original string to replace
    return origstr[:startpos] + replacestr + origstr[startpos+replacelen:]

def datalstupdate(filename,newsize):
    '''Updates data.lst'''
    filedata = get_data('data.lst')
    pos = 0x9020
    filename = filename + '\x00'*(8-len(filename))
    while True:
        pos = filedata.find(filename,pos+1)
        if pos == -1:
            print '{} not found in data.lst'.format(filename)
        elif filedata[pos+15] == '\x0d': #Indicates .arb file extension
            newsize = struct.pack('<I',newsize)
            filedata = replacestr(filedata,newsize,pos+8,4)
            with open('data.lst','wb') as f:
                f.write(filedata)
            break

inputdata = []
with open('arbdata.csv','rb') as f:
    for line in f:
        line = line.translate(None,"\r\n") #Trims newline characters
        line = line.split('\t')
        inputdata.append((line[0],int(line[1],16),line[3]))

oldfile = ''
firstpass = True
for i,(filename,addr,newstring) in enumerate(inputdata):
    if filename != oldfile:
        if firstpass:
            firstpass = False
        else:
            outfile = open(os.getcwd() + '\\' + oldfile + '\\' + oldfile + '.arb','wb')
            outfile.write(filedata)
            outfile.close()
            datalstupdate(oldfile,len(filedata))
        filedata = get_data(os.getcwd() + '\\' + filename + '.orig')
        offset = 0
    addr = addr + offset
    oldsize = struct.unpack('<I',filedata[addr:addr+4])[0] + 4
    if filename != oldfile:
        newsize = len(newstring) + 6
        newstring = struct.pack('<I',len(newstring) + 2) + ' ' + newstring + '\x00'
    else:
        newsize = len(newstring) + 5
        newstring = struct.pack('<I',len(newstring) + 1) + newstring + '\x00'
    filedata = replacestr(filedata,newstring,addr,oldsize)
    offset = offset + newsize - oldsize
    oldfile = filename

outfile = open(os.getcwd() + '\\' + filename + '\\' + filename + \
               '.arb','wb')
outfile.write(filedata)
outfile.close()
datalstupdate(filename,len(filedata))