Iterate 2 large text files across lines and replace lines in second file

[medatib531]

Worked, needed a minor tweak to make it work for the very last line as well

with open("file1", 'r') as fileA, open("file2", 'r+') as fileB:
    line_start = fileB.tell()
    line1 = fileA.readline()
    line2 = fileB.readline()
    while line1 and line2:
        if line1 == line2:
            replace = "00"
            fileB.seek(line_start)
            fileB.write(replace) 
        line_start = fileB.tell()
        if line1 != line2: line1 = fileA.readline()
        line2 = fileB.readline()

Thanks!