Help - python compare files

file1 = "file1.txt"
file2 = "file2.txt"
outputfile = "file3.txt"

f1lines = []
f2stripped = []
f2linesoriginal = []
output = []

with open(file1, "r") as f1:
    for line in f1:
        f1lines.append(line.strip())

with open(file2, "r") as f2:
    for line in f2:
        f2linesoriginal.append(line)
        compdata = line.split(";")[-2:]   #removing the [ag] in start of line.....
        datastring = ""
        for element in compdata:
            datastring += element.strip() + ";" #putting the string back toghether adding ";"

        f2stripped.append(datastring[:-1])

for line in f1lines:
    if line in f2stripped:
        output.append(f2linesoriginal[f2stripped.index(line)].strip()) #if match add the original f2 line
    else:
        output.append(line) # else keep the f1 line

print(output)