Python : Compare two csv files and print out differences

Question

I need to compare two CSV files and print out differences in a third CSV file. In my case, the first CSV is a old list of hash named old.csv and the second CSV is the new list of hash which contains both old and new hash.

Here is my code :

import csv
t1 = open('old.csv', 'r')
t2 = open('new.csv', 'r')
fileone = t1.readlines()
filetwo = t2.readlines()
t1.close()
t2.close()

outFile = open('update.csv', 'w')
x = 0
for i in fileone:
    if i != filetwo[x]:
        outFile.write(filetwo[x])
    x += 1
outFile.close()

The third file is a copy of the old one and not the update. What's wrong ? I Hope you can help me, many thanks !!

PS : i don't want to use diff

Answer 1

The problem is that you are comparing each line in fileone to the same line in filetwo. As soon as there is an extra line in one file you will find that the lines are never equal again. Try this:

with open('old.csv', 'r') as t1, open('new.csv', 'r') as t2:
    fileone = t1.readlines()
    filetwo = t2.readlines()

with open('update.csv', 'w') as outFile:
    for line in filetwo:
        if line not in fileone:
            outFile.write(line)