Compare 2 seperate csv files and write difference to a new csv file - Python 2.7

Question

I am trying to compare two csv files in python and save the difference to a third csv file in python 2.7.

import csv

f1 = open (\"olddata/file1.csv\")
oldFile1         


        

Answer 1

What do you mean by difference? The answer to that gives you two distinct possibilities.

If a row is considered same when all columns are same, then you can get your answer via the following code:

import csv

f1 = open ("olddata/file1.csv")
oldFile1 = csv.reader(f1)
oldList1 = []
for row in oldFile1:
    oldList1.append(row)

f2 = open ("newdata/file2.csv")
oldFile2 = csv.reader(f2)
oldList2 = []
for row in oldFile2:
    oldList2.append(row)

f1.close()
f2.close()

print [row for row in oldList1 if row not in oldList2]

However, if two rows are same if a certain key field (i.e. column) is same, then the following code will give you your answer:

import csv

f1 = open ("olddata/file1.csv")
oldFile1 = csv.reader(f1)
oldList1 = []
for row in oldFile1:
    oldList1.append(row)

f2 = open ("newdata/file2.csv")
oldFile2 = csv.reader(f2)
oldList2 = []
for row in oldFile2:
    oldList2.append(row)

f1.close()
f2.close()

keyfield = 0 # Change this for choosing the column number

oldList2keys = [row[keyfield] for row in oldList2]
print [row for row in oldList1 if row[keyfield] not in oldList2keys]

Note: The above code might run slow for extremely large files. If instead, you wish to speed up code through hashing, you can use set after converting the oldLists using the following code:

set1 = set(tuple(row) for row in oldList1)
set2 = set(tuple(row) for row in oldList2)

After this, you can use set1.difference(set2)