Compare two csv files in Python and return matching results in new csv file without duplicates

Question

I have two csv files, one called web_file with 25,000 lines, the other file called inv_file contains 320,000 lines.

I need to read through each row from column 1 of web_file and find all matching values from each row in column 1 of inv_file and write the row from inv_file into a new csv file.

Using example files with only 5-10 lines doesn't show the issue as well, so I've listed a bunch of random numbers for example below.

Example web_file:

Inv_SKU,Web_SKU,Brand,Barcode
225481-34,225481-34,brand1,987654321
0486592,0486592,brand2,654871233
AB56412,AB56412,brand2,651273214
LL-123456,LL-123456,brand3,748912349
JLPD-65,JLPD-65,brand6,341541648
20143966,20143966,brand3,82193714
39585824,39585824,brand5,36837329
78066099,78066099,brand4,98398987
44381051,44381051,brand1,9090428
86529443,86529443,brand4,6861670
DF 5645 12,DF 5645 12,brand1,489456138
9845671325,9845671325,brand4,498451315
59634923,59634923,brand4,35828574
85290760,85290760,brand2,64562216
41217184,41217184,brand4,12816236
AE48915,AE48915,brand1,342536125
93981723,93981723,brand2,58155601

Example inv_file:

Inv_SKU,Web_SKU,Brand,Barcode
0486592,0486592,brand2,654871233
LL-123456,LL-123456,brand3,748912349
9845671325,9845671325,brand4,498451315
OI3248967,OI3248967,brand2,891513211
AB56412,AB56412,brand2,651273214
DF 5645 12,DF 5645 12,brand1,489456138
225481-34,225481-34,brand1,987654321
123456789,123456789,brand5,654986413
9841531,9841531,brand3,543254512
AE48915,AE48915,brand1,342536125
JLPD-65,JLPD-65,brand6,341541648
MMMM,MMMM,brand7,384941542
23481-4323,23481-4323,brand3,489123157
98451321,98451321,brand4,498121354
23454152,23454152,brand2,894165123
10275690,10275690,brand2,25612670
20143966,20143966,brand3,82193714
59634923,59634923,brand4,35828574
65800253,65800253,brand5,72318134
67722613,67722613,brand6,93290033
92617199,92617199,brand7,95078073
15379652,15379652,brand1,56281224
85290760,85290760,brand2,64562216
78066099,78066099,brand4,98398987
41217184,41217184,brand4,12816236
87152990,87152990,brand4,95058925
73813369,73813369,brand1,2395994
50201544,50201544,brand1,9167830
93981723,93981723,brand2,58155601
39585824,39585824,brand5,36837329
29082963,29082963,brand3,23393947
23856043,23856043,brand8,57295562
74249006,74249006,brand8,83219065
94376071,94376071,brand8,94887004
14553763,14553763,brand8,14223230
44381051,44381051,brand1,9090428
7598085,7598085,brand1,48967969
56383025,56383025,brand2,68864452
44338055,44338055,brand4,47043853
86529443,86529443,brand4,6861670

I tried using this code, but ended up with many duplicated lines, which I want to avoid since the file I am actually using is so large I end up with millions of lines.

with open('inv_file.csv', 'r') as f1, open('web_file.csv', 'r') as f2:
    inv_file = f1.readlines()
    web_file = f2.readlines()


with open('result.csv', 'r+') as f3:
    result_file = f3.readlines()

    while len(result_file) < len(web_file):
        for row in inv_file:
            for row1 in web_file:
                if row[0] in row1[0]:
                    f3.write(row1)
        break

Answer 1

You should really be parsing csv files using the csv library. One approach would be to store a list of the web skus (hopefully I've got this right way round) and then check the inv skus against it. This can be done efficiently with a generator passed to the csv writerows() method.

import csv
with open('inv_file.csv', 'r') as f1, open('web_file.csv', 'r') as f2, open('result.csv', 'w') as f3:
    web_skus = [row[0] for row in csv.reader(f2)]
    # web_skus = set([row[0] for row in csv.reader(f2)])  # uncomment to remove dupliate web skus
    inv_file = csv.reader(f1)
    rows = (row for row in inv_file if row[0] in web_skus)

    writer = csv.writer(f3)
    writer.writerows(rows)