Compare two csv files and write the matching entries in third file python

Question

I've seen many questions like this one here, but none of the solutions helped me. So lets say we have two csv files like this:

file1

12345:somevalue:anothervalue
23456:yetanother:onemore
23423:somevalue:anothervalue
41231:somevalue:anothervalue

file2

12345:desiredvalue:desiredvalue:andonemore
22334:desiredvalue:desiredvalue:andonemore
23456:desiredvalue:desiredvalue:andonemore
32213:desiredvalue:desiredvalue:andonemore
54412:desiredvalue:desiredvalue:andonemore
64123:desiredvalue:desiredvalue:andonemore

And the desired result is this: based on the number in the first column in file1 I need to get the row with the same starting number in file2 and write the matching values to file3 that should look like this:

12345:desiredvalue:desiredvalue:andonemore
23456:desiredvalue:desiredvalue:andonemore

I tried like this:

with open('file1', 'r', newline='') as file1, open('file2.csv', 'r', newline='') as file2:
    reader = csv.reader(file1, delimiter=':')
    reader1 = csv.reader(file2, delimiter=':')

    for row in reader:
        for row1 in reader1:
            if row[0] in row1[0]:
                with open('file3.csv', "w", encoding='utf-8',newline='') as csv_file:
                    writer = csv.writer(csv_file, delimiter=':')
                    writer.writerow(row1)

The sample code works .. partially. The result I get is just one row while there are more matches. What am I missing?

EDIT: I found it - I was missing the break after the second for loop:

if row[0] == row1[0]:
   writer.writerow(row1)
   break

does this answer your question? https://stackoverflow.com/a/5269042/13273054 — kiranr, Apr 05 '21 at 10:21

score 1 · Accepted Answer · answered Apr 05 '21 at 10:23

1

You are rewriting output file each time. Change "w" to "a+":

with open('file3.csv', "a+", encoding....

answered Apr 05 '21 at 10:23

Bohdan

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18

Thanks but I was missing the `break` after the second `for` loop. – Tony Apr 05 '21 at 11:12

Compare two csv files and write the matching entries in third file python

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