How to append one csv file to another with python

Question

I have two .csv files that I need to either join into a new file or append one to the other:

filea:

jan,feb,mar
80,50,52
74,73,56

fileb:

apr,may,jun
64,75,64
75,63,63

What I need is:

jan,feb,mar,apr,may,jun
80,50,52,64,75,64
74,73,56,75,63,63

What I'm getting:

jan,feb,mar
80,50,52
74,73,56
apr,may,jun
64,75,64
75,63,63

I'm using the simplest code I can find. A bit too simple I guess:

sourceFile = open('fileb.csv', 'r')
data = sourceFile.read()
with open('filea.csv', 'a') as destFile:
    destFile.write(data

I'd be very grateful if anyone could tell me what I'm doing wrong and how to get them to append 'horizontally' instead of 'vertically'.

Not in python at all, but there is a bash command exactly for that: paste -d',' filea.csv fileb.csv — fredtantini
– fredtantini, Commented Oct 7, 2014 at 8:16
You are passing strings to writerow, hence it is iterating over and comma-separating characters. You need to build a list representing each row ['Jan', 'Feb', ..., 'Jun'] and pass that to writerow. — jonrsharpe
– jonrsharpe, Commented Oct 7, 2014 at 8:16
Apologies, I edited my question and didn't realise I had already gotten comments. Thanks very much everyone. — Meelah
– Meelah, Commented Oct 7, 2014 at 8:34
fredtantini - Thank you but I tried that and got the exact same result. @Padraic - the files I'm trying it with are equal length but I need something that could also work with files different in length and possibly containing blank fields. — Meelah
– Meelah, Commented Oct 7, 2014 at 8:42

Padraic Cunningham · Accepted Answer · 2014-10-07 09:11:51Z

1

from itertools import izip_longest
with open("filea.csv") as source1,open("fileb.csv")as source2,open("filec.csv","a") as dest2:
    zipped = izip_longest(source1,source2) # use izip_longest which will add None as a fillvalue where we have uneven length files
    for line in zipped:
        if line[1]: # if we have two lines to join
            dest2.write("{},{}\n".format(line[0][:-1],line[1][:-1]))
        else: # else we are into the longest file, just treat line as a single item tuple
             dest2.write("{}".format(line[0]))

answered Oct 7, 2014 at 9:11

Padraic Cunningham

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Yanuar Kusnadi · Accepted Answer · 2014-10-07 13:05:14Z

0

In case your files have the same length or at least contain blank fields:

filea.csv

jan,feb,mar
80,50,52
74,73,56
,,

fileb.csv

apr,may,jun
64,75,64
75,63,63
77,88,99

Script :

with open("filea.csv", "r") as source1, open("fileb.csv", "r") as source2, open("filec.csv","w") as dest:
    for line1, line2 in zip(source1, source2):
        dest.write(line1.strip()+','+line2)

If you need more compact version :

with open("filea.csv", "r") as source1, open("fileb.csv", "r") as source2, open("filec.csv","w") as dest:
    [dest.write(line1.strip()+','+line2) for line1, line2 in zip(source1, source2)]

Result (filec.csv):

jan,feb,mar,apr,may,jun
80,50,52,64,75,64
74,73,56,75,63,63
,,,77,88,99

edited Oct 7, 2014 at 13:05

answered Oct 7, 2014 at 9:33

Yanuar Kusnadi

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