fairyberry fairyberry -4 years ago 246
Python Question

How to add a new column to a CSV file using Python?

I have several CSV files that look like this:

Input
Name Code
blackberry 1
wineberry 2
rasberry 1
blueberry 1
mulberry 2


I would like to add a new column to all CSV files so that it would look like this:

Output
Name Code Berry
blackberry 1 blackberry
wineberry 2 wineberry
rasberry 1 rasberry
blueberry 1 blueberry
mulberry 2 mulberry


The script I have so far is this:

import csv
with open(input.csv,'r') as csvinput:
with open(output.csv, 'w') as csvoutput:
writer = csv.writer(csvoutput)
for row in csv.reader(csvinput):
writer.writerow(row+['Berry'])


(Python 3.2)

But in the output, the script skips every line and the new column has only Berry in it:

Output
Name Code Berry
blackberry 1 Berry

wineberry 2 Berry

rasberry 1 Berry

blueberry 1 Berry

mulberry 2 Berry

Answer Source

This should give you an idea of what to do:

>>> v = open('C:/test/test.csv')
>>> r = csv.reader(v)
>>> row0 = r.next()
>>> row0.append('berry')
>>> print row0
['Name', 'Code', 'berry']
>>> for item in r:
...     item.append(item[0])
...     print item
...     
['blackberry', '1', 'blackberry']
['wineberry', '2', 'wineberry']
['rasberry', '1', 'rasberry']
['blueberry', '1', 'blueberry']
['mulberry', '2', 'mulberry']
>>> 

Edit, note in py3k you must use next(r)

Thanks for accepting the answer. Here you have a bonus (your working script):

import csv

with open('C:/test/test.csv','r') as csvinput:
    with open('C:/test/output.csv', 'w') as csvoutput:
        writer = csv.writer(csvoutput, lineterminator='\n')
        reader = csv.reader(csvinput)

        all = []
        row = next(reader)
        row.append('Berry')
        all.append(row)

        for row in reader:
            row.append(row[0])
            all.append(row)

        writer.writerows(all)

Please note

  1. the lineterminator parameter in csv.writer. By default it is set to '\r\n' and this is why you have double spacing.
  2. the use of a list to append all the lines and to write them in one shot with writerows. If your file is very, very big this probably is not a good idea (RAM) but for normal files I think it is faster because there is less I/O.
  3. As indicated in the comments to this post, note that instead of nesting the two with statements, you can do it in the same line:

    with open('C:/test/test.csv','r') as csvinput, open('C:/test/output.csv', 'w') as csvoutput:

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