How to write data onto csv file over a loop in python

Question

I want to write data onto several rows in a csv file. With this code I am only getting a single line written onto file. Need help.

for i in range(1,10):
    with open("output.csv",'wb') as f:
        writer = csv.writer(f, dialect='excel')
        writer.writerow([value1, value2, value3])

score 3 · Accepted Answer · answered Nov 19 '15 at 20:36

3

You are encountering the error because it is re-writing a csv for every iteration in your loop. You should move the with open() statement outside of your loop block.

answered Nov 19 '15 at 20:36

Jeff Mandell

863
7
16

1

And the creation of the `csv.writer` should be outside the loop for that matter (it would probably work without that change, but rebuilding the `writer` over and over is slow/wasteful). – ShadowRanger Nov 19 '15 at 20:38
Is there a way I can append the data without overwriting it? – Rakesh Nittur Nov 19 '15 at 20:39
yes. There is a post by Alvaro here that shows the code – Jeff Mandell Nov 19 '15 at 20:40
I changed it to "with open("output.csv",'a') as f:" and it solved the problem! – Rakesh Nittur Nov 19 '15 at 20:54

score 3 · Answer 2 · answered Nov 19 '15 at 20:39

3

Try opening the file only once and then doing the loop:

with open("output.csv",'wb') as f:
    writer = csv.writer(f, dialect='excel')
    for item in list_A:
        writer.writerow([value1, value2, value3])

answered Nov 19 '15 at 20:39

Alvaro Bataller

487
8
29

score 1 · Answer 3 · answered Apr 08 '21 at 23:02

1

You would need to use the w option in open as follows:

import csv
list_A = [0,0,0,0,0,0,0,0,0,0,0]
with open("output.csv",'w') as f:
    writer = csv.writer(f)
    for item in list_A:
        writer.writerow([1,0,0,0])

answered Apr 08 '21 at 23:02

GigaWatts

101
8

How to write data onto csv file over a loop in python

3 Answers3