Output enumerate function to csv in Python

September 1, 2022

I am trying to understand how to enumerate through a csv file and output the results to another csv file. For example, I have a csv file that has 2 columns, group_id and node_id. I want to use this csv file, loop through it and output the group_id, node_id and a formatted string that mentions the node_id. My code does exactly this when I use the print function however when I attempt to write to another csv file, only the last row is written.

Here is my code:

import csv

with open('input.csv', 'r') as f:

config_csv = csv.reader(f)

for row, column in enumerate(config_csv):

if row == 0:
    continue

group_id = int(column[0])
sub_id = column[1]
node = f"The sub ID for this node is {sub_id}."
full_output=[group_id, sub_id, node]
print(full_output)

with open('output.csv', 'w') as file:
    writer=csv.writer(file)
    writer.writerow(full_output)

csv file (input.csv):

GROUP_ID,SUB_ID
675233,111
877531,222
455632,333

And my print output is:

[675233, 111, 'The sub ID for this node is 111.']
[877531, 222, 'The sub ID for this node is 222.']
[455632, 333, 'The sub ID for this node is 333.']

However my output file (output.csv) only shows the last line:

[455632, 333, 'The sub ID for this node is 333.']

What am I doing wrong? Why doesn’t it output the same to the csv file that I see in the print function?

>Solution :

Open both files and write the lines as they are processed. It’s not clear from your incorrect indentation but you are probably writing a new file each time through the loop so only end up with the last line.

Also, make sure to use newline='' when opening as per csv documentation:

import csv

with (open('input.csv', 'r', newline='') as fin,
      open('output.csv', 'w', newline='') as fout):

    reader = csv.reader(fin)
    writer = csv.writer(fout)

    # read and copy header to output
    header = next(reader)
    header.append('COMMENT')
    print(header)
    writer.writerow(header)

    for row in reader:
        node = f"The sub ID for this node is {row[1]}."
        row.append(node)
        print(row)
        writer.writerow(row)

Console output:

['GROUP_ID', 'SUB_ID', 'COMMENT']
['675233', '111', 'The sub ID for this node is 111.']
['877531', '222', 'The sub ID for this node is 222.']
['455632', '333', 'The sub ID for this node is 333.']

output.csv:

GROUP_ID,SUB_ID,COMMENT
675233,111,The sub ID for this node is 111.
877531,222,The sub ID for this node is 222.
455632,333,The sub ID for this node is 333.