How can I replace a value in an existing excel csv file using a python program?

Question

How can I update a value in an existing .csv file using a python program. At the moment the file is read into the program but I need to be able to change this value using my program, and for the change to be made to the file. How can I do this? This is my program:

import csv
file = open("productcodes.csv", "r")
for row in csv.reader(file):
   print(row[1])

How can I override the value row[1]?

Answer 1

import csv
r = csv.reader(open('productcodes.csv'))
lines = [l for l in r]
for l in lines:
    l[1] = "new value"

writer = csv.writer(open('productcodes.csv', 'w'))
writer.writerows(lines)

You can't really replace values in the existing file. Instead, you need to:

1. read in existing file
2. alter file in memory
3. write out new file(overwriting existing file)