read and write on same csv file

Question

I am trying to read and write on the same CSV file:

file1 = open(file.csv, \'rb\')
file2 = open(file.csv, \'wb\')
read         


        

Answer 1

You can't open a file in both read and write modes at once.

Your code could be modified as follows:-

# Do the reading
file1 = open(file.csv, 'rb')
reader = csv.reader(file1)
new_rows_list = []
for row in reader:
   if row[2] == 'Test':
      new_row = [row[0], row[1], 'Somevalue']
      new_rows_list.append(new_row)
file1.close()   # <---IMPORTANT

# Do the writing
file2 = open(file.csv, 'wb')
writer = csv.writer(file2)
writer.writerows(new_rows_list)
file2.close()

As Jason points out, if your CSV is too big for your memory, then you'll need to write to a different filename and then rename it. This will likely be a bit slower.