Adding Columns to CSV using iterator

[buran]

import csv

in_file = 'InputFile.csv' # change this accordingly
out_file = 'PSL_Combined_Sorted.csv'
with open(in_file, 'r') as in_f, open(out_file, 'w', newline='') as out_f:
    rdr =  csv.DictReader(in_f)
    fieldnames =  ['Purchase Order Number', 'Purchases Transaction ID Number']
    fieldnames.extend(rdr.fieldnames)
    wrtr = csv.DictWriter(out_f, fieldnames=fieldnames)
    wrtr.writeheader()
    for row_id, row in enumerate(rdr, start=1):
        row['Purchase Order Number'] = '{:0>7}'.format(row_id)
        row['Purchases Transaction ID Number'] = '{:0>6}'.format(row_id)
        wrtr.writerow(row)