merge_csv.py

import sys
import csv
import argparse


def find_first_common_column(list_1, list_2):
    column = list(set(list_1) & set(list_2))[0]
    for i, col in enumerate(list_1):
        if col == column:
            place_1 = i
    for i, col in enumerate(list_2):
        if col == column:
            place_2 = i
    return (column, place_1, place_2)


def merge_files(path_1, path_2,):
    csv_1 = open(path_1)
    csv_2 = open(path_2)
    csv_output = open('output.csv', 'w')

    reader_1 = csv.reader(csv_1, delimiter=',')
    reader_2 = csv.reader(csv_2, delimiter=',')
    writer = csv.writer(csv_output, delimiter=',')

    header_1 = reader_1.next()
    header_2 = reader_2.next()

    common_column = find_first_common_column(header_1, header_2)
    print('Uses "%s" as common column' % common_column[0])

    data = {}
    for row in reader_1:
        key = row.pop(common_column[1])
        data[key] = row

    header_1.pop(common_column[1])

    print('Writing file')
    writer.writerow(header_2 + header_1)
    for row in reader_2:
        try:
            writer.writerow(row + data[row[common_column[2]]])
            sys.stdout.write('.')
        except KeyError:
            sys.stdout.write('F')

    csv_1.close()
    csv_2.close()
    csv_output.close()

if __name__ == '__main__':
    parser = argparse.ArgumentParser()
    parser.add_argument(
        'file_1',
        help='File containing lookup data. The common column should be '
             'unique in this file.'
    )
    parser.add_argument('file_2')

    args = parser.parse_args()
    merge_files(args.file_1, args.file_2)