agregate / csv / itertools

import csv
from itertools import chain, groupby


def agregate(*csv_files, output_file):
    grouping = lambda row: row[1]
    fds = [open(file) for file in csv_files]
    csv_files = (csv.reader(f, delimiter=';') for f in fds)
    csv_data = sorted(chain.from_iterable(csv_files), key=grouping)
    [file.close() for file in fds]
    with open(output_file, 'w') as fd:
        writer = csv.writer(fd, delimiter=';')
        for group, elements in groupby(csv_data, key=grouping):
            produkte = []
            for produkt, e_mail, vorname, nachname in elements:
                produkte.append(produkt)
            produkte = '\n'.join(produkte)
            writer.writerow((produkte, e_mail, vorname, nachname))