import py chunks = 7 p = py.path.local(py.std.sys.argv[1]) data = [l.strip() for l in p.readlines()] result = data[:2] data = data[2:] acc = 0 for i, line in enumerate(data): print line line = line.split(',') acc += int(line[1]) if i % chunks == chunks - 1: line[1] = str(acc) result.append(", ".join(line)) acc = 0 p.write("\n".join(result))