Example solution

port1 = 0
for port2 in range(1, 5):
    for port3 in range(1, 5):
        for port4 in range(1, 5):
            for port5 in range(1, 5):
                route = [port1, port2, port3, port4, port5]
                if 0 in route and 1 in route and 2 in route and 3 in route and 4 in route:
                    distance = D[route[0]][route[1]] + D[route[1]][route[2]] + \
                        D[route[2]][route[3]] + D[route[3]][route[4]]               
                    emissions = distance * co2
                    print(' '.join([portnames[i] for i in route]) + " %.1f kg" % emissions)