does overbooking validation

filter.py

def validate_reservation_booking(initial_day, final_day, ship_object):
    """does reservation avoiding overbooking"""
    response = False
    message = []
    now_time = date.today()
#     initial is less than final
    if final_day < initial_day:
        message.append('La fecha final no puede ser menor a la fecha inicial, por favor modifica las fechas de tu reserva.')
        return response, ','.join(message)
#     initial and final are in the future
    if final_day < now_time or initial_day < now_time:
        message.append('No es posible reservar en una fecha pasada, por favor modifica las fechas de tu reserva.')
        return response, ','.join(message)
#     for all reservations, check new one is not inside them or contain them
    overbooking = False
    # get all reservations that end in future from now
    reservations = Reservation.objects.filter(end_day__gt=now_time, product=ship_object)
    for item in reservations:
        if (initial_day < final_day < item.start_day) or (item.end_day < initial_day < final_day):
            pass
        else:
            overbooking = True
            message.append('Hay una reserva de: <<{0.start_day}>> al <<{0.end_day}>>'.format(item))
            break
#    if overbooking, dont accept add the reservation
    if not overbooking:
        message.append('not overbooking')
        response = True
    else:
        message.append('overbooking, por favor modifica las fechas de tu reserva.')

    print(message)
    return response, ','.join(message)