A simple script I use for reconnecting my wifi device on a network that has a portal. Used on Ubuntu 16.04.3 LTS.

reconnect_wifi_on_portal_wifi.py

from os import system as run
from random import random
import requests
from time import sleep

def reconnect_wifi():
    run("nmcli d connect wlp3s0")
    run("firefox http://bbc.co.uk/?"+str(random()))

def are_we_connected():
    try:
        r = requests.get('http://bbc.co.uk')
        assert('News' in r.text)
        return True
    except:
        print('Not connected')
        return False

while True:
    if are_we_connected() is False:
        reconnect_wifi()
    sleep(30)