这是我的密码

def parser():
    flag = True
    url = 'https://quotes.toscrape.com'
    while flag:
        responce = requests.get(url)
        soup = BeautifulSoup(responce.text, 'html.parser')
        quote_l = soup.find_all('span', {'class': 'text'})
        q_count = 0
        for i in range(len(quote_l)):
            if q_count >= 5:
                flag = False
                break
            quote = soup.find_all('span', {'class': 'text'})[i]
            if not Quote.objects.filter(quote=quote.string).exists():
                author = soup.find_all('small', {'class': 'author'})[i]
                if not Author.objects.filter(name=author.string).exists():
                    a = Author.objects.create(name=author.string)
                    Quote.objects.create(quote=quote.string, author_id=a.id)
                    q_count += 1
                else:
                    a = Author.objects.get(name=author.string)
                    Quote.objects.create(quote=quote.string, author_id=a.id)
                    q_count += 1

        url += soup.find('li', {'class': 'next'}).a['href']

我需要取得下一页，但我有这个例外。'NoneType'对象没有属性'a'
如何解决这个问题，也许我可以如何优化我的代码。Thx