from urllib2 import urlopenfrom bs4 import BeautifulSoup# Get the next page url from the current page urldef get_next_page_url(url):    page = urlopen(url)    soup_page = BeautifulSoup(page, 'lxml')    page.close()    # Get current page and next page tag    current_page_tag = soup_page.find(class_="current")    next_page_tag = current_page_tag.find_next_sibling()    # Check if the current page is the last one    if next_page_tag is None:        next_page_url = None    else:        next_page_url = next_page_tag['href']    return next_page_url # Get the book detail urls by page url def get_book_detail_urls(url):    page = urlopen(url)    soup = BeautifulSoup(page, 'lxml')    page.close()    urls = []    book_header_tags = soup.find_all(class_="entry-title")    for book_header_tag in book_header_tags:        urls.append(book_header_tag.a['href'])    return urls    # Get the book detail info by book detail urldef get_book_isbn(url):    page = urlopen(url)    book_isbn_soup = BeautifulSoup(page, 'lxml')    page.close()    title_tag = book_isbn_soup.find(class_="single-title")    title = title_tag.string    isbn_key_tag = book_isbn_soup.find(text="ISBN-10:").parent    isbn_tag = isbn_key_tag.find_next_sibling()    isbn = isbn_tag.string.strip() # Remove the whitespace with the strip method    return { 'title': title, 'ISBN': isbn }def start():    url = "http://www.allitebooks.com/certification/"    book_info_list = []    def next_page(page_url):        book_detail_urls = get_book_detail_urls(page_url)        for book_detail_url in book_detail_urls: #print all books ISBN one by one            # print(book_detail_url)            book_info = get_book_isbn(book_detail_url)            print(book_info)   # print ISBD            book_info_list.append(book_info)        next_page_url = get_next_page_url(page_url)        if next_page_url is not None:            next_page(next_page_url)        else:            return 0    next_page(url)