docs: def generate_key(url, page_number):
      """
      >>> url_a = 'http://localhost:5009/search?keywords=a'
      >>> generate_key(url_a, 10)
      'http://localhost:5009/search?keywords=a&page=10'
      >>> url_b = 'http://localhost:5009/search?keywords=b&page=1'
      >>> generate_key(url_b, 10)
      'http://localhost:5009/search?keywords=b&page=10'
      """
      index = url.rfind('page')
      if index != -1:
          result = url[0:index]
          result += 'page=%s' % page_number
      else:
          result = url
          result += '&page=%s' % page_number
      return result