frontera运行link_follower.py 报错：doesn't define any object named 'FIFO'

from __future__ import print_function



import re



import requests



from frontera.contrib.requests.manager import RequestsFrontierManager

# from frontera.contrib.requests.manager import RequestsFrontierManager

from frontera import Settings



from six.moves.urllib.parse import urljoin





SETTINGS = Settings()

SETTINGS.BACKEND = 'frontera.contrib.backends.memory.FIFO'

# SETTINGS.BACKEND = 'frontera.contrib.backends.memory.MemoryDistributedBackend'



SETTINGS.LOGGING_MANAGER_ENABLED = True

SETTINGS.LOGGING_BACKEND_ENABLED = True

SETTINGS.MAX_REQUESTS = 100

SETTINGS.MAX_NEXT_REQUESTS = 10



SEEDS = [

    'http://www.imdb.com',

]



LINK_RE = re.compile(r'<a.+?href="(.*?)".?>', re.I)





def extract_page_links(response):

    return [urljoin(response.url, link) for link in LINK_RE.findall(response.text)]



if __name__ == '__main__':



    frontier = RequestsFrontierManager(SETTINGS)

    frontier.add_seeds([requests.Request(url=url) for url in SEEDS])

    while True:

        next_requests = frontier.get_next_requests()

        if not next_requests:

            break

        for request in next_requests:

                try:

                    response = requests.get(request.url)

                    links = [

                        requests.Request(url=url)

                        for url in extract_page_links(response)

                    ]

                    frontier.page_crawled(response)

                    print('Crawled', response.url, '(found', len(links), 'urls)')



                    if links:

                        frontier.links_extracted(request, links)

                except requests.RequestException as e:

                    error_code = type(e).__name__

                    frontier.request_error(request, error_code)

                    print('Failed to process request', request.url, 'Error:', e)

raise NameError("Module '%s' doesn't define any object named '%s'" % (module, name))

NameError: Module 'frontera.contrib.backends.memory' doesn't define any object named 'FIFO'