How to use scrapy to crawl multiple pages?

Question

All examples i found of Scrapy talk about how to crawl a single page, pages with the same url schema or all the pages of a website. I need to crawl series of pages A, B, C where in A you got the link to B and so on.. For example the website structure is:

A
----> B
---------> C
D
E

I need to crawl all the C pages, but to get link to C i need to crawl before A and B. Any hints?

Answer 1

see scrapy Request structure, to crawl such chain you'll have to use the callback parameter like the following:

class MySpider(BaseSpider):
    ...
    # spider starts here
    def parse(self, response):
        ...
        # A, D, E are done in parallel, A -> B -> C are done serially
        yield Request(url=<A url>,
                      ...
                      callback=parseA)
        yield Request(url=<D url>,
                      ...
                      callback=parseD)
        yield Request(url=<E url>,
                      ...
                      callback=parseE)

    def parseA(self, response):
        ...
        yield Request(url=<B url>,
                      ...
                      callback=parseB)

    def parseB(self, response):
        ...
        yield Request(url=<C url>,
                      ...
                      callback=parseC)

    def parseC(self, response):
        ...

    def parseD(self, response):
        ...

    def parseE(self, response):
        ...

Answer 2

Here is an example spider I wrote for a project of mine:

from scrapy.contrib.spiders import CrawlSpider
from scrapy.selector import HtmlXPathSelector
from scrapy.http import Request
from yoMamaSpider.items import JokeItem
from yoMamaSpider.striputils import stripcats, stripjokes
import re

class Jokes4UsSpider(CrawlSpider):
    name = 'jokes4us'
    allowed_domains = ['jokes4us.com']
    start_urls = ["http://www.jokes4us.com/yomamajokes/"]

    def parse(self, response):
        hxs = HtmlXPathSelector(response)
        links = hxs.select('//a')
        for link in links:
            url = ''.join(link.select('./@href').extract())
            relevant_urls = re.compile(
                'http://www\.jokes4us\.com/yomamajokes/yomamas([a-zA-Z]+)')
            if relevant_urls.match(url):
                yield Request(url, callback=self.parse_page)

    def parse_page(self, response):
        hxs = HtmlXPathSelector(response)
        categories = stripcats(hxs.select('//title/text()').extract())
        joke_area = hxs.select('//p/text()').extract()
        for joke in joke_area:
            joke = stripjokes(joke)
            if len(joke) > 15:
                yield JokeItem(joke=joke, categories=categories)

I think the parse method is what you are after: It looks at every link on the start_urls page, it then uses some regex to decide if it is a relevant_url (i.e. a url i would like to scrape), if it is relevant - it scrapes the page using yield Request(url, callback=self.parse_page), which calls the parse_page method.

Is this the kind of thing you are after?