爬虫课堂（二十八）|Spider和CrawlSpider的源码分析

    def __init__(self, name=None, **kwargs):
        if name is not None:
            self.name = name
        elif not getattr(self, 'name', None):
            raise ValueError("%s must have a name" % type(self).__name__)
        self.__dict__.update(kwargs)
        if not hasattr(self, 'start_urls'):
            self.start_urls = []

    def start_requests(self):
        cls = self.__class__
        if method_is_overridden(cls, Spider, 'make_requests_from_url'):
            warnings.warn(
                "Spider.make_requests_from_url method is deprecated; it "
                "won't be called in future Scrapy releases. Please "
                "override Spider.start_requests method instead (see %s.%s)." % (
                    cls.__module__, cls.__name__
                ),
            )
            for url in self.start_urls:
                yield self.make_requests_from_url(url)
        else:
            for url in self.start_urls:
                yield Request(url, dont_filter=True)

    def make_requests_from_url(self, url):
        """ This method is deprecated. """
        return Request(url, dont_filter=True)

    def parse(self, response):
        raise NotImplementedError

"""
Base class for Scrapy spiders

See documentation in docs/topics/spiders.rst
"""
import logging
import warnings

from scrapy import signals
from scrapy.http import Request
from scrapy.utils.trackref import object_ref
from scrapy.utils.url import url_is_from_spider
from scrapy.utils.deprecate import create_deprecated_class
from scrapy.exceptions import ScrapyDeprecationWarning
from scrapy.utils.deprecate import method_is_overridden

#所有爬虫的基类，用户定义的爬虫必须从这个类继承
class Spider(object_ref):
    """Base class for scrapy spiders. All spiders must inherit from this
    class.
    """

    #1、定义spider名字的字符串。spider的名字定义了Scrapy如何定位（并初始化）spider，所以其必须是唯一的。
    #2、name是spider最重要的属性，而且是必须的。一般做法是以该网站的域名来命名spider。例如我们在爬取豆瓣读书爬虫时使用‘name = "douban_book_spider"’  
    name = None
    custom_settings = None

    #初始化爬虫名字和start_urls列表。上面已经提到。
    def __init__(self, name=None, **kwargs):
        #初始化爬虫名字
        if name is not None:
            self.name = name
        elif not getattr(self, 'name', None):
            raise ValueError("%s must have a name" % type(self).__name__)
        self.__dict__.update(kwargs)

        #初始化start_urls列表，当没有指定的URL时，spider将从该列表中开始进行爬取。 因此，第一个被获取到的页面的URL将是该列表之一，后续的URL将会从获取到的数据中提取。  
        if not hasattr(self, 'start_urls'):
            self.start_urls = []

    @property
    def logger(self):
        logger = logging.getLogger(self.name)
        return logging.LoggerAdapter(logger, {'spider': self})

    def log(self, message, level=logging.DEBUG, **kw):
        """Log the given message at the given log level

        This helper wraps a log call to the logger within the spider, but you
        can use it directly (e.g. Spider.logger.info('msg')) or use any other
        Python logger too.
        """
        self.logger.log(level, message, **kw)

    @classmethod
    def from_crawler(cls, crawler, *args, **kwargs):
        spider = cls(*args, **kwargs)
        spider._set_crawler(crawler)
        return spider

    def set_crawler(self, crawler):
        warnings.warn("set_crawler is deprecated, instantiate and bound the "
                      "spider to this crawler with from_crawler method "
                      "instead.",
                      category=ScrapyDeprecationWarning, stacklevel=2)
        assert not hasattr(self, 'crawler'), "Spider already bounded to a " \
                                             "crawler"
        self._set_crawler(crawler)

    def _set_crawler(self, crawler):
        self.crawler = crawler
        self.settings = crawler.settings
        crawler.signals.connect(self.close, signals.spider_closed)

    #该方法将读取start_urls列表内的地址，为每一个地址生成一个Request对象，并返回这些对象的迭代器。
    #注意：该方法只会调用一次。
    def start_requests(self):
        cls = self.__class__
        if method_is_overridden(cls, Spider, 'make_requests_from_url'):
            warnings.warn(
                "Spider.make_requests_from_url method is deprecated; it "
                "won't be called in future Scrapy releases. Please "
                "override Spider.start_requests method instead (see %s.%s)." % (
                    cls.__module__, cls.__name__
                ),
            )
            for url in self.start_urls:
                yield self.make_requests_from_url(url)
        else:
            for url in self.start_urls:
                yield Request(url, dont_filter=True)

    #1、start_requests()中调用，实际生成Request的函数。
    #2、Request对象默认的回调函数为parse()，提交的方式为get。
    def make_requests_from_url(self, url):
        """ This method is deprecated. """
        return Request(url, dont_filter=True)

    #默认的Request对象回调函数，处理返回的response。  
    #生成Item或者Request对象。这个类需要我们自己去实现。
    def parse(self, response):
        raise NotImplementedError

    @classmethod
    def update_settings(cls, settings):
        settings.setdict(cls.custom_settings or {}, priority='spider')

    @classmethod
    def handles_request(cls, request):
        return url_is_from_spider(request.url, cls)

    @staticmethod
    def close(spider, reason):
        closed = getattr(spider, 'closed', None)
        if callable(closed):
            return closed(reason)

    def __str__(self):
        return "<%s %r at 0x%0x>" % (type(self).__name__, self.name, id(self))

    __repr__ = __str__


BaseSpider = create_deprecated_class('BaseSpider', Spider)


class ObsoleteClass(object):
    def __init__(self, message):
        self.message = message

    def __getattr__(self, name):
        raise AttributeError(self.message)

spiders = ObsoleteClass(
    '"from scrapy.spider import spiders" no longer works - use '
    '"from scrapy.spiderloader import SpiderLoader" and instantiate '
    'it with your project settings"'
)

# Top-level imports
from scrapy.spiders.crawl import CrawlSpider, Rule
from scrapy.spiders.feed import XMLFeedSpider, CSVFeedSpider
from scrapy.spiders.sitemap import SitemapSpider

rules = (
    # 提取匹配 'category.php' (但不匹配 'subsection.php') 的链接并跟进链接(没有callback意味着follow默认为True)
    Rule(LinkExtractor(allow=('category\.php', ), deny=('subsection\.php', ))),

    # 提取匹配 'item.php' 的链接并使用spider的parse_item方法进行分析
    Rule(LinkExtractor(allow=('item\.php', )), callback='parse_item',  follow=True),
)

def parse_start_url(self, response):  
    return []  

def parse(self, response):
        raise NotImplementedError

def parse(self, response):
        return self._parse_response(response, self.parse_start_url, cb_kwargs={}, follow=True)

class JianshuCrawl(CrawlSpider):
    name = "jianshu_spider_crawl"
    # 可选，加上会有一个爬取的范围
    allowed_domains = ["jianshu.com"]
    start_urls = ['https://www.jianshu.com/']

    # response中提取链接的匹配规则，得出符合条件的链接
    pattern = '.*jianshu.com/u/*.'
    pagelink = LinkExtractor(allow=pattern)

    # 可以写多个rule规则
    rules = [
        # 只要符合匹配规则，在rule中都会发送请求，同时调用回调函数处理响应。
        # rule就是批量处理请求。
        Rule(pagelink, callback='parse_item', follow=True),
    ]

    # 不能写parse方法，因为源码中已经有了，会覆盖导致程序不能跑
    def parse_item(self, response):
        for each in response.xpath("//div[@class='main-top']"):
    ......

class CrawlSpider(Spider):

    rules = ()

    def __init__(self, *a, **kw):
        super(CrawlSpider, self).__init__(*a, **kw)
        self._compile_rules()

    #1、首先调用parse()方法来处理start_urls中返回的response对象。
    #2、parse()将这些response对象传递给了_parse_response()函数处理，并设置回调函数为parse_start_url()。
    #3、设置了跟进标志位True，即follow=True。
    #4、返回response。
    def parse(self, response):
        return self._parse_response(response, self.parse_start_url, cb_kwargs={}, follow=True)

    #处理start_url中返回的response，需要重写。
    def parse_start_url(self, response):
        return []

    def process_results(self, response, results):
        return results

    def _build_request(self, rule, link):
         #构造Request对象，并将Rule规则中定义的回调函数作为这个Request对象的回调函数。这个‘_build_request’函数在下面调用。
        r = Request(url=link.url, callback=self._response_downloaded)
        r.meta.update(rule=rule, link_text=link.text)
        return r

    #从response中抽取符合任一用户定义'规则'的链接，并构造成Resquest对象返回。
    def _requests_to_follow(self, response):
        if not isinstance(response, HtmlResponse):
            return
        seen = set()
        #抽取所有链接，只要通过任意一个'规则'，即表示合法。 
        for n, rule in enumerate(self._rules):
            links = [lnk for lnk in rule.link_extractor.extract_links(response)
                     if lnk not in seen]
            if links and rule.process_links:
                links = rule.process_links(links)
            #将链接加入seen集合，为每个链接生成Request对象，并设置回调函数为_repsonse_downloaded()。
            for link in links:
                seen.add(link)
                #构造Request对象，并将Rule规则中定义的回调函数作为这个Request对象的回调函数。这个‘_build_request’函数在上面定义。
                r = self._build_request(n, link)
                #对每个Request调用process_request()函数。该函数默认为indentify，即不做任何处理，直接返回该Request。
                yield rule.process_request(r)

    #处理通过rule提取出的连接，并返回item以及request。
    def _response_downloaded(self, response):
        rule = self._rules[response.meta['rule']]
        return self._parse_response(response, rule.callback, rule.cb_kwargs, rule.follow)

    #解析response对象，使用callback解析处理他，并返回request或Item对象。
    def _parse_response(self, response, callback, cb_kwargs, follow=True):
        #1、首先判断是否设置了回调函数。（该回调函数可能是rule中的解析函数，也可能是 parse_start_url函数）  
        #2、如果设置了回调函数（parse_start_url()），那么首先用parse_start_url()处理response对象，  
        #3、然后再交给process_results处理。返回cb_res的一个列表。  
        if callback:
            cb_res = callback(response, **cb_kwargs) or ()
            cb_res = self.process_results(response, cb_res)
            for requests_or_item in iterate_spider_output(cb_res):
                yield requests_or_item

        #如果需要跟进，那么使用定义的Rule规则提取并返回这些Request对象。
        if follow and self._follow_links:
            #返回每个Request对象。
            for request_or_item in self._requests_to_follow(response):
                yield request_or_item

    def _compile_rules(self):
        def get_method(method):
            if callable(method):
                return method
            elif isinstance(method, six.string_types):
                return getattr(self, method, None)

        self._rules = [copy.copy(r) for r in self.rules]
        for rule in self._rules:
            rule.callback = get_method(rule.callback)
            rule.process_links = get_method(rule.process_links)
            rule.process_request = get_method(rule.process_request)

    @classmethod
    def from_crawler(cls, crawler, *args, **kwargs):
        spider = super(CrawlSpider, cls).from_crawler(crawler, *args, **kwargs)
        spider._follow_links = crawler.settings.getbool(
            'CRAWLSPIDER_FOLLOW_LINKS', True)
        return spider

    def set_crawler(self, crawler):
        super(CrawlSpider, self).set_crawler(crawler)
        self._follow_links = crawler.settings.getbool('CRAWLSPIDER_FOLLOW_LINKS', True)