Scrapy之迭代爬取网页中失效问题分析

引言

在Scrapy中，在很多种情况下，需要一层层地进行爬取网页数据，就是基于url爬取网页，然后在从网页中提取url，继续爬取，循环往复。 本文将讲述一个在迭代爬取中，只能爬取第一层网页的问题。

问题的提出

scrapy crawl enrolldata
Scrapy代码执行结果输出如下：
“`
2018-05-06 17:23:06 [scrapy.utils.log] INFO: Scrapy 1.5.0 started (bot: enrolldata)
2018-05-06 17:23:06 [scrapy.utils.log] INFO: Versions: lxml 4.2.1.0, libxml2 2.9.8, cssselect 1.0.3, parsel 1.4.0, w3lib 1.19.0, Twisted 18.4.0, Python 3.6.1 (default, Apr 24 2017, 23:31:02) - [GCC 6.2.0 20161005], pyOpenSSL 17.5.0 (OpenSSL 1.1.0h 27 Mar 2018), cryptography 2.2.2, Platform Linux-4.15.0-20-generic-x86_64-with-debian-buster-sid
2018-05-06 17:23:06 [scrapy.crawler] INFO: Overridden settings: {‘BOT_NAME’: ‘enrolldata’, ‘CONCURRENT_REQUESTS’: 60, ‘CONCURRENT_REQUESTS_PER_IP’: 60, ‘DEPTH_LIMIT’: 5, ‘NEWSPIDER_MODULE’: ‘enrolldata.spiders’, ‘SPIDER_MODULES’: [‘enrolldata.spiders’]}
2018-05-06 17:23:06 [scrapy.middleware] INFO: Enabled extensions:
[‘scrapy.extensions.corestats.CoreStats’,
‘scrapy.extensions.telnet.TelnetConsole’,
‘scrapy.extensions.memusage.MemoryUsage’,
‘scrapy.extensions.logstats.LogStats’]
2018-05-06 17:23:06 [scrapy.middleware] INFO: Enabled downloader middlewares:
[‘scrapy.downloadermiddlewares.httpauth.HttpAuthMiddleware’,
‘scrapy.downloadermiddlewares.downloadtimeout.DownloadTimeoutMiddleware’,
‘scrapy.downloadermiddlewares.defaultheaders.DefaultHeadersMiddleware’,
‘scrapy.downloadermiddlewares.useragent.UserAgentMiddleware’,
‘scrapy.downloadermiddlewares.retry.RetryMiddleware’,
‘scrapy.downloadermiddlewares.redirect.MetaRefreshMiddleware’,
‘scrapy.downloadermiddlewares.httpcompression.HttpCompressionMiddleware’,
‘scrapy.downloadermiddlewares.redirect.RedirectMiddleware’,
‘scrapy.downloadermiddlewares.cookies.CookiesMiddleware’,
‘scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware’,
‘scrapy.downloadermiddlewares.stats.DownloaderStats’]
2018-05-06 17:23:06 [scrapy.middleware] INFO: Enabled spider middlewares:
[‘scrapy.spidermiddlewares.httperror.HttpErrorMiddleware’,
‘scrapy.spidermiddlewares.offsite.OffsiteMiddleware’,
‘scrapy.spidermiddlewares.referer.RefererMiddleware’,
‘scrapy.spidermiddlewares.urllength.UrlLengthMiddleware’,
‘scrapy.spidermiddlewares.depth.DepthMiddleware’]
2018-05-06 17:23:06 [scrapy.middleware] INFO: Enabled item pipelines:
[‘enrolldata.pipelines.EnrolldataPipeline’]
2018-05-06 17:23:06 [scrapy.core.engine] INFO: Spider opened
open spider ………..pipeline
2018-05-06 17:23:06 [scrapy.extensions.logstats] INFO: Crawled 0 pages (at 0 pages/min), scraped 0 items (at 0 items/min)
2018-05-06 17:23:06 [py.warnings] WARNING: /home/bladestone/codebase/python36env/lib/python3.6/site-packages/scrapy/spidermiddlewares/offsite.py:59: URLWarning: allowed_domains accepts only domains, not URLs. Ignoring URL entry http://www.heao.gov.cn/ in allowed_domains.
warnings.warn(“allowed_domains accepts only domains, not URLs. Ignoring URL entry %s in allowed_domains.” % domain, URLWarning)

2018-05-06 17:23:06 [scrapy.extensions.telnet] DEBUG: Telnet console listening on 127.0.0.1:6023
end of start requests
2018-05-06 17:23:14 [scrapy.core.engine] DEBUG: Crawled (200)

 -*- coding: utf-8 -*-
import scrapy
from enrolldata.items import EnrolldataItem

from scrapy.http import FormRequest

class SchoolspiderSpider(scrapy.Spider):
    name = 'enrolldata'
    cookies = {}
    allowed_domains = ['http://www.heao.gov.cn/']
    start_urls = ['http://www.heao.gov.cn/JHCX/PZ/enrollplan/SchoolList.aspx']
   .............
   def start_requests(self):
        formdata = {}
        formdata['PagesUpDown$edtPage'] = '1'
        formdata['__EVENTTARGET'] = 'PagesUpDown$lbtnGO'
        formdata['__EVENTARGUMENT'] = ''
        formdata['__VIEWSTATE'] = '/wEPDwUKMjA1MTU4MDA1Ng9kFgICBQ9kFgICAQ8PFggeDGZDdXJyZW50UGFnZQIBHhFmVG90YWxSZWNvcmRDb3VudAK4ER4KZlBhZ2VDb3VudAKVAR4JZlBhZ2VTaXplAg9kZGSI36vb/TsBmDT8pwwx37ajH1x0og=='
        formdata['__VIEWSTATEGENERATOR'] = 'AABB4DD8'
        formdata['__EVENTVALIDATION'] = '/wEWBQLYvvTTCwK2r/yJBQK6r/CJBgLqhPDLCwLQ0r3uCMy0KhJCAT8jebTQL0eNdj7uk4L5'
        for i in range(1, 2):
            formdata['PagesUpDown$edtPage'] = str(i)
            yield FormRequest(url=self.start_urls[0], headers=self.headers, formdata=formdata, callback=self.parse_school)
        print("end of start requests")

    def parse(self, response):
        print("parse method is invoked")
        pass

    def parse_school(self, response):
        print("parse school data.....")
        urls = response.xpath('//*[@id="SpanSchoolList"]/div/div[2]/ul/li/a/@href').extract();
        print("print out all the matched urls")
        print(urls)

        for url in urls:
            request_url = self.base_url + url
            print("request_url in major:" + request_url)
            yield scrapy.Request(request_url, headers=self.request_headers,  cookies=self.cookies, callback=self.parse_major_enroll, meta=self.meta)
......

代码没有报错，只是输出了第一层的Web的爬取结果。但是第二层没有执行爬取。

问题分析

从日志来进行分析，没有发现错误信息；第一层代码爬取正确，但是第二层web爬取，没有被执行，代码的编写应该没有问题的。
那问题是什么呢？会不会代码没有被执行呢？通过添加日志，但是对应的代码并没有执行，日志也被正常输出。是不是被过滤或者拦截了，从而代码没有被执行？
经过代码审查之后，发现allowed_domains设置的问题，由于起设置不正确，导致其余的链接被直接过滤了。
关于allowed_domains需要是一组域名，而非一组urls。

问题的解决

需要将之前的domain name修改一下：

allowed_domains = [‘http://www.heao.gov.cn/‘]

将起修改为：

allowed_domains = [‘heao.gov.cn’]

重新执行爬虫，发现多个层次是可以被正确爬取的。

总结

关于scrapy是一整套的解决方案，其中很多的设置和配置需要通过不同的实例来反复理解和应用的，才能如鱼得水，庖丁解牛般快速定位问题。