Scrapy之start_urls、爬虫中间件之深度，优先级以及源码流程

start_urls内部原理　　

步骤

1 scrapy引擎来爬虫中取起始的url:
2     1.调用start_requests并获取返回值
3     2.v = iter(返回值)
4     3.
5         req1 = 执行v.__next__()
6         req2 = 执行v.__next__()
7         req3 = 执行v.__next__()
8     4.全部放到调度器中

编写

 1 class ChoutiSpider(scrapy.Spider):
 2 name = 'chouti'
 3 # 爬取定向的网页 只允许这个域名的
 4 allowed_domains = ['chouti.com']
 5 start_urls = ['https://dig.chouti.com/']
 6 cookie_dict = {}
 7 
 8     def start_requests(self):
 9     # 方式1
10     # for url in self.start_urls:
11     #     yield Request(url=url)
12     # 方式2
13     # req_list = []
14     # for url in self.start_urls:
15     #     req_list.append(Request(url=url))
16     # return req_list
17     pass

用到的知识

　　可迭代对象或者生成器直接iter方法变成迭代器，以后定制start_urls的时候可以自己直接发post请求，内置默认用的get方法，拿url也可以到缓存redis中拿。

源码部分：

深度、优先级

源码流程分析

结合我的这篇博文深度https://www.cnblogs.com/Alexephor/p/11437061.html

 1 # -*- coding: utf-8 -*-
 2 import scrapy
 3 from wyb.items import WybItem
 4 from scrapy.dupefilters import RFPDupeFilter
 5 from scrapy.http.response.html import HtmlResponse
 6 from scrapy.http.cookies import CookieJar
 7 from urllib.parse import urlencode
 8 from scrapy.http import Request
 9 
10 
11 class ChoutiSpider(scrapy.Spider):
12     name = 'chouti'
13     # 爬取定向的网页 只允许这个域名的
14     allowed_domains = ['chouti.com']
15     start_urls = ['https://dig.chouti.com/']
16 
17     def start_requests(self):
18         import os
19         # 代理设置 downloadmiddleware中httppxoxy
20         os.environ['HTTP_PROXY'] = '1.1.1.2'
21         os.environ['HTTPS_PROXY'] = 'http://root:woshinizuzong@192.168.10.10:8888/'
22         # 方式1
23         for url in self.start_urls:
24             yield Request(url=url)
25         # 方式2
26         # req_list = []
27         # for url in self.start_urls:
28         #     req_list.append(Request(url=url))
29         # return req_list
30 
31     def parse(self, response):
32         """
33         第一次访问响应response
34         :param response:
35         :return:
36         """
37         from scrapy.spidermiddlewares.depth import DepthMiddleware
38         from scrapy.http import Response
39         # response.request
40         # response.request.meta 此时None
41         print(response.meta.get('depth', 0))
42         # response.request.meta['depth'] = 0
43 
44         page_list = response.xpath('//div[@id="dig_lcpage"]//a/@href').extract()
45         for page in page_list:
46             from scrapy.http import Request
47             page = "https://dig.chouti.com"+page
48             # 继续发请求，回调函数parse
49             yield Request(url=page, callback=self.parse, meta={"proxy": "http://root:woshinizuzong@192.168.10.10:8888/"})

源码看下

总结：　　

　　- 深度最开始是0每次在yield原来请求中的depth自加1
　 - 优先级 -= 深度*配置文件优先级默认0