爬虫 - scrapy之中间件

中间件的简介

1.中间件的作用

          在scrapy运行的整个过程中,对scrapy框架运行的某些步骤做一些适配自己项目的动作.

  　　 例如scrapy内置的HttpErrorMiddleware,可以在http请求出错时做一些处理.

 2.中间件的使用方法

         配置settings.py.详见scrapy文档 https://doc.scrapy.org

中间件的分类

　　scrapy的中间件理论上有三种(Schduler Middleware,Spider Middleware,Downloader Middleware),在应用上一般有以下两种

       1.爬虫中间件Spider Middleware

         主要功能是在爬虫运行过程中进行一些处理.

　　2.下载器中间件Downloader Middleware

         主要功能在请求到网页后,页面被下载时进行一些处理.

 中间件的方法

      1.Spider Middleware有以下几个函数被管理:

       - process_spider_input 接收一个response对象并处理,

         位置是Downloader-->process_spider_input-->Spiders(Downloader和Spiders是scrapy官方结构图中的组件)

       - process_spider_exception spider出现的异常时被调用

       - process_spider_output 当Spider处理response返回result时,该方法被调用

       - process_start_requests 当spider发出请求时,被调用

　　  位置是Spiders-->process_start_requests-->Scrapy Engine(Scrapy Engine是scrapy官方结构图中的组件)         

 　 2.Downloader Middleware有以下几个函数被管理

　　 - process_request  request通过下载中间件时，该方法被调用

　　 - process_response 下载结果经过中间件时被此方法处理

　　 - process_exception 下载过程中出现异常时被调用

      编写中间件时,需要思考要实现的功能最适合在那个过程处理,就编写哪个方法.

      中间件可以用来处理请求,处理结果或者结合信号协调一些方法的使用等.也可以在原有的爬虫上添加适应项目的其他功能,这一点在扩展中编写也可以达到目的,实际上扩展更加去耦合化,推荐使用扩展.

代码示例

下载中间件代码示例

 1 from scrapy.http import HtmlResponse
 2 from scrapy.http import Request
 3  
 4 class Md1(object):
 5     @classmethod
 6     def from_crawler(cls, crawler):
 7         # This method is used by Scrapy to create your spiders.
 8         s = cls()
 9         return s
10  
11     def process_request(self, request, spider):
12         # Called for each request that goes through the downloader
13         # middleware.
14  
15         # Must either:
16         # - return None: continue processing this request
17         # - or return a Response object
18         # - or return a Request object
19         # - or raise IgnoreRequest: process_exception() methods of
20         #   installed downloader middleware will be called
21         print('md1.process_request',request)
22         # 1. 返回Response
23         # import requests
24         # result = requests.get(request.url)
25         # return HtmlResponse(url=request.url, status=200, headers=None, body=result.content)
26         # 2. 返回Request
27         # return Request('https://dig.chouti.com/r/tec/hot/1')
28  
29         # 3. 抛出异常
30         # from scrapy.exceptions import IgnoreRequest
31         # raise IgnoreRequest
32  
33         # 4. 对请求进行加工(*)
34         # request.headers['user-agent'] = "Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36"
35  
36  
37     def process_response(self, request, response, spider):
38         # Called with the response returned from the downloader.
39  
40         # Must either;
41         # - return a Response object
42         # - return a Request object
43         # - or raise IgnoreRequest
44         print('m1.process_response',request,response)
45         return response
46  
47     def process_exception(self, request, exception, spider):
48         # Called when a download handler or a process_request()
49         # (from other downloader middleware) raises an exception.
50  
51         # Must either:
52         # - return None: continue processing this exception
53         # - return a Response object: stops process_exception() chain
54         # - return a Request object: stops process_exception() chain

配置

1 DOWNLOADER_MIDDLEWARES = {
2    #'xdb.middlewares.XdbDownloaderMiddleware': 543,
3     # 'xdb.proxy.XdbProxyMiddleware':751,
4     'xdb.md.Md1':666,
5     'xdb.md.Md2':667,
6 }

爬虫中间件下载示例

编写类

class Sd1(object):
# Not all methods need to be defined. If a method is not defined,
# scrapy acts as if the spider middleware does not modify the
# passed objects.
 
@classmethod
def from_crawler(cls, crawler):
    # This method is used by Scrapy to create your spiders.
    s = cls()
    return s
 
def process_spider_input(self, response, spider):
    # Called for each response that goes through the spider
    # middleware and into the spider.
 
    # Should return None or raise an exception.
    return None
 
def process_spider_output(self, response, result, spider):
    # Called with the results returned from the Spider, after
    # it has processed the response.
 
    # Must return an iterable of Request, dict or Item objects.
    for i in result:
        yield i
 
def process_spider_exception(self, response, exception, spider):
    # Called when a spider or process_spider_input() method
    # (from other spider middleware) raises an exception.
 
    # Should return either None or an iterable of Response, dict
    # or Item objects.
    pass
 
# 只在爬虫启动时，执行一次。
def process_start_requests(self, start_requests, spider):
    # Called with the start requests of the spider, and works
    # similarly to the process_spider_output() method, except
    # that it doesn’t have a response associated.
 
    # Must return only requests (not items).
    for r in start_requests:
        yield r

配置

1 SPIDER_MIDDLEWARES = {
2    # 'xdb.middlewares.XdbSpiderMiddleware': 543,
3     'xdb.sd.Sd1': 666,
4     'xdb.sd.Sd2': 667,
5 }