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.
配置settings.py.详见scrapy文档 https://doc.scrapy.org
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
主要功能是在爬虫运行过程中进行一些处理.
主要功能在请求到网页后,页面被下载时进行一些处理.
- 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)
- 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.
from scrapy.http import HtmlResponse
from scrapy.http import Request
class Md1(object):
@classmethod
def from_crawler(cls, crawler):
# This method is used by Scrapy to create your spiders.
s = cls()
return s
def process_request(self, request, spider):
# Called for each request that goes through the downloader
# middleware.
# Must either:
# - return None: continue processing this request
# - or return a Response object
# - or return a Request object
# - or raise IgnoreRequest: process_exception() methods of
# installed downloader middleware will be called
print('md1.process_request',request)
# 1. 返回Response
# import requests
# result = requests.get(request.url)
# return HtmlResponse(url=request.url, status=200, headers=None, body=result.content)
# 2. 返回Request
# return Request('https://dig.chouti.com/r/tec/hot/1')
# 3. 抛出异常
# from scrapy.exceptions import IgnoreRequest
# raise IgnoreRequest
# 4. 对请求进行加工(*)
# 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"
def process_response(self, request, response, spider):
# Called with the response returned from the downloader.
# Must either;
# - return a Response object
# - return a Request object
# - or raise IgnoreRequest
print('m1.process_response',request,response)
return response
def process_exception(self, request, exception, spider):
# Called when a download handler or a process_request()
# (from other downloader middleware) raises an exception.
# Must either:
# - return None: continue processing this exception
# - return a Response object: stops process_exception() chain
# - return a Request object: stops process_exception() chain
pass
DOWNLOADER_MIDDLEWARES = {
#'xdb.middlewares.XdbDownloaderMiddleware': 543,
# 'xdb.proxy.XdbProxyMiddleware':751,
'xdb.md.Md1':666,
'xdb.md.Md2':667,
}
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
SPIDER_MIDDLEWARES = {
# 'xdb.middlewares.XdbSpiderMiddleware': 543,
'xdb.sd.Sd1': 666,
'xdb.sd.Sd2': 667,
}