scrapy之 downloader middleware

一. 功能说明

Downloader Middleware有三个核心的方法

process_request(request, spider)

process_response(request, response, spider)

process_exception(request, exception, spider)  

二. 本次实验实现两个功能

1. 修改请求时的user-agent

方法一：修改settings里面的USER_AGENT变量，加一行USER_AGENT = '....'即可

方法二：修改middleware.py，这里实现得到一个随机的user-agent，在里面定义一个RandomUserAgentMiddleware类，并写一个process_request()函数

2. 修改网页响应时的返回码

在middleware.py中定义一个process_response()函数

三. 具体实现

scrapy startproject httpbintest

cd httpbintest && scrapy genspider httpbin httpbin.org

修改httpbin.py代码

 -*- coding: utf-8 -*-
import scrapy


class HttpbinSpider(scrapy.Spider):
    name = 'httpbin'
    allowed_domains = ['httpbin.org']
    start_urls = ['http://httpbin.org/get']

    def parse(self, response):
       # print(response.text)
        self.logger.debug(response.text)
        self.logger.debug('status code: ' + str(response.status))

在middlewares.py添加如下代码

其中的process_request函数是得到一个随机的user-agent； process_response函数是修改网页返回码为201

import random


class RandomUserAgentMiddleware():
    def __init__(self):
        self.user_agents = [
            'Mozilla/5.0 (Windows; U; MSIE 9.0; Windows NT 9.0; en-US)',
            'Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.2 (KHTML, like Gecko) Chrome/22.0.1216.0 Safari/537.2',
            'Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:15.0) Gecko/20100101 Firefox/15.0.1'
        ]

    def process_request(self, request, spider):
        request.headers['User-Agent'] = random.choice(self.user_agents)

    def process_response(self, request, response, spider):
        response.status = 201
        return response

settings.py中添加如下代码，使上面修改生效

DOWNLOADER_MIDDLEWARES = {
    'httpbintest.middlewares.RandomUserAgentMiddleware': 543,
}