Pyspider 中采用了 tornado 库来做 http 请求，在请求过程中可以添加各种参数，例如请求链接超时时间，请求传输数据超时时间，请求头等等，但是根据pyspider的原始框架，给爬虫添加参数只能通过 crawl_config这个Python字典来完成(如下所示)，框架代码将这个字典中的参数转换成 task 数据，进行http请求。这个参数的缺点是不方便给每一次请求做随机请求头。

crawl_config = {
"user_agent": "Mozilla/5.0 (Windows NT 6.3; WOW) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36",
"timeout": 120,
"connect_timeout": 60,
"retries": 5,
"fetch_type": 'js',
"auto_recrawl": True,
}

这里写出给爬虫添加随机请求头的方法：

1、编写脚本，将脚本放置在 pyspider 的 libs 文件夹下，命名为 header_switch.py

#!/usr/bin/env python
# -*- coding:utf-8 -*-
# Created on 2017-10-18 11:52:26
import random
import time
class HeadersSelector(object):
 """
 Header 中缺少几个字段 Host 和 Cookie
 """
 headers_1 = {
 "Proxy-Connection": "keep-alive",
 "Pragma": "no-cache",
 "Cache-Control": "no-cache",
 "User-Agent": "Mozilla/5.0 (Windows NT 6.3; WOW) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36",
 "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8",
 "DNT": "1",
 "Accept-Encoding": "gzip, deflate, sdch",
 "Accept-Language": "zh-CN,zh;q=0.8,en-US;q=0.6,en;q=0.4",
 "Referer": "https://www.baidu.com/s?wd=%BC%96%E7%A0%81&rsv_spt=1&rsv_iqid=0x9fcbc99a0000b5d7&issp=1&f=8&rsv_bp=1&rsv_idx=2&ie=utf-8&rqlang=cn&tn=baiduhome_pg&rsv_enter=0&oq=If-None-Match&inputT=7282&rsv_t",
 "Accept-Charset": "gb2312,gbk;q=0.7,utf-8;q=0.7,*;q=0.7",
 } # 网上找的浏览器
 headers_2 = {
 "Proxy-Connection": "keep-alive",
 "Pragma": "no-cache",
 "Cache-Control": "no-cache",
 "User-Agent": "Mozilla/5.0 (Windows NT 6.1; WOW) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.221 Safari/537.36 SE 2.X MetaSr 1.0",
 "Accept": "image/gif,image/x-xbitmap,image/jpeg,application/x-shockwave-flash,application/vnd.ms-excel,application/vnd.ms-powerpoint,application/msword,*/*",
 "DNT": "1",
 "Referer": "https://www.baidu.com/link?url=c-FMHf06-ZPhoRM4tWduhraKXhnSm_RzjXZ-ZTFnPAvZN",
 "Accept-Encoding": "gzip, deflate, sdch",
 "Accept-Language": "zh-CN,zh;q=0.8,en-US;q=0.6,en;q=0.4",
 } # window 7 系统浏览器
 headers_3 = {
 "Proxy-Connection": "keep-alive",
 "Pragma": "no-cache",
 "Cache-Control": "no-cache",
 "User-Agent": "Mozilla/5.0 (X11; Linux x86_; rv:52.0) Gecko/20100101 Firefox/52.0",
 "Accept": "image/x-xbitmap,image/jpeg,application/x-shockwave-flash,application/vnd.ms-excel,application/vnd.ms-powerpoint,application/msword,*/*",
 "DNT": "1",
 "Referer": "https://www.baidu.com/s?wd=http%B4%20Pragma&rsf=1&rsp=4&f=1&oq=Pragma&tn=baiduhome_pg&ie=utf-8&usm=3&rsv_idx=2&rsv_pq=e9bd5e5000010",
 "Accept-Encoding": "gzip, deflate, sdch",
 "Accept-Language": "zh-CN,zh;q=0.8,en-US;q=0.7,en;q=0.6",
 } # Linux 系统 firefox 浏览器
 headers_4 = {
 "Proxy-Connection": "keep-alive",
 "Pragma": "no-cache",
 "Cache-Control": "no-cache",
 "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win; x; rv:55.0) Gecko/20100101 Firefox/55.0",
 "Accept": "*/*",
 "DNT": "1",
 "Referer": "https://www.baidu.com/link?url=c-FMHf06-ZPhoRM4tWduhraKXhnSm_RzjXZ-ZTFnP",
 "Accept-Encoding": "gzip, deflate, sdch",
 "Accept-Language": "zh-CN,zh;q=0.9,en-US;q=0.7,en;q=0.6",
 } # Win10 系统 firefox 浏览器
 headers_5 = {
 "Connection": "keep-alive",
 "Pragma": "no-cache",
 "Cache-Control": "no-cache",
 "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win; x;) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36 Edge/15.15063",
 "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8",
 "Referer": "https://www.baidu.com/link?url=c-FMHf06-ZPhoRM4tWduhraKXhnSm_RzjXZ-",
 "Accept-Encoding": "gzip, deflate, sdch",
 "Accept-Language": "zh-CN,zh;q=0.9,en-US;q=0.7,en;q=0.6",
 "Accept-Charset": "gb2312,gbk;q=0.7,utf-8;q=0.7,*;q=0.7",
 } # Win10 系统 Chrome 浏览器
 headers_6 = {
 "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8",
 "Accept-Encoding": "gzip, deflate, sdch",
 "Accept-Language": "zh-CN,zh;q=0.8",
 "Pragma": "no-cache",
 "Cache-Control": "no-cache",
 "Connection": "keep-alive",
 "DNT": "1",
 "Referer": "https://www.baidu.com/s?wd=If-None-Match&rsv_spt=1&rsv_iqid=0x9fcbc99a0000b5d7&issp=1&f=8&rsv_bp=1&rsv_idx=2&ie=utf-8&rq",
 "Accept-Charset": "gb2312,gbk;q=0.7,utf-8;q=0.7,*;q=0.7",
 "User-Agent": "Mozilla/5.0 (Windows NT 10.0; WOW) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.221 Safari/537.36 SE 2.X MetaSr 1.0",
 } # win10 系统浏览器
 def __init__(self):
 pass
 def select_header(self):
 n = random.randint(1, 6)
 switch={
 1: self.headers_1
 2: self.headers_2
 3: self.headers_3
 4: self.headers_4
 5: self.headers_5
 6: self.headers_6
 }
 headers = switch[n]
 return headers

其中，我只写了6个请求头，如果爬虫的量非常大，完全可以写更多的请求头，甚至上百个，然后将 random的随机范围扩大，进行选择。

2、在pyspider 脚本中编写如下代码：

#!/usr/bin/env python
# -*- encoding: utf-8 -*-
# Created on 2017-08-18 11:52:26
from pyspider.libs.base_handler import *
from pyspider.addings.headers_switch import HeadersSelector
import sys
defaultencoding = 'utf-8'
if sys.getdefaultencoding() != defaultencoding:
 reload(sys)
 sys.setdefaultencoding(defaultencoding)
class Handler(BaseHandler):
 crawl_config = {
 "user_agent": "Mozilla/5.0 (Windows NT 6.3; WOW) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/52.0.2743.116 Safari/537.36",
 "timeout": 120,
 "connect_timeout": 60,
 "retries": 5,
 "fetch_type": 'js',
 "auto_recrawl": True,
 }
 @every(minutes=24 * 60)
 def on_start(self):
 header_slt = HeadersSelector()
 header = header_slt.select_header() # 获取一个新的 header
 # header["X-Requested-With"] = "XMLHttpRequest"
 orig_href = 'http://sww.bjxch.gov.cn/gggs.html'
 self.crawl(orig_href,
 callback=self.index_page,
 headers=header) # 请求头必须写在 crawl 里，cookies 从 response.cookies 中找
 @config(age=24 * 60 * 60)
 def index_page(self, response):
 header_slt = HeadersSelector()
 header = header_slt.select_header() # 获取一个新的 header
 # header["X-Requested-With"] = "XMLHttpRequest"
 if response.cookies:
 header["Cookies"] = response.cookies

其中最重要的就是在每个回调函数 on_start，index_page 等等 当中，每次调用时，都会实例化一个 header 选择器，给每一次请求添加不一样的 header。要注意添加的如下代码：

 header_slt = HeadersSelector()
 header = header_slt.select_header() # 获取一个新的 header
 # header["X-Requested-With"] = "XMLHttpRequest"
 header["Host"] = "www.baidu.com"
 if response.cookies:
 header["Cookies"] = response.cookies

当使用 XHR 发送 AJAX 请求时会带上 Header，常被用来判断是不是 Ajax 请求， headers 要添加 {‘X-Requested-With': ‘XMLHttpRequest'} 才能抓取到内容。

确定了 url 也就确定了请求头中的 Host，需要按需添加，urlparse包里给出了根据 url解析出 host的方法函数，直接调用netloc即可。

如果响应中有 cookie，就需要将 cookie 添加到请求头中。

如果还有别的伪装需求，自行添加。

如此即可实现随机请求头，完。