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Python网络爬虫4 - scrapy入门

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scrapy作为一款强大的爬虫框架,当然要好好学习一番,本文便是本人学习和使用scrapy过后的一个总结,内容比较基础,算是入门笔记吧,主要讲述scrapy的基本概念和使用方法。

scrapy framework

首先附上scrapy经典图如下:

scrapy framework

scrapy框架包含以下几个部分

  1. Scrapy Engine 引擎
  2. Spiders 爬虫
  3. Scheduler 调度器
  4. Downloader 下载器
  5. Item Pipeline 项目管道
  6. Downloader Middlewares 下载器中间件
  7. Spider Middlewares 爬虫中间件

spider process

其爬取过程简述如下:

  1. 引擎从爬虫获取首个待爬取的链接url,并传递给调度器
  2. 调度器将链接存入队列
  3. 引擎向调度器请求要爬取的链接,并将请求得到的链接经下载器中间件传递给下载器
  4. 下载器从网上下载网页,下载后的网页经下载器中间件传递给引擎
  5. 引擎将网页经爬虫中间件传递给爬虫
  6. 爬虫对网页进行解析,将得到的Items和新的链接经爬虫中间件交给引擎
  7. 引擎将从爬虫得到的Items交给项目管道,将新的链接请求requests交给调度器
  8. 此后循环2~7步,直到没有待爬取的链接为止

需要说明的是,项目管道(Item Pipeline)主要完成数据清洗,验证,持久化存储等工作;下载器中间件(Downloader Middlewares)作为下载器和引擎之间的的钩子(hook),用于监听或修改下载请求或已下载的网页,比如修改请求包的头部信息等;爬虫中间件(Spider Middlewares)作为爬虫和引擎之间的钩子(hook),用于处理爬虫的输入输出,即网页response和爬虫解析网页后得到的Itemsrequests

Items

至于什么是Items,个人认为就是经爬虫解析后得到的一个数据单元,包含一组数据,比如爬取的是某网站的商品信息,那么每爬取一个网页可能会得到多组商品信息,每组信息包含商品名称,价格,生产日期,商品样式等,那我们便可以定义一组Item

from scrapy.item import Item
from scrapy.item import Field

class GoodsItem(Item):
    name = Field()
    price = Field()
    date = Field()
    types = Field()
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Field()实质就是一个字典Dict()类型的扩展,如上代码所示,一组Item对应一个商品信息,单个网页可能包含一个或多个商品,所有Item信息都需要在Spider中赋值,然后经引擎交给Item Pipeline。具体实现在后续博文的实例中会有体现,本文旨在简单记述scrapy的基本概念和使用方法。

Install

with pip

pip install scrapy
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or conda

conda install -c conda-forge scrapy
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基本指令如下:

D:\WorkSpace>scrapy --help
Scrapy 1.5.0 - no active project

Usage:
  scrapy <command> [options] [args]

Available commands:
  bench         Run quick benchmark test
  fetch         Fetch a URL using the Scrapy downloader
  genspider     Generate new spider using pre-defined templates
  runspider     Run a self-contained spider (without creating a project)
  settings      Get settings values
  shell         Interactive scraping console
  startproject  Create new project
  version       Print Scrapy version
  view          Open URL in browser, as seen by Scrapy

  [ more ]      More commands available when run from project directory

Use "scrapy <command> -h" to see more info about a command
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如果需要使用虚拟环境,需要安装virtualenv

pip install virtualenv
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scrapy startproject

scrapy startproject <project-name> [project-dir]
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使用该指令可以生成一个新的scrapy项目,以demo为例

$ scrapy startproject demo
...
You can start your first spider with:
    cd demo
    scrapy genspider example example.com

$ cd demo
$ tree
.
├── demo
│   ├── __init__.py
│   ├── items.py
│   ├── middlewares.py
│   ├── pipelines.py
│   ├── __pycache__
│   ├── settings.py
│   └── spiders
│       ├── __init__.py
│       └── __pycache__
└── scrapy.cfg

4 directories, 7 files
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可以看到startproject自动生成了一些文件夹和文件,其中:

  1. scrapy.cfg: 项目配置文件,一般不用修改
  2. items.py: 定义items的文件,例如上述的GoodsItem
  3. middlewares.py: 中间件代码,默认包含下载器中间件和爬虫中间件
  4. pipelines.py: 项目管道,用于处理spider返回的items,包括清洗,验证,持久化等
  5. settings.py: 全局配置文件,包含各类全局变量
  6. spiders: 该文件夹用于存储所有的爬虫文件,注意一个项目可以包含多个爬虫
  7. __init__.py: 该文件指示当前文件夹属于一个python模块
  8. __pycache__: 存储解释器生成的.pyc文件(一种跨平台的字节码byte code),在python2中该类文件与.py保存在相同文件夹

scrapy genspider

项目生成以后,可以使用scrapy genspider指令自动生成一个爬虫文件,比如,如果要爬取花瓣网首页,执行以下指令:

$ cd demo
$ scrapy genspider huaban www.huaban.com
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默认生成的爬虫文件huaban.py如下:

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


class HuabanSpider(scrapy.Spider):
    name = 'huaban'
    allowed_domains = ['www.huaban.com']
    start_urls = ['http://www.huaban.com/']

    def parse(self, response):
        pass
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  • 爬虫类继承于scrapy.Spider
  • name是必须存在的参数,用以标识该爬虫
  • allowed_domains指代允许爬虫爬取的域名,指定域名之外的链接将被丢弃
  • start_urls存储爬虫的起始链接,该参数是列表类型,所以可以同时存储多个链接

如果要自定义起始链接,也可以重写scrapy.Spider类的start_requests函数,此处不予细讲。

parse函数是一个默认的回调函数,当下载器下载网页后,会调用该函数进行解析,response就是请求包的响应数据。至于网页内容的解析方法,scrapy内置了几种选择器(Selector),包括xpath选择器、CSS选择器和正则匹配。下面是一些选择器的使用示例,方便大家更加直观的了解选择器的用法。

# xpath selector
response.xpath('//a')
response.xpath('./img').extract()
response.xpath('//*[@id="huaban"]').extract_first()
repsonse.xpath('//*[@id="Profile"]/div[1]/a[2]/text()').extract_first()

# css selector
response.css('a').extract()
response.css('#Profile > div.profile-basic').extract_first()
response.css('a[href="test.html"]::text').extract_first()

# re selector
response.xpath('.').re('id:\s*(\d+)')
response.xpath('//a/text()').re_first('username: \s(.*)')
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需要说明的是,response不能直接调用re,re_first.

scrapy crawl

假设爬虫编写完了,那就可以使用scrapy crawl指令开始执行爬取任务了。

当进入一个创建好的scrapy项目目录时,使用scrapy -h可以获得相比未创建之前更多的帮助信息,其中就包括用于启动爬虫任务的scrapy crawl

$ scrapy -h
Scrapy 1.5.0 - project: huaban

Usage:
  scrapy <command> [options] [args]

Available commands:
  bench         Run quick benchmark test
  check         Check spider contracts
  crawl         Run a spider
  edit          Edit spider
  fetch         Fetch a URL using the Scrapy downloader
  genspider     Generate new spider using pre-defined templates
  list          List available spiders
  parse         Parse URL (using its spider) and print the results
  runspider     Run a self-contained spider (without creating a project)
  settings      Get settings values
  shell         Interactive scraping console
  startproject  Create new project
  version       Print Scrapy version
  view          Open URL in browser, as seen by Scrapy

Use "scrapy <command> -h" to see more info about a command
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$ scrapy crawl -h
Usage
=====
  scrapy crawl [options] <spider>

Run a spider

Options
=======
--help, -h              show this help message and exit
-a NAME=VALUE           set spider argument (may be repeated)
--output=FILE, -o FILE  dump scraped items into FILE (use - for stdout)
--output-format=FORMAT, -t FORMAT
                        format to use for dumping items with -o

Global Options
--------------
--logfile=FILE          log file. if omitted stderr will be used
--loglevel=LEVEL, -L LEVEL
                        log level (default: DEBUG)
--nolog                 disable logging completely
--profile=FILE          write python cProfile stats to FILE
--pidfile=FILE          write process ID to FILE
--set=NAME=VALUE, -s NAME=VALUE
                        set/override setting (may be repeated)
--pdb                   enable pdb on failure
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scrapy crawl的帮助信息可以看出,该指令包含很多可选参数,但必选参数只有一个,就是spider,即要执行的爬虫名称,对应每个爬虫的名称(name)。

scrapy crawl huaban
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至此,一个scrapy爬虫任务的创建和执行过程就介绍完了,至于实例,后续博客会陆续介绍。

scrapy shell

最后简要说明一下指令scrapy shell,这是一个交互式的shell,类似于命令行形式的python,当我们刚开始学习scrapy或者刚开始爬虫某个陌生的站点时,可以使用它熟悉各种函数操作或者选择器的使用,用它来不断试错纠错,熟练掌握scrapy各种用法。

$ scrapy shell www.huaban.com
2018-05-29 23:58:49 [scrapy.utils.log] INFO: Scrapy 1.5.0 started (bot: scrapybot)
2018-05-29 23:58:49 [scrapy.utils.log] INFO: Versions: lxml 4.2.1.0, libxml2 2.9.5, cssselect 1.0.3, parsel 1.4.0, w3lib 1.19.0, Twisted 17.9.0, Python 3.6.3 (v3.6.3:2c5fed8, Oct  3
2017, 17:26:49) [MSC v.1900 32 bit (Intel)], pyOpenSSL 17.5.0 (OpenSSL 1.1.0h  27 Mar 2018), cryptography 2.2.2, Platform Windows-10-10.0.17134-SP0
2018-05-29 23:58:49 [scrapy.crawler] INFO: Overridden settings: {'DUPEFILTER_CLASS': 'scrapy.dupefilters.BaseDupeFilter', 'LOGSTATS_INTERVAL': 0}
2018-05-29 23:58:49 [scrapy.middleware] INFO: Enabled extensions:
['scrapy.extensions.corestats.CoreStats',
 'scrapy.extensions.telnet.TelnetConsole']
2018-05-29 23:58:50 [scrapy.middleware] INFO: Enabled downloader middlewares:
['scrapy.downloadermiddlewares.httpauth.HttpAuthMiddleware',
 'scrapy.downloadermiddlewares.downloadtimeout.DownloadTimeoutMiddleware',
 'scrapy.downloadermiddlewares.defaultheaders.DefaultHeadersMiddleware',
 'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware',
 'scrapy.downloadermiddlewares.retry.RetryMiddleware',
 'scrapy.downloadermiddlewares.redirect.MetaRefreshMiddleware',
 'scrapy.downloadermiddlewares.httpcompression.HttpCompressionMiddleware',
 'scrapy.downloadermiddlewares.redirect.RedirectMiddleware',
 'scrapy.downloadermiddlewares.cookies.CookiesMiddleware',
 'scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware',
 'scrapy.downloadermiddlewares.stats.DownloaderStats']
2018-05-29 23:58:50 [scrapy.middleware] INFO: Enabled spider middlewares:
['scrapy.spidermiddlewares.httperror.HttpErrorMiddleware',
 'scrapy.spidermiddlewares.offsite.OffsiteMiddleware',
 'scrapy.spidermiddlewares.referer.RefererMiddleware',
 'scrapy.spidermiddlewares.urllength.UrlLengthMiddleware',
 'scrapy.spidermiddlewares.depth.DepthMiddleware']
2018-05-29 23:58:50 [scrapy.middleware] INFO: Enabled item pipelines:
[]
2018-05-29 23:58:50 [scrapy.extensions.telnet] DEBUG: Telnet console listening on 127.0.0.1:6023
2018-05-29 23:58:50 [scrapy.core.engine] INFO: Spider opened
2018-05-29 23:58:50 [scrapy.downloadermiddlewares.redirect] DEBUG: Redirecting (301) to <GET http://huaban.com/> from <GET http://www.huaban.com>
2018-05-29 23:58:50 [scrapy.core.engine] DEBUG: Crawled (200) <GET http://huaban.com/> (referer: None)
[s] Available Scrapy objects:
[s]   scrapy     scrapy module (contains scrapy.Request, scrapy.Selector, etc)
[s]   crawler    <scrapy.crawler.Crawler object at 0x03385CB0>
[s]   item       {}
[s]   request    <GET http://www.huaban.com>
[s]   response   <200 http://huaban.com/>
[s]   settings   <scrapy.settings.Settings object at 0x04CC4D10>
[s]   spider     <DefaultSpider 'default' at 0x4fa6bf0>
[s] Useful shortcuts:
[s]   fetch(url[, redirect=True]) Fetch URL and update local objects (by default, redirects are followed)
[s]   fetch(req)                  Fetch a scrapy.Request and update local objects
[s]   shelp()           Shell help (print this help)
[s]   view(response)    View response in a browser
In [1]: view(response)
Out[1]: True

In [2]: response.xpath('//a')
Out[2]:
[<Selector xpath='//a' data='<a id="elevator" class="off" onclick="re'>,
 <Selector xpath='//a' data='<a class="plus"></a>'>,
 <Selector xpath='//a' data='<a onclick="app.showUploadDialog();">添加采'>,
 <Selector xpath='//a' data='<a class="add-board-item">添加画板<i class="'>,
 <Selector xpath='//a' data='<a href="/about/goodies/">安装采集工具<i class'>,
 <Selector xpath='//a' data='<a class="huaban_security_oauth" logo_si'>]

In [3]: response.xpath('//a').extract()
Out[3]:
['<a id="elevator" class="off" onclick="return false;" title="回到顶部"></a>',
 '<a class="plus"></a>',
 '<a onclick="app.showUploadDialog();">添加采集<i class="upload"></i></a>',
 '<a class="add-board-item">添加画板<i class="add-board"></i></a>',
 '<a href="/about/goodies/">安装采集工具<i class="goodies"></i></a>',
 '<a class="huaban_security_oauth" logo_size="124x47" logo_type="realname" href="//www.anquan.org" rel="nofollow"> <script src="//static.anquan.org/static/outer/js/aq_auth.js"></script> </a>']

In [4]: response.xpath('//img')
Out[4]: [<Selector xpath='//img' data='<img src="https://d5nxst8fruw4z.cloudfro'>]

In [5]: response.xpath('//a/text()')
Out[5]:
[<Selector xpath='//a/text()' data='添加采集'>,
 <Selector xpath='//a/text()' data='添加画板'>,
 <Selector xpath='//a/text()' data='安装采集工具'>,
 <Selector xpath='//a/text()' data=' '>,
 <Selector xpath='//a/text()' data=' '>]

In [6]: response.xpath('//a/text()').extract()
Out[6]: ['添加采集', '添加画板', '安装采集工具', ' ', ' ']

In [7]: response.xpath('//a/text()').extract_first()
Out[7]: '添加采集'
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