yield

　　1. 带有 yield 的函数不再是一个普通函数，而是一个生成器generator，可用于迭代

　　2. yield 是一个类似 return 的关键字，迭代一次遇到yield时就返回yield后面(右边)的值。重点是：下一次迭代时，从上一次迭代遇到的yield后面的代码(下一行)开始执行

　　3. 简要理解：yield就是 return 返回一个值，并且记住这个返回的位置，下次迭代就从这个位置后(下一行)开始

案例：

　　1.当当网 （1）yield（2）管道封装（3）多条管道下载 （4）多页数据下载[下节介绍]

创建项目

>scrapy startproject scrapy_dangdang

创建爬虫文件

scrapy_dangdang\scrapy_dangdang\scrapy> **scrapy genspider dang http://category.dangdang.com/cp01.01.02.00.00.00.html**

测试运行，爬虫文件

scrapy_dangdang\scrapy_dangdang\scrapy> scrapy crawl dang

---

items.py定义数据结构

获取数据

找到书图片的src

找到书名称的alt

找到书价格的span

获取selector对象列表的data属性值

发现src相同，解决懒加载情况

网页向下滑动

使用图片的data-original值

解决第一张图的src使用none空

第一张图的img只有src属性

第二张图及之后图的img有data-original属性

导入

yield返回值到pipelines管道

settings中开启管道

---

pipelines管道的封装

追加写入数据

爬虫文件执行之，前执行

爬虫文件执行之，后执行

解决操作文件过于频繁

book.json

---

多条管道的下载

开启图片下载管道

运行

---

项目结构，及代码

dang.py爬虫核心功能

import scrapy
from scrapy_dangdang.items import ScrapyDangdang095Item

class DangSpider(scrapy.Spider):

# 如果是多页下载的话 那么必须要调整的是allowed\_domains的范围 一般情况下只写域名  
allowed\_domains = \['http://category.dangdang.com/cp01.01.02.00.00.00.html'\]  
start\_urls = \['http://category.dangdang.com/cp01.01.02.00.00.00.html'\]

def parse(self, response):  

pipelines 下载数据

items 定义数据结构的

src = //ul[@id="component_59"]/li//img/@src

alt = //ul[@id="component_59"]/li//img/@alt

price = //ul[@id="component_59"]/li//p[@class="price"]/span[1]/text()

所有的seletor的对象 都可以再次调用xpath方法

    li\_list = response.xpath('//ul\[@id="component\_59"\]/li')

    for li in li\_list:  
        src = li.xpath('.//img/@data-original').extract\_first()  
        # 第一张图片和其他的图片的标签的属性是不一样的  
        # 第一张图片的src是可以使用的  其他的图片的地址是data-original  
        if src:  
            src = src  
        else:  
            src = li.xpath('.//img/@src').extract\_first()

        name = li.xpath('.//img/@alt').extract\_first()  
        price = li.xpath('.//p\[@class="price"\]/span\[1\]/text()').extract\_first()

        book = ScrapyDangdang095Item(src=src,name=name,price=price)

        # 获取一个book就将book交给pipelines  
        yield book

items.py定义数据结构，类

# Define here the models for your scraped items

See documentation in:

https://docs.scrapy.org/en/latest/topics/items.html

import scrapy

class ScrapyDangdang095Item(scrapy.Item):
# define the fields for your item here like:
# name = scrapy.Field()
# 通俗的说就是你要下载的数据都有什么

# 图片  
src = scrapy.Field()  
# 名字  
name = scrapy.Field()  
# 价格  
price = scrapy.Field()

pipelines.py管道功能

# Define your item pipelines here

Don't forget to add your pipeline to the ITEM_PIPELINES setting

See: https://docs.scrapy.org/en/latest/topics/item-pipeline.html

useful for handling different item types with a single interface

from itemadapter import ItemAdapter

如果想使用管道的话 那么就必须在settings中开启管道

class ScrapyDangdang095Pipeline:
# 在爬虫文件开始的之前就执行的一个方法
def open_spider(self,spider):
self.fp = open('book.json','w',encoding='utf-8')

# item就是yield后面的book对象  
def process\_item(self, item, spider):  
    # 以下这种模式不推荐  因为每传递过来一个对象 那么就打开一次文件  对文件的操作过于频繁

    # # (1) write方法必须要写一个字符串 而不能是其他的对象  
    # # (2) w模式 会每一个对象都打开一次文件 覆盖之前的内容  
    # with open('book.json','a',encoding='utf-8')as fp:  
    #     fp.write(str(item))

    self.fp.write(str(item))

    return item

# 在爬虫文件执行完之后  执行的方法  
def close\_spider(self,spider):  
    self.fp.close()

import urllib.request

多条管道开启

(1) 定义管道类

（2） 在settings中开启管道

'scrapy_dangdang_095.pipelines.DangDangDownloadPipeline':301

class DangDangDownloadPipeline:
def process_item(self, item, spider):

    url = 'http:' + item.get('src')  
    filename = './books/' + item.get('name') + '.jpg'

    urllib.request.urlretrieve(url = url, filename= filename)

    return item

settings.py配置管道

# Scrapy settings for scrapy_dangdang_095 project

For simplicity, this file contains only settings considered important or

commonly used. You can find more settings consulting the documentation:

https://docs.scrapy.org/en/latest/topics/settings.html

https://docs.scrapy.org/en/latest/topics/downloader-middleware.html

https://docs.scrapy.org/en/latest/topics/spider-middleware.html

BOT_NAME = 'scrapy_dangdang'

SPIDER_MODULES = ['scrapy_dangdang.spiders']
NEWSPIDER_MODULE = 'scrapy_dangdang.spiders'

Crawl responsibly by identifying yourself (and your website) on the user-agent

#USER_AGENT = 'scrapy_dangdang_095 (+http://www.yourdomain.com)'

Obey robots.txt rules

ROBOTSTXT_OBEY = True

Configure maximum concurrent requests performed by Scrapy (default: 16)

#CONCURRENT_REQUESTS = 32

Configure a delay for requests for the same website (default: 0)

See https://docs.scrapy.org/en/latest/topics/settings.html#download-delay

See also autothrottle settings and docs

#DOWNLOAD_DELAY = 3

The download delay setting will honor only one of:

#CONCURRENT_REQUESTS_PER_DOMAIN = 16
#CONCURRENT_REQUESTS_PER_IP = 16

Disable cookies (enabled by default)

#COOKIES_ENABLED = False

Disable Telnet Console (enabled by default)

#TELNETCONSOLE_ENABLED = False

Override the default request headers:

#DEFAULT_REQUEST_HEADERS = {

'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8',

'Accept-Language': 'en',

#}

Enable or disable spider middlewares

See https://docs.scrapy.org/en/latest/topics/spider-middleware.html

#SPIDER_MIDDLEWARES = {

'scrapy_dangdang_095.middlewares.ScrapyDangdang095SpiderMiddleware': 543,

#}

Enable or disable downloader middlewares

See https://docs.scrapy.org/en/latest/topics/downloader-middleware.html

#DOWNLOADER_MIDDLEWARES = {

'scrapy_dangdang_095.middlewares.ScrapyDangdang095DownloaderMiddleware': 543,

#}

Enable or disable extensions

See https://docs.scrapy.org/en/latest/topics/extensions.html

#EXTENSIONS = {

'scrapy.extensions.telnet.TelnetConsole': None,

#}

Configure item pipelines

See https://docs.scrapy.org/en/latest/topics/item-pipeline.html

**ITEM_PIPELINES = {
# 管道可以有很多个 那么管道是有优先级的 优先级的范围是1到1000 值越小优先级越高
'scrapy_dangdang.pipelines.ScrapyDangdang095Pipeline': 300,

DangDangDownloadPipeline

'scrapy_dangdang.pipelines.DangDangDownloadPipeline':301
}** # Enable and configure the AutoThrottle extension (disabled by default)

See https://docs.scrapy.org/en/latest/topics/autothrottle.html

#AUTOTHROTTLE_ENABLED = True

The initial download delay

#AUTOTHROTTLE_START_DELAY = 5

The maximum download delay to be set in case of high latencies

#AUTOTHROTTLE_MAX_DELAY = 60

The average number of requests Scrapy should be sending in parallel to

each remote server

#AUTOTHROTTLE_TARGET_CONCURRENCY = 1.0

Enable showing throttling stats for every response received:

#AUTOTHROTTLE_DEBUG = False

Enable and configure HTTP caching (disabled by default)

See https://docs.scrapy.org/en/latest/topics/downloader-middleware.html#httpcache-middleware-settings

#HTTPCACHE_ENABLED = True
#HTTPCACHE_EXPIRATION_SECS = 0
#HTTPCACHE_DIR = 'httpcache'
#HTTPCACHE_IGNORE_HTTP_CODES = []
#HTTPCACHE_STORAGE = 'scrapy.extensions.httpcache.FilesystemCacheStorage'