<scrapy爬虫>基本操作

scrapy选择器的用法
	//selector可以加可以不加
	response.selector.xpath("//title/text()").extract_first()
	response.selector.css("title::text").extract_first()
	response.xpath("//title/text()").extract_first()

	response.xpath("//div[@id='images']").xpath("//img/@src").extract()[0]
	response.css('a::text').re('Name:(.*)')---需要特殊处理的数据后面可以加re模块
	response.xpath("//a/text()").re('Name:(.*)')--这个后面对象变了,不能再加extract()方法了

spiders的用法
	完成爬虫的逻辑url
	解析网页数据parse

	1.初始start_urls→解析parse→scrapy.Request(url,callback=parse)
		item
		url继续给下载器

	2.name---唯一标识爬虫
	3.allowed_domains---允许爬取列表
	4.start_urls---从这里开始,可以构造列表,一个一个请求(遍历),get形式的请求
	5.custom_settings---字典形式的写法(参考headers设置)可以覆盖项目settings---可以吧请求头写在myspider中
	6.crawler---
	7.settings---
	8.from_crawler----可以拿到全局配置
	9.start_requests()----利用改写start_requests()方法,使第一次请求用post请求
		def start_requests(self):
			yield scrapy.Request(url='http://httpbin.org/post',method='post',callback=self.parse_post)

		def parse_post(self,response):
			print('OK',response.status)

	10.make_requests_from_url--改变默认回调函数,将回调函数改写,,如果先写start_requests函数就不会在调用此函数
			
		def make_requests_from_url(self,url):
			yield scrapy.Request(url=url,callback=self.parse_index)

		def parse_index(self,response):
			print('OK',response.status)

	11.parse默认回调函数--item 可迭代对象,request(加到下载队列)

	12.log日志文件--info --debug
		self.logger.info(response.status)

	13.closed--myspider关闭时调用
item Pipeline的用法
	数据清洗
	重复检查
	存储到数据库
	1.process_item---item会自动传给它

	2.open_spider---spider开启的时候调用
	3.close_spider--spider关闭的时候调用

	4.from-crawler----类方法,获取项目中的setting
		from scrapy.exceptions import DropItem
标准写法:

class PricePipeline(object):

    vat_factor = 1.15

    def process_item(self, item, spider):
        if item.get('price'):
            if item.get('price_excludes_vat'):
                item['price'] = item['price'] * self.vat_factor
            return item
        else:
            raise DropItem("Missing price in %s" % item)

写到json:
import json

class JsonWriterPipeline(object):

    def open_spider(self, spider):
        self.file = open('items.jl', 'w')

    def close_spider(self, spider):
        self.file.close()

    def process_item(self, item, spider):
        line = json.dumps(dict(item)) + "
"
        self.file.write(line)
        return item

存储到MongoDB:
import pymongo

class MongoPipeline(object):

    collection_name = 'scrapy_items'

    def __init__(self, mongo_uri, mongo_db):
        self.mongo_uri = mongo_uri
        self.mongo_db = mongo_db

    @classmethod
    def from_crawler(cls, crawler):
        return cls(
            mongo_uri=crawler.settings.get('MONGO_URI'),
            mongo_db=crawler.settings.get('MONGO_DATABASE', 'items')
        )

    def open_spider(self, spider):
        self.client = pymongo.MongoClient(self.mongo_uri)
        self.db = self.client[self.mongo_db]

    def close_spider(self, spider):
        self.client.close()

    def process_item(self, item, spider):
        self.db[self.collection_name].insert_one(dict(item))
        return item
请求网页,保存图片
import scrapy
import hashlib
from urllib.parse import quote


class ScreenshotPipeline(object):
    """Pipeline that uses Splash to render screenshot of
    every Scrapy item."""

    SPLASH_URL = "http://localhost:8050/render.png?url={}"

    def process_item(self, item, spider):
        encoded_item_url = quote(item["url"])
        screenshot_url = self.SPLASH_URL.format(encoded_item_url)
        request = scrapy.Request(screenshot_url)
        dfd = spider.crawler.engine.download(request, spider)
        dfd.addBoth(self.return_item, item)
        return dfd

    def return_item(self, response, item):
        if response.status != 200:
            # Error happened, return item.
            return item

        # Save screenshot to file, filename will be hash of url.
        url = item["url"]
        url_hash = hashlib.md5(url.encode("utf8")).hexdigest()
        filename = "{}.png".format(url_hash)
        with open(filename, "wb") as f:
            f.write(response.body)

        # Store filename in item.
        item["screenshot_filename"] = filename
        return item
去重操作:
from scrapy.exceptions import DropItem

class DuplicatesPipeline(object):

    def __init__(self):
        self.ids_seen = set()

    def process_item(self, item, spider):
        if item['id'] in self.ids_seen:
            raise DropItem("Duplicate item found: %s" % item)
        else:
            self.ids_seen.add(item['id'])
            return item
使用item pipeline:
ITEM_PIPELINES = {
    'myproject.pipelines.PricePipeline': 300,
    'myproject.pipelines.JsonWriterPipeline': 800,
}