【scrapy】Item及Spider
Items
Item objects are simple containers used to collect the scraped data.They provide a dictionary-like api with a convenient syntax for declaring their available fields.
import scrapy;
class Product(scrapy.Item):
name=scrapy.Field()
price=scrapy.Field()
stock=scrapy.Field()
last_updated=scrapy.Field(serializer=str)
Extending Items
you can extend Items(to add more fields or to change some metadata for some fields)by declaring a subclass of your original Item.
class DiscountedProduct(Product):
discount_percent=scrapy.Field(serializer=str)
You can also extend fields metadata by using the previous field metadata and appending more values,or changind existing values.
class SpecificProduct(Product):
name=scrapy.Field(Product.fields['name'],serializer=my_serializer)
Item Objects
1.class scrapy.item.Item([arg])
Return a new Item optionally initialized from the given argument
The only additional attribute provided by Items is:fields
2.Field objects
class scrapy.item.Field([arg])
The Field class is just an alias to the built-in dict class and doesn't provide any extra functionality or attributes.
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Built-in spiders reference
Scrapy comes with some useful generic spiders that you can use,to subclass your spiders from .Their aim is to provide convenient functionality for a few common scraping cases.
class scrapy.spider.Spider
This is the simplest spider,and the one from which every other spider must inherit from.
重要属性：
name
A string which defines the name for this spider.it must be unique.This is the most important spider attribute and it is required.
allowed_domains
An optional list of strings containing domains that this spider is allowed to crawl.Requests for URLs not belonging to the domain names specified in this list won't be followed if offsiteMiddleware is enabled.
start_urls
A list of URLs where the spider will begin to crawl from,when no particular URLs are specified.
start_requests()
This is the method called by Scrapy when the spider is opened for scraping when no particular URLs are specified.If particular URLs are specified,the make_requests_from_url() is used instead to create the Requests.
make_requests_from_url(url)
A method that receives a URL and returns a Request object to scrape.Unless overridden,this method returns Requests with the parse() method as their callback function.
parse(response)
The parse method is in charge of processing the response and returning scraped data .
log(message[,level,component])
Log a message
closed(reason)
called when the spider closes.
class scrapy.contrib.spiders.CrawlSpider
This is the most commonly used spider for crawling regular websites,as it provides a convenient mechanism for following links by defining a set of rules.
除了继承自Spider的属性，CrawlSpider还提供以下属性。
rules
Which is a list of one or more Rule objects.Each Rule defines a certain behaviour for crawling the site.
关于Rule对象：
class scrapy.contrib.spiders.Rule(link_extractor,callback=None,cb_kwargs=None,follow=None,process_links=None,process_request=None)
link_extractor is a Link Extractor object which defines how links will be extracted from each crawled page.
callback is a callable or a string to be called for each link extracted with the specified link_extractor.
注意：when writing crawl spider rules,avoid using parse as callback,since the CrawlSpider uses the parse method itself to implement its logic.
cb_kwargs is a dict containing the keyword arguments to be passed to the callback function.
follow is a boolean which specifies if links should be followed from each response extracted with this rule.If callback is None,follow defaults to true(即继续爬取这个链接),otherwise it default to false.
process_request is a callable or a string which will be called with every request extracted by this rule,and must return a request or None.
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LinkExtractors are objects whose only purpose is to extract links from web pages(scrapy.http.Response objects).
Scrapy内置的Link Extractors有两个，可以根据需要自己来写。
All availble link extractors classes bundled with scrapy are provided in the scrapy.contrib.linkextractors module.
SgmlLinkExtractor
class scrapy.contrib.linkextractors.sgml.SgmlLinkExtractor(allow,...)
The SgmlLinkExtrator extends the base BaseSgmlLinkExtractor by providing additional filters that you can specify to extract links.
allow(a (a list of)regular expression):a single regular expression that the urls must match in order to be extracted,if not given,it will match all links.
