Items有哪些知識？

1、宣告

import scrapy

class Product(scrapy.Item):
    name = scrapy.Field()
    price = scrapy.Field()
    stock = scrapy.Field()
    last_updated = scrapy.Field(serializer=str)

2、建立item

>>> product = Product(name='Desktop PC', price=1000)
>>> print
 
 product
Product(name='Desktop PC', price=1000)

3、dict API

>>> product.keys()
['price', 'name']

>>> product.items()
[('price', 1000), ('name', 'Desktop PC')]

4、複製item

 product3 = product2.copy()

5、擴充套件Item（繼承）

class
 
 DiscountedProduct(Product):
    discount_percent = scrapy.Field(serializer=str)
    discount_expiration_date = scrapy.Field()

怎麼使用持久化？這章沒講，看下面內容吧！