3.7 KiB
Item Pipeline
After an item has been scraped by a spider, it is sent to the Item Pipeline which process it through several components that are executed sequentially.
Item pipelines are usually implemented on each project. Typical usage for item pipelines consists of:
- HTML cleansing
- validation
- persistence (storing the scraped item)
Writing your own item pipeline
Writing your own item pipeline is easy. Each item pipeline component is a single Python class that must implement the following method:
System Message: ERROR/3 (<stdin>, line 24)
Unknown directive type "method".
.. method:: process_item(item, spider) This method is called for every item pipeline component and must either return a :class:`~scrapy.item.Item` (or any descendant class) object or raise a :exc:`~scrapy.exceptions.DropItem` exception. Dropped items are no longer processed by further pipeline components. :param item: the item scraped :type item: :class:`~scrapy.item.Item` object :param spider: the spider which scraped the item :type spider: :class:`~scrapy.spider.BaseSpider` object
Additionally, they may also implement the following methods:
System Message: ERROR/3 (<stdin>, line 39)
Unknown directive type "method".
.. method:: open_spider(spider) This method is called when the spider is opened. :param spider: the spider which was opened :type spider: :class:`~scrapy.spider.BaseSpider` object
System Message: ERROR/3 (<stdin>, line 46)
Unknown directive type "method".
.. method:: close_spider(spider) This method is called when the spider is closed. :param spider: the spider which was closed :type spider: :class:`~scrapy.spider.BaseSpider` object
Item pipeline example
Let's take a look at the following hypothetic pipeline that adjusts the price attribute for those items that do not include VAT (price_excludes_vat attribute), and drops those items which don't contain a price:
from scrapy.exceptions import DropItem
class PricePipeline(object):
vat_factor = 1.15
def process_item(self, item, spider):
if item['price']:
if item['price_excludes_vat']:
item['price'] = item['price'] * self.vat_factor
return item
else:
raise DropItem("Missing price in %s" % item)
Activating an Item Pipeline component
To activate an Item Pipeline component you must add its class to the :setting:`ITEM_PIPELINES` list, like in the following example:
System Message: ERROR/3 (<stdin>, line 79); backlink
Unknown interpreted text role "setting".ITEM_PIPELINES = [
'myproject.pipeline.PricePipeline',
]
Item pipeline example with resources per spider
Sometimes you need to keep resources about the items processed grouped per spider, and delete those resource when a spider finishes.
An example is a filter that looks for duplicate items, and drops those items that were already processed. Let say that our items have an unique id, but our spider returns multiples items with the same id:
from scrapy.xlib.pydispatch import dispatcher
from scrapy import signals
from scrapy.exceptions import DropItem
class DuplicatesPipeline(object):
def __init__(self):
self.duplicates = {}
dispatcher.connect(self.spider_opened, signals.spider_opened)
dispatcher.connect(self.spider_closed, signals.spider_closed)
def spider_opened(self, spider):
self.duplicates[spider] = set()
def spider_closed(self, spider):
del self.duplicates[spider]
def process_item(self, item, spider):
if item['id'] in self.duplicates[spider]:
raise DropItem("Duplicate item found: %s" % item)
else:
self.duplicates[spider].add(item['id'])
return item