2.8 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 pipeline are usually implemented on each project. Typical usage for item pipelines are:
- 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 define the following method:
System Message: ERROR/3 (<stdin>, line 24)
Unknown directive type "method".
.. method:: process_item(domain, item)
domain is a string with the domain of the spider which scraped the item
item is a :class:`scrapy.item.ScrapedItem` with the item scraped
System Message: ERROR/3 (<stdin>, line 28); backlink
Unknown interpreted text role "class".This method is called for every item pipeline component and must either return a ScrapedItem (or any descendant class) object or raise a :exception:`DropItem` exception. Dropped items are no longer processed by further pipeline components.
System Message: ERROR/3 (<stdin>, line 30); backlink
Unknown interpreted text role "exception".Item pipeline example
Let's take a look at 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.core.exceptions import DropItem
class PricePipeline(object):
vat_factor = 1.15
def process_item(self, domain, item):
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)
Item pipeline example with resources per domain
Sometimes you need to keep resources about the items processed grouped per domain, and delete those resource when a domain finish.
An example is a filter that looks for duplicate items, and drops those items that were already processed. Let say that our items has an unique id, but our spider returns multiples items with the same id:
from pydispatch import dispatcher
from scrapy.core import signals
from scrapy.core.exceptions import DropItem
class DuplicatesPipeline(object):
def __init__(self):
self.domaininfo = {}
dispatcher.connect(self.domain_open, signals.domain_open)
dispatcher.connect(self.domain_closed, signals.domain_closed)
def domain_open(self, domain):
self.duplicates[domain] = set()
def domain_closed(self, domain):
del self.duplicates[domain]
def process_item(self, domain, item):
if item.id in self.duplicates[domain]:
raise DropItem("Duplicate item found: %s" % item)
else:
self.duplicates[domain].add(item.id)
return item