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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.
Each item pipeline component (sometimes referred as just "Item Pipeline") is a Python class that implements a simple method. They receive an Item and perform an action over it, also deciding if the Item should continue through the pipeline or be dropped and no longer processed.
Typical use for item pipelines are:
- cleansing HTML data
- validating scraped data (checking that the items contain certain fields)
- checking for duplicates (and dropping them)
- storing the scraped item in a database
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 29)
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.Spider` object
Additionally, they may also implement the following methods:
System Message: ERROR/3 (<stdin>, line 44)
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.Spider` object
System Message: ERROR/3 (<stdin>, line 51)
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.Spider` object
Item pipeline example
Price validation and dropping items with no prices
Let's take a look at the following hypothetical 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)
Write items to a JSON file
The following pipeline stores all scraped items (from all spiders) into a a single items.jl file, containing one item per line serialized in JSON format:
import json
class JsonWriterPipeline(object):
def __init__(self):
self.file = open('items.jl', 'wb')
def process_item(self, item, spider):
line = json.dumps(dict(item)) + "\n"
self.file.write(line)
return item
Note
The purpose of JsonWriterPipeline is just to introduce how to write item pipelines. If you really want to store all scraped items into a JSON file you should use the :ref:`Feed exports <topics-feed-exports>`.
System Message: ERROR/3 (<stdin>, line 103); backlink
Unknown interpreted text role "ref".Duplicates filter
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 import signals
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
Activating an Item Pipeline component
To activate an Item Pipeline component you must add its class to the :setting:`ITEM_PIPELINES` setting, like in the following example:
System Message: ERROR/3 (<stdin>, line 134); backlink
Unknown interpreted text role "setting".ITEM_PIPELINES = {
'myproject.pipeline.PricePipeline': 300,
'myproject.pipeline.JsonWriterPipeline': 800,
}
The integer values you assign to classes in this setting determine the order they run in- items go through pipelines from order number low to high. It's customary to define these numbers in the 0-1000 range.