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:
This method is called for every item pipeline component and must either return a
Item(or any descendant class) object or raise a
DropItemexception. Dropped items are no longer processed by further pipeline components.
Additionally, they may also implement the following methods:
This method is called when the spider is opened.
Parameters: spider (
BaseSpiderobject) – the spider which was opened
Item pipeline example¶
Price validation and dropping items with no prices¶
Let’s take a look at the following hypothetical pipeline that adjusts the
attribute for those items that do not include 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
items.jl file, containing one item per line serialized in JSON
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
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 Feed exports.
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