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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:

`process_item`(item, spider)

  This method is called for every item pipeline component and must either return a `Item` (or any descendant class) object or raise a `DropItem` exception. Dropped items are no longer processed by further pipeline components.

  Parameters:

  - item (`Item` object) – the item scraped
  - spider (`BaseSpider` object) – the spider which scraped the item

Additionally, they may also implement the following methods:

`open_spider`(spider)

  This method is called when the spider is opened.

  Parameters: | spider (`BaseSpider` object) – the spider which was opened

`close_spider`(spider)

  This method is called when the spider is closed.

  Parameters: | spider (`BaseSpider` object) – the spider which was closed

## Item pipeline example

### Price validation and dropping items with no prices

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)

### 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 Feed exports.

## Activating an Item Pipeline component

To activate an Item Pipeline component you must add its class to the `ITEM_PIPELINES` list, like in the following example:

    ITEM_PIPELINES = [
        'myproject.pipeline.PricePipeline',
        'myproject.pipeline.JsonWriterPipeline',
    ]

## 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

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