11 KiB
Items
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.. module:: scrapy.item :synopsis: Item and Field classes
The main goal in scraping is to extract structured data from unstructured sources, typically, web pages. :ref:`Spiders <topics-spiders>` may return the extracted data as items, Python objects that define key-value pairs.
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Unknown interpreted text role "ref".Scrapy supports :ref:`multiple types of items <item-types>`. When you create an item, you may use whichever type of item you want. When you write code that receives an item, your code should :ref:`work for any item type <supporting-item-types>`.
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Unknown interpreted text role "ref".Item Types
Scrapy supports the following types of items, via the itemadapter library: :ref:`dictionaries <dict-items>`, :ref:`Item objects <item-objects>`, :ref:`dataclass objects <dataclass-items>`, and :ref:`attrs objects <attrs-items>`.
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As an item type, :class:`dict` is convenient and familiar.
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:class:`Item` provides a :class:`dict`-like API plus additional features that make it the most feature-complete item type:
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Unknown interpreted text role "class".:class:`Item` objects replicate the standard :class:`dict` API, including its __init__ method.
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Unknown interpreted text role "class".:class:`Item` allows defining field names, so that:
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Unknown interpreted text role "class".:class:`KeyError` is raised when using undefined field names (i.e. prevents typos going unnoticed)
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:ref:`Item exporters <topics-exporters>` can export all fields by default even if the first scraped object does not have values for all of them
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:class:`Item` also allows defining field metadata, which can be used to :ref:`customize serialization <topics-exporters-field-serialization>`.
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Unknown interpreted text role "ref".:mod:`trackref` tracks :class:`Item` objects to help find memory leaks (see :ref:`topics-leaks-trackrefs`).
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Unknown interpreted text role "ref".:class:`Item` objects also provide the following additional API members:
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.. automethod:: copy
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.. automethod:: deepcopy
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.. attribute:: fields
A dictionary containing *all declared fields* for this Item, not only
those populated. The keys are the field names and the values are the
:class:`Field` objects used in the :ref:`Item declaration
<topics-items-declaring>`.
Example:
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.. code-block:: python
from scrapy.item import Item, Field
class CustomItem(Item):
one_field = Field()
another_field = Field()
Dataclass objects
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.. versionadded:: 2.2
:func:`~dataclasses.dataclass` allows defining item classes with field names, so that :ref:`item exporters <topics-exporters>` can export all fields by default even if the first scraped object does not have values for all of them.
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Unknown interpreted text role "ref".Additionally, dataclass items also allow to:
define the type and default value of each defined field.
define custom field metadata through :func:`dataclasses.field`, which can be used to :ref:`customize serialization <topics-exporters-field-serialization>`.
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Example:
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.. code-block:: python
from dataclasses import dataclass
@dataclass
class CustomItem:
one_field: str
another_field: int
Note
Field types are not enforced at run time.
attr.s objects
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.. versionadded:: 2.2
:func:`attr.s` allows defining item classes with field names, so that :ref:`item exporters <topics-exporters>` can export all fields by default even if the first scraped object does not have values for all of them.
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Unknown interpreted text role "ref".Additionally, attr.s items also allow to:
define the type and default value of each defined field.
define custom field :ref:`metadata <attrs:metadata>`, which can be used to :ref:`customize serialization <topics-exporters-field-serialization>`.
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In order to use this type, the :doc:`attrs package <attrs:index>` needs to be installed.
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.. code-block:: python
import attr
@attr.s
class CustomItem:
one_field = attr.ib()
another_field = attr.ib()
Working with Item objects
Declaring Item subclasses
Item subclasses are declared using a simple class definition syntax and :class:`Field` objects. Here is an example:
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.. code-block:: python
import scrapy
class Product(scrapy.Item):
name = scrapy.Field()
price = scrapy.Field()
stock = scrapy.Field()
tags = scrapy.Field()
last_updated = scrapy.Field(serializer=str)
Note
Those familiar with Django will notice that Scrapy Items are declared similar to Django Models, except that Scrapy Items are much simpler as there is no concept of different field types.
Declaring fields
:class:`Field` objects are used to specify metadata for each field. For example, the serializer function for the last_updated field illustrated in the example above.
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Unknown interpreted text role "class".You can specify any kind of metadata for each field. There is no restriction on the values accepted by :class:`Field` objects. For this same reason, there is no reference list of all available metadata keys. Each key defined in :class:`Field` objects could be used by a different component, and only those components know about it. You can also define and use any other :class:`Field` key in your project too, for your own needs. The main goal of :class:`Field` objects is to provide a way to define all field metadata in one place. Typically, those components whose behaviour depends on each field use certain field keys to configure that behaviour. You must refer to their documentation to see which metadata keys are used by each component.
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Unknown interpreted text role "class".It's important to note that the :class:`Field` objects used to declare the item do not stay assigned as class attributes. Instead, they can be accessed through the :attr:`Item.fields` attribute.
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Unknown interpreted text role "attr".The :class:`Field` class is just an alias to the built-in :class:`dict` class and doesn't provide any extra functionality or attributes. In other words, :class:`Field` objects are plain-old Python dicts. A separate class is used to support the :ref:`item declaration syntax <topics-items-declaring>` based on class attributes.
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Field metadata can also be declared for dataclass and attrs items. Please refer to the documentation for dataclasses.field and attr.ib for additional information.
Working with Item objects
Here are some examples of common tasks performed with items, using the Product item :ref:`declared above <topics-items-declaring>`. You will notice the API is very similar to the :class:`dict` API.
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>>> product = Product(name='Desktop PC', price=1000) >>> print(product) Product(name='Desktop PC', price=1000)
Getting field values
>>> product['name']
Desktop PC
>>> product.get('name')
Desktop PC
>>> product['price'] 1000
>>> product['last_updated']
Traceback (most recent call last):
...
KeyError: 'last_updated'
>>> product.get('last_updated', 'not set')
not set
>>> product['lala'] # getting unknown field
Traceback (most recent call last):
...
KeyError: 'lala'
>>> product.get('lala', 'unknown field')
'unknown field'
>>> 'name' in product # is name field populated? True
>>> 'last_updated' in product # is last_updated populated? False
>>> 'last_updated' in product.fields # is last_updated a declared field? True
>>> 'lala' in product.fields # is lala a declared field? False
Setting field values
>>> product['last_updated'] = 'today' >>> product['last_updated'] today
>>> product['lala'] = 'test' # setting unknown field
Traceback (most recent call last):
...
KeyError: 'Product does not support field: lala'
Accessing all populated values
To access all populated values, just use the typical :class:`dict` API:
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>>> product.items()
[('price', 1000), ('name', 'Desktop PC')]
Copying items
To copy an item, you must first decide whether you want a shallow copy or a deep copy.
If your item contains :term:`mutable` values like lists or dictionaries, a shallow copy will keep references to the same mutable values across all different copies.
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Unknown interpreted text role "term".For example, if you have an item with a list of tags, and you create a shallow copy of that item, both the original item and the copy have the same list of tags. Adding a tag to the list of one of the items will add the tag to the other item as well.
If that is not the desired behavior, use a deep copy instead.
See :mod:`copy` for more information.
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Unknown interpreted text role "mod".To create a shallow copy of an item, you can either call :meth:`~scrapy.Item.copy` on an existing item (product2 = product.copy()) or instantiate your item class from an existing item (product2 = Product(product)).
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Unknown interpreted text role "meth".To create a deep copy, call :meth:`~scrapy.Item.deepcopy` instead (product2 = product.deepcopy()).
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Unknown interpreted text role "meth".Other common tasks
Creating dicts from items:
>>> dict(product) # create a dict from all populated values
{'price': 1000, 'name': 'Desktop PC'}
Creating items from dicts:
>>> Product({'name': 'Laptop PC', 'price': 1500})
Product(price=1500, name='Laptop PC')
>>> Product({'name': 'Laptop PC', 'lala': 1500}) # warning: unknown field in dict
Traceback (most recent call last):
...
KeyError: 'Product does not support field: lala'
Extending Item subclasses
You can extend Items (to add more fields or to change some metadata for some fields) by declaring a subclass of your original Item.
For example:
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.. code-block:: python
class DiscountedProduct(Product):
discount_percent = scrapy.Field(serializer=str)
discount_expiration_date = scrapy.Field()
You can also extend field metadata by using the previous field metadata and appending more values, or changing existing values, like this:
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.. code-block:: python
class SpecificProduct(Product):
name = scrapy.Field(Product.fields["name"], serializer=my_serializer)
That adds (or replaces) the serializer metadata key for the name field, keeping all the previously existing metadata values.
Supporting All Item Types
In code that receives an item, such as methods of :ref:`item pipelines <topics-item-pipeline>` or :ref:`spider middlewares <topics-spider-middleware>`, it is a good practice to use the :class:`~itemadapter.ItemAdapter` class and the :func:`~itemadapter.is_item` function to write code that works for any :ref:`supported item type <item-types>`:
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.. autoclass:: itemadapter.ItemAdapter
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.. autofunction:: itemadapter.is_item