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.. _topics-exporters:
==============
Item Exporters
==============
.. module:: scrapy.exporters
:synopsis: Item Exporters
Once you have scraped your items, you often want to persist or export those
items, to use the data in some other application. That is, after all, the whole
purpose of the scraping process.
For this purpose Scrapy provides a collection of Item Exporters for different
output formats, such as XML, CSV or JSON.
Using Item Exporters
====================
If you are in a hurry, and just want to use an Item Exporter to output scraped
data see the :ref:`topics-feed-exports`. Otherwise, if you want to know how
Item Exporters work or need more custom functionality (not covered by the
default exports), continue reading below.
In order to use an Item Exporter, you must instantiate it with its required
args. Each Item Exporter requires different arguments, so check each exporter
documentation to be sure, in :ref:`topics-exporters-reference`. After you have
instantiated your exporter, you have to:
1. call the method :meth:`~BaseItemExporter.start_exporting` in order to
signal the beginning of the exporting process
2. call the :meth:`~BaseItemExporter.export_item` method for each item you want
to export
3. and finally call the :meth:`~BaseItemExporter.finish_exporting` to signal
the end of the exporting process
Here you can see an :doc:`Item Pipeline <item-pipeline>` which uses multiple
Item Exporters to group scraped items to different files according to the
value of one of their fields:
.. code-block:: python
from itemadapter import ItemAdapter
from scrapy.exporters import XmlItemExporter
class PerYearXmlExportPipeline:
"""Distribute items across multiple XML files according to their 'year' field"""
def open_spider(self, spider):
self.year_to_exporter = {}
def close_spider(self, spider):
for exporter, xml_file in self.year_to_exporter.values():
exporter.finish_exporting()
xml_file.close()
def _exporter_for_item(self, item):
adapter = ItemAdapter(item)
year = adapter["year"]
if year not in self.year_to_exporter:
xml_file = open(f"{year}.xml", "wb")
exporter = XmlItemExporter(xml_file)
exporter.start_exporting()
self.year_to_exporter[year] = (exporter, xml_file)
return self.year_to_exporter[year][0]
def process_item(self, item, spider):
exporter = self._exporter_for_item(item)
exporter.export_item(item)
return item
.. _topics-exporters-field-serialization:
Serialization of item fields
============================
By default, the field values are passed unmodified to the underlying
serialization library, and the decision of how to serialize them is delegated
to each particular serialization library.
However, you can customize how each field value is serialized *before it is
passed to the serialization library*.
There are two ways to customize how a field will be serialized, which are
described next.
.. _topics-exporters-serializers:
1. Declaring a serializer in the field
--------------------------------------
If you use :class:`~scrapy.Item` you can declare a serializer in the
:ref:`field metadata <topics-items-fields>`. The serializer must be
a callable which receives a value and returns its serialized form.
Example:
.. code-block:: python
import scrapy
def serialize_price(value):
return f"$ {str(value)}"
class Product(scrapy.Item):
name = scrapy.Field()
price = scrapy.Field(serializer=serialize_price)
2. Overriding the serialize_field() method
------------------------------------------
You can also override the :meth:`~BaseItemExporter.serialize_field()` method to
customize how your field value will be exported.
Make sure you call the base class :meth:`~BaseItemExporter.serialize_field()` method
after your custom code.
Example:
.. code-block:: python
from scrapy.exporters import XmlItemExporter
class ProductXmlExporter(XmlItemExporter):
def serialize_field(self, field, name, value):
if name == "price":
return f"$ {str(value)}"
return super().serialize_field(field, name, value)
.. _topics-exporters-reference:
Built-in Item Exporters reference
=================================
Here is a list of the Item Exporters bundled with Scrapy. Some of them contain
output examples, which assume you're exporting these two items:
.. skip: next
.. code-block:: python
Item(name="Color TV", price="1200")
Item(name="DVD player", price="200")
BaseItemExporter
----------------
.. class:: BaseItemExporter(fields_to_export=None, export_empty_fields=False, encoding='utf-8', indent=0, dont_fail=False)
This is the (abstract) base class for all Item Exporters. It provides
support for common features used by all (concrete) Item Exporters, such as
defining what fields to export, whether to export empty fields, or which
encoding to use.
These features can be configured through the ``__init__`` method arguments which
populate their respective instance attributes: :attr:`fields_to_export`,
:attr:`export_empty_fields`, :attr:`encoding`, :attr:`indent`.
.. versionadded:: 2.0
The *dont_fail* parameter.
.. method:: export_item(item)
Exports the given item. This method must be implemented in subclasses.
.. method:: serialize_field(field, name, value)
Return the serialized value for the given field. You can override this
method (in your custom Item Exporters) if you want to control how a
particular field or value will be serialized/exported.
By default, this method looks for a serializer :ref:`declared in the item
field <topics-exporters-serializers>` and returns the result of applying
that serializer to the value. If no serializer is found, it returns the
value unchanged.
:param field: the field being serialized. If the source :ref:`item object
<item-types>` does not define field metadata, *field* is an empty
:class:`dict`.
:type field: :class:`~scrapy.Field` object or a :class:`dict` instance
:param name: the name of the field being serialized
:type name: str
:param value: the value being serialized
.. method:: start_exporting()
Signal the beginning of the exporting process. Some exporters may use
this to generate some required header (for example, the
:class:`XmlItemExporter`). You must call this method before exporting any
items.
.. method:: finish_exporting()
Signal the end of the exporting process. Some exporters may use this to
generate some required footer (for example, the
:class:`XmlItemExporter`). You must always call this method after you
have no more items to export.
.. attribute:: fields_to_export
Fields to export, their order [1]_ and their output names.
Possible values are:
- ``None`` (all fields [2]_, default)
- A list of fields::
['field1', 'field2']
- A dict where keys are fields and values are output names::
{'field1': 'Field 1', 'field2': 'Field 2'}
.. [1] Not all exporters respect the specified field order.
.. [2] When using :ref:`item objects <item-types>` that do not expose
all their possible fields, exporters that do not support exporting
a different subset of fields per item will only export the fields
found in the first item exported.
.. attribute:: export_empty_fields
Whether to include empty/unpopulated item fields in the exported data.
Defaults to ``False``. Some exporters (like :class:`CsvItemExporter`)
ignore this attribute and always export all empty fields.
This option is ignored for dict items.
.. attribute:: encoding
The output character encoding.
.. attribute:: indent
Amount of spaces used to indent the output on each level. Defaults to ``0``.
* ``indent=None`` selects the most compact representation,
all items in the same line with no indentation
* ``indent<=0`` each item on its own line, no indentation
* ``indent>0`` each item on its own line, indented with the provided numeric value
PythonItemExporter
------------------
.. autoclass:: PythonItemExporter
.. highlight:: none
XmlItemExporter
---------------
.. class:: XmlItemExporter(file, item_element='item', root_element='items', **kwargs)
Exports items in XML format to the specified file object.
:param file: the file-like object to use for exporting the data. Its ``write`` method should
accept ``bytes`` (a disk file opened in binary mode, a ``io.BytesIO`` object, etc)
:param root_element: The name of root element in the exported XML.
:type root_element: str
:param item_element: The name of each item element in the exported XML.
:type item_element: str
The additional keyword arguments of this ``__init__`` method are passed to the
:class:`BaseItemExporter` ``__init__`` method.
A typical output of this exporter would be::
<?xml version="1.0" encoding="utf-8"?>
<items>
<item>
<name>Color TV</name>
<price>1200</price>
</item>
<item>
<name>DVD player</name>
<price>200</price>
</item>
</items>
Unless overridden in the :meth:`serialize_field` method, multi-valued fields are
exported by serializing each value inside a ``<value>`` element. This is for
convenience, as multi-valued fields are very common.
For example, the item::
Item(name=['John', 'Doe'], age='23')
Would be serialized as::
<?xml version="1.0" encoding="utf-8"?>
<items>
<item>
<name>
<value>John</value>
<value>Doe</value>
</name>
<age>23</age>
</item>
</items>
CsvItemExporter
---------------
.. class:: CsvItemExporter(file, include_headers_line=True, join_multivalued=',', errors=None, **kwargs)
Exports items in CSV format to the given file-like object. If the
:attr:`fields_to_export` attribute is set, it will be used to define the
CSV columns, their order and their column names. The
:attr:`export_empty_fields` attribute has no effect on this exporter.
:param file: the file-like object to use for exporting the data. Its ``write`` method should
accept ``bytes`` (a disk file opened in binary mode, a ``io.BytesIO`` object, etc)
:param include_headers_line: If enabled, makes the exporter output a header
line with the field names taken from
:attr:`BaseItemExporter.fields_to_export` or the first exported item fields.
:type include_headers_line: bool
:param join_multivalued: The char (or chars) that will be used for joining
multi-valued fields, if found.
:type include_headers_line: str
:param errors: The optional string that specifies how encoding and decoding
errors are to be handled. For more information see
:class:`io.TextIOWrapper`.
:type errors: str
The additional keyword arguments of this ``__init__`` method are passed to the
:class:`BaseItemExporter` ``__init__`` method, and the leftover arguments to the
:func:`csv.writer` function, so you can use any :func:`csv.writer` function
argument to customize this exporter.
A typical output of this exporter would be::
product,price
Color TV,1200
DVD player,200
PickleItemExporter
------------------
.. class:: PickleItemExporter(file, protocol=0, **kwargs)
Exports items in pickle format to the given file-like object.
:param file: the file-like object to use for exporting the data. Its ``write`` method should
accept ``bytes`` (a disk file opened in binary mode, a ``io.BytesIO`` object, etc)
:param protocol: The pickle protocol to use.
:type protocol: int
For more information, see :mod:`pickle`.
The additional keyword arguments of this ``__init__`` method are passed to the
:class:`BaseItemExporter` ``__init__`` method.
Pickle isn't a human readable format, so no output examples are provided.
PprintItemExporter
------------------
.. class:: PprintItemExporter(file, **kwargs)
Exports items in pretty print format to the specified file object.
:param file: the file-like object to use for exporting the data. Its ``write`` method should
accept ``bytes`` (a disk file opened in binary mode, a ``io.BytesIO`` object, etc)
The additional keyword arguments of this ``__init__`` method are passed to the
:class:`BaseItemExporter` ``__init__`` method.
A typical output of this exporter would be::
{'name': 'Color TV', 'price': '1200'}
{'name': 'DVD player', 'price': '200'}
Longer lines (when present) are pretty-formatted.
JsonItemExporter
----------------
.. class:: JsonItemExporter(file, **kwargs)
Exports items in JSON format to the specified file-like object, writing all
objects as a list of objects. The additional ``__init__`` method arguments are
passed to the :class:`BaseItemExporter` ``__init__`` method, and the leftover
arguments to the :class:`~json.JSONEncoder` ``__init__`` method, so you can use any
:class:`~json.JSONEncoder` ``__init__`` method argument to customize this exporter.
:param file: the file-like object to use for exporting the data. Its ``write`` method should
accept ``bytes`` (a disk file opened in binary mode, a ``io.BytesIO`` object, etc)
A typical output of this exporter would be::
[{"name": "Color TV", "price": "1200"},
{"name": "DVD player", "price": "200"}]
.. _json-with-large-data:
.. warning:: JSON is very simple and flexible serialization format, but it
doesn't scale well for large amounts of data since incremental (aka.
stream-mode) parsing is not well supported (if at all) among JSON parsers
(on any language), and most of them just parse the entire object in
memory. If you want the power and simplicity of JSON with a more
stream-friendly format, consider using :class:`JsonLinesItemExporter`
instead, or splitting the output in multiple chunks.
JsonLinesItemExporter
---------------------
.. class:: JsonLinesItemExporter(file, **kwargs)
Exports items in JSON format to the specified file-like object, writing one
JSON-encoded item per line. The additional ``__init__`` method arguments are passed
to the :class:`BaseItemExporter` ``__init__`` method, and the leftover arguments to
the :class:`~json.JSONEncoder` ``__init__`` method, so you can use any
:class:`~json.JSONEncoder` ``__init__`` method argument to customize this exporter.
:param file: the file-like object to use for exporting the data. Its ``write`` method should
accept ``bytes`` (a disk file opened in binary mode, a ``io.BytesIO`` object, etc)
A typical output of this exporter would be::
{"name": "Color TV", "price": "1200"}
{"name": "DVD player", "price": "200"}
Unlike the one produced by :class:`JsonItemExporter`, the format produced by
this exporter is well suited for serializing large amounts of data.
MarshalItemExporter
-------------------
.. autoclass:: MarshalItemExporter
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