.. _topics-debug:

=================
Debugging Spiders
=================

This document explains the most common techniques for debugging spiders.
Consider the following scrapy spider below::

    import scrapy
    from myproject.items import MyItem

    class MySpider(scrapy.Spider):
        name = 'myspider'
        start_urls = (
            'http://example.com/page1',
            'http://example.com/page2',
            )

        def parse(self, response):
            # collect `item_urls`
            for item_url in item_urls:
                yield scrapy.Request(item_url, self.parse_item)

        def parse_item(self, response):
            item = MyItem()
            # populate `item` fields
            # and extract item_details_url
            yield scrapy.Request(item_details_url, self.parse_details, meta={'item': item})

        def parse_details(self, response):
            item = response.meta['item']
            # populate more `item` fields
            return item

Basically this is a simple spider which parses two pages of items (the
start_urls). Items also have a details page with additional information, so we
use the ``meta`` functionality of :class:`~scrapy.http.Request` to pass a
partially populated item.


Parse Command
=============

The most basic way of checking the output of your spider is to use the
:command:`parse` command. It allows to check the behaviour of different parts
of the spider at the method level. It has the advantage of being flexible and
simple to use, but does not allow debugging code inside a method.

In order to see the item scraped from a specific url::

    $ scrapy parse --spider=myspider -c parse_item -d 2 <item_url>
    [ ... scrapy log lines crawling example.com spider ... ]

    >>> STATUS DEPTH LEVEL 2 <<<
    # Scraped Items  ------------------------------------------------------------
    [{'url': <item_url>}]

    # Requests  -----------------------------------------------------------------
    []

Using the ``--verbose`` or ``-v`` option we can see the status at each depth level::

    $ scrapy parse --spider=myspider -c parse_item -d 2 -v <item_url>
    [ ... scrapy log lines crawling example.com spider ... ]

    >>> DEPTH LEVEL: 1 <<<
    # Scraped Items  ------------------------------------------------------------
    []

    # Requests  -----------------------------------------------------------------
    [<GET item_details_url>]


    >>> DEPTH LEVEL: 2 <<<
    # Scraped Items  ------------------------------------------------------------
    [{'url': <item_url>}]

    # Requests  -----------------------------------------------------------------
    []

Checking items scraped from a single start_url, can also be easily achieved
using::

    $ scrapy parse --spider=myspider -d 3 'http://example.com/page1'


Scrapy Shell
============

While the :command:`parse` command is very useful for checking behaviour of a
spider, it is of little help to check what happens inside a callback, besides
showing the response received and the output. How to debug the situation when
``parse_details`` sometimes receives no item?

Fortunately, the :command:`shell` is your bread and butter in this case (see
:ref:`topics-shell-inspect-response`)::

    from scrapy.shell import inspect_response

    def parse_details(self, response):
        item = response.meta.get('item', None)
        if item:
            # populate more `item` fields
            return item
        else:
            inspect_response(response, self)

See also: :ref:`topics-shell-inspect-response`.

Open in browser
===============

Sometimes you just want to see how a certain response looks in a browser, you
can use the ``open_in_browser`` function for that. Here is an example of how
you would use it::

    from scrapy.utils.response import open_in_browser

    def parse_details(self, response):
        if "item name" not in response.body:
            open_in_browser(response)

``open_in_browser`` will open a browser with the response received by Scrapy at
that point, adjusting the `base tag`_ so that images and styles are displayed
properly.

Logging
=======

Logging is another useful option for getting information about your spider run.
Although not as convenient, it comes with the advantage that the logs will be
available in all future runs should they be necessary again::

    def parse_details(self, response):
        item = response.meta.get('item', None)
        if item:
            # populate more `item` fields
            return item
        else:
            self.logger.warning('No item received for %s', response.url)

For more information, check the :ref:`topics-logging` section.

.. _base tag: http://www.w3schools.com/tags/tag_base.asp