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.
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'
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`.
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 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.