The extensions framework provides a mechanism for inserting your own custom functionality into Scrapy.
Extensions are just regular classes that are instantiated at Scrapy startup, when extensions are initialized.
Extensions use the :ref:`Scrapy settings <topics-settings>` to manage their settings, just like any other Scrapy code.
It is customary for extensions to prefix their settings with their own name, to avoid collision with existing (and future) extensions. For example, an hypothetic extension to handle Google Sitemaps would use settings like GOOGLESITEMAP_ENABLED, GOOGLESITEMAP_DEPTH, and so on.
Extensions are loaded and activated at startup by instantiating a single
instance of the extension class. Therefore, all the extension initialization
code must be performed in the class constructor (__init__
method).
To make an extension available, add it to the :setting:`EXTENSIONS` setting in your Scrapy settings. In :setting:`EXTENSIONS`, each extension is represented by a string: the full Python path to the extension's class name. For example:
EXTENSIONS = { 'scrapy.extensions.corestats.CoreStats': 500, 'scrapy.telnet.TelnetConsole': 500, }
As you can see, the :setting:`EXTENSIONS` setting is a dict where the keys are the extension paths, and their values are the orders, which define the extension loading order. Extensions orders are not as important as middleware orders though, and they are typically irrelevant, ie. it doesn't matter in which order the extensions are loaded because they don't depend on each other [1].
However, this feature can be exploited if you need to add an extension which depends on other extensions already loaded.
[1] This is is why the :setting:`EXTENSIONS_BASE` setting in Scrapy (which
contains all built-in extensions enabled by default) defines all the extensions
with the same order (500
).
Not all available extensions will be enabled. Some of them usually depend on a particular setting. For example, the HTTP Cache extension is available by default but disabled unless the :setting:`HTTPCACHE_ENABLED` setting is set.
In order to disable an extension that comes enabled by default (ie. those
included in the :setting:`EXTENSIONS_BASE` setting) you must set its order to
None
. For example:
EXTENSIONS = { 'scrapy.extensions.corestats.CoreStats': None, }
Each extension is a Python class. The main entry point for a Scrapy extension
(this also includes middlewares and pipelines) is the from_crawler
class method which receives a Crawler
instance. Through the Crawler object
you can access settings, signals, stats, and also control the crawling behaviour.
Typically, extensions connect to :ref:`signals <topics-signals>` and perform tasks triggered by them.
Finally, if the from_crawler
method raises the
:exc:`~scrapy.exceptions.NotConfigured` exception, the extension will be
disabled. Otherwise, the extension will be enabled.
Here we will implement a simple extension to illustrate the concepts described in the previous section. This extension will log a message every time:
- a spider is opened
- a spider is closed
- a specific number of items are scraped
The extension will be enabled through the MYEXT_ENABLED
setting and the
number of items will be specified through the MYEXT_ITEMCOUNT
setting.
Here is the code of such extension:
import logging from scrapy import signals from scrapy.exceptions import NotConfigured logger = logging.getLogger(__name__) class SpiderOpenCloseLogging(object): def __init__(self, item_count): self.item_count = item_count self.items_scraped = 0 @classmethod def from_crawler(cls, crawler): # first check if the extension should be enabled and raise # NotConfigured otherwise if not crawler.settings.getbool('MYEXT_ENABLED'): raise NotConfigured # get the number of items from settings item_count = crawler.settings.getint('MYEXT_ITEMCOUNT', 1000) # instantiate the extension object ext = cls(item_count) # connect the extension object to signals crawler.signals.connect(ext.spider_opened, signal=signals.spider_opened) crawler.signals.connect(ext.spider_closed, signal=signals.spider_closed) crawler.signals.connect(ext.item_scraped, signal=signals.item_scraped) # return the extension object return ext def spider_opened(self, spider): logger.info("opened spider %s", spider.name) def spider_closed(self, spider): logger.info("closed spider %s", spider.name) def item_scraped(self, item, spider): self.items_scraped += 1 if self.items_scraped % self.item_count == 0: logger.info("scraped %d items", self.items_scraped)
.. module:: scrapy.extensions.logstats :synopsis: Basic stats logging
Log basic stats like crawled pages and scraped items.
.. module:: scrapy.extensions.corestats :synopsis: Core stats collection
Enable the collection of core statistics, provided the stats collection is enabled (see :ref:`topics-stats`).
.. module:: scrapy.telnet :synopsis: Telnet console
Provides a telnet console for getting into a Python interpreter inside the currently running Scrapy process, which can be very useful for debugging.
The telnet console must be enabled by the :setting:`TELNETCONSOLE_ENABLED` setting, and the server will listen in the port specified in :setting:`TELNETCONSOLE_PORT`.
.. module:: scrapy.extensions.memusage :synopsis: Memory usage extension
Note
This extension does not work in Windows.
Monitors the memory used by the Scrapy process that runs the spider and:
- sends a notification e-mail when it exceeds a certain value
- closes the spider when it exceeds a certain value
The notification e-mails can be triggered when a certain warning value is reached (:setting:`MEMUSAGE_WARNING_MB`) and when the maximum value is reached (:setting:`MEMUSAGE_LIMIT_MB`) which will also cause the spider to be closed and the Scrapy process to be terminated.
This extension is enabled by the :setting:`MEMUSAGE_ENABLED` setting and can be configured with the following settings:
- :setting:`MEMUSAGE_LIMIT_MB`
- :setting:`MEMUSAGE_WARNING_MB`
- :setting:`MEMUSAGE_NOTIFY_MAIL`
- :setting:`MEMUSAGE_REPORT`
.. module:: scrapy.extensions.memdebug :synopsis: Memory debugger extension
An extension for debugging memory usage. It collects information about:
- objects uncollected by the Python garbage collector
- objects left alive that shouldn't. For more info, see :ref:`topics-leaks-trackrefs`
To enable this extension, turn on the :setting:`MEMDEBUG_ENABLED` setting. The info will be stored in the stats.
.. module:: scrapy.extensions.closespider :synopsis: Close spider extension
Closes a spider automatically when some conditions are met, using a specific closing reason for each condition.
The conditions for closing a spider can be configured through the following settings:
- :setting:`CLOSESPIDER_TIMEOUT`
- :setting:`CLOSESPIDER_ITEMCOUNT`
- :setting:`CLOSESPIDER_PAGECOUNT`
- :setting:`CLOSESPIDER_ERRORCOUNT`
.. setting:: CLOSESPIDER_TIMEOUT
Default: 0
An integer which specifies a number of seconds. If the spider remains open for
more than that number of second, it will be automatically closed with the
reason closespider_timeout
. If zero (or non set), spiders won't be closed by
timeout.
.. setting:: CLOSESPIDER_ITEMCOUNT
Default: 0
An integer which specifies a number of items. If the spider scrapes more than
that amount if items and those items are passed by the item pipeline, the
spider will be closed with the reason closespider_itemcount
. If zero (or
non set), spiders won't be closed by number of passed items.
.. setting:: CLOSESPIDER_PAGECOUNT
.. versionadded:: 0.11
Default: 0
An integer which specifies the maximum number of responses to crawl. If the spider
crawls more than that, the spider will be closed with the reason
closespider_pagecount
. If zero (or non set), spiders won't be closed by
number of crawled responses.
.. setting:: CLOSESPIDER_ERRORCOUNT
.. versionadded:: 0.11
Default: 0
An integer which specifies the maximum number of errors to receive before
closing the spider. If the spider generates more than that number of errors,
it will be closed with the reason closespider_errorcount
. If zero (or non
set), spiders won't be closed by number of errors.
.. module:: scrapy.extensions.statsmailer :synopsis: StatsMailer extension
This simple extension can be used to send a notification e-mail every time a domain has finished scraping, including the Scrapy stats collected. The email will be sent to all recipients specified in the :setting:`STATSMAILER_RCPTS` setting.
.. module:: scrapy.extensions.debug :synopsis: Extensions for debugging Scrapy
Dumps information about the running process when a SIGQUIT or SIGUSR2 signal is received. The information dumped is the following:
- engine status (using
scrapy.utils.engine.get_engine_status()
) - live references (see :ref:`topics-leaks-trackrefs`)
- stack trace of all threads
After the stack trace and engine status is dumped, the Scrapy process continues running normally.
This extension only works on POSIX-compliant platforms (ie. not Windows), because the SIGQUIT and SIGUSR2 signals are not available on Windows.
There are at least two ways to send Scrapy the SIGQUIT signal:
By pressing Ctrl-while a Scrapy process is running (Linux only?)
By running this command (assuming
<pid>
is the process id of the Scrapy process):kill -QUIT <pid>
Invokes a Python debugger inside a running Scrapy process when a SIGUSR2 signal is received. After the debugger is exited, the Scrapy process continues running normally.
For more info see Debugging in Python.
This extension only works on POSIX-compliant platforms (ie. not Windows).