# Extensions

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.

## Extension settings

Extensions use the Scrapy 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, a hypothetic extension to handle Google Sitemaps would use settings like GOOGLESITEMAP_ENABLED, GOOGLESITEMAP_DEPTH, and so on.

## Loading & activating extensions

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 `EXTENSIONS` setting in your Scrapy settings. In `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.extensions.telnet.TelnetConsole': 500,
    }

As you can see, the `EXTENSIONS` setting is a dict where the keys are the extension paths, and their values are the orders, which define the extension loading order. The `EXTENSIONS` setting is merged with the `EXTENSIONS_BASE` setting defined in Scrapy (and not meant to be overridden) and then sorted by order to get the final sorted list of enabled extensions.

As extensions typically do not depend on each other, their loading order is irrelevant in most cases. This is why the `EXTENSIONS_BASE` setting defines all extensions with the same order (`0`). However, this feature can be exploited if you need to add an extension which depends on other extensions already loaded.

## Available, enabled and disabled extensions

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 `HTTPCACHE_ENABLED` setting is set.

## Disabling an extension

In order to disable an extension that comes enabled by default (ie. those included in the `EXTENSIONS_BASE` setting) you must set its order to `None`. For example:

    EXTENSIONS = {
        'scrapy.extensions.corestats.CoreStats': None,
    }

## Writing your own extension

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 signals and perform tasks triggered by them.

Finally, if the `from_crawler` method raises the `NotConfigured` exception, the extension will be disabled. Otherwise, the extension will be enabled.

### Sample extension

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)

## Built-in extensions reference

### General purpose extensions

#### Log Stats extension

class `scrapy.extensions.logstats.``LogStats`

Log basic stats like crawled pages and scraped items.

#### Core Stats extension

class `scrapy.extensions.corestats.``CoreStats`

Enable the collection of core statistics, provided the stats collection is enabled (see Stats Collection).

#### Telnet console extension

class `scrapy.extensions.telnet.``TelnetConsole`

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 `TELNETCONSOLE_ENABLED` setting, and the server will listen in the port specified in `TELNETCONSOLE_PORT`.

#### Memory usage extension

class `scrapy.extensions.memusage.``MemoryUsage`

Note

This extension does not work in Windows.

Monitors the memory used by the Scrapy process that runs the spider and:

1. sends a notification e-mail when it exceeds a certain value
2. closes the spider when it exceeds a certain value

The notification e-mails can be triggered when a certain warning value is reached (`MEMUSAGE_WARNING_MB`) and when the maximum value is reached (`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 `MEMUSAGE_ENABLED` setting and can be configured with the following settings:

- `MEMUSAGE_LIMIT_MB`
- `MEMUSAGE_WARNING_MB`
- `MEMUSAGE_NOTIFY_MAIL`
- `MEMUSAGE_CHECK_INTERVAL_SECONDS`

#### Memory debugger extension

class `scrapy.extensions.memdebug.``MemoryDebugger`

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 Debugging memory leaks with trackref

To enable this extension, turn on the `MEMDEBUG_ENABLED` setting. The info will be stored in the stats.

#### Close spider extension

class `scrapy.extensions.closespider.``CloseSpider`

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:

- `CLOSESPIDER_TIMEOUT`
- `CLOSESPIDER_ITEMCOUNT`
- `CLOSESPIDER_PAGECOUNT`
- `CLOSESPIDER_ERRORCOUNT`

##### 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.

##### CLOSESPIDER_ITEMCOUNT

Default: `0`

An integer which specifies a number of items. If the spider scrapes more than that amount and those items are passed by the item pipeline, the spider will be closed with the reason `closespider_itemcount`. Requests which are currently in the downloader queue (up to `CONCURRENT_REQUESTS` requests) are still processed. If zero (or non set), spiders won’t be closed by number of passed items.

##### CLOSESPIDER_PAGECOUNT

New in version 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.

##### CLOSESPIDER_ERRORCOUNT

New in version 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.

#### StatsMailer extension

class `scrapy.extensions.statsmailer.``StatsMailer`

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 `STATSMAILER_RCPTS` setting.

### Debugging extensions

#### Stack trace dump extension

class `scrapy.extensions.debug.``StackTraceDump`

Dumps information about the running process when a SIGQUIT or SIGUSR2 signal is received. The information dumped is the following:

1. engine status (using `scrapy.utils.engine.get_engine_status()`)
2. live references (see Debugging memory leaks with trackref)
3. 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:

1. By pressing Ctrl-while a Scrapy process is running (Linux only?)

2. By running this command (assuming `<pid>` is the process id of the Scrapy process):

       kill -QUIT <pid>

#### Debugger extension

class `scrapy.extensions.debug.``Debugger`

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).