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# Spider Middleware

The spider middleware is a framework of hooks into Scrapy’s spider processing mechanism where you can plug custom functionality to process the requests that are sent to Spiders for processing and to process the responses and items that are generated from spiders.

## Activating a spider middleware

To activate a spider middleware component, add it to the `SPIDER_MIDDLEWARES` setting, which is a dict whose keys are the middleware class path and their values are the middleware orders.

Here’s an example:

    SPIDER_MIDDLEWARES = {
        'myproject.middlewares.CustomSpiderMiddleware': 543,
    }

The `SPIDER_MIDDLEWARES` setting is merged with the `SPIDER_MIDDLEWARES_BASE` setting defined in Scrapy (and not meant to be overridden) and then sorted by order to get the final sorted list of enabled middlewares: the first middleware is the one closer to the engine and the last is the one closer to the spider.

To decide which order to assign to your middleware see the `SPIDER_MIDDLEWARES_BASE` setting and pick a value according to where you want to insert the middleware. The order does matter because each middleware performs a different action and your middleware could depend on some previous (or subsequent) middleware being applied.

If you want to disable a builtin middleware (the ones defined in `SPIDER_MIDDLEWARES_BASE`, and enabled by default) you must define it in your project `SPIDER_MIDDLEWARES` setting and assign None as its value. For example, if you want to disable the off-site middleware:

    SPIDER_MIDDLEWARES = {
        'myproject.middlewares.CustomSpiderMiddleware': 543,
        'scrapy.contrib.spidermiddleware.offsite.OffsiteMiddleware': None,
    }

Finally, keep in mind that some middlewares may need to be enabled through a particular setting. See each middleware documentation for more info.

## Writing your own spider middleware

Writing your own spider middleware is easy. Each middleware component is a single Python class that defines one or more of the following methods:

class `scrapy.contrib.spidermiddleware.``SpiderMiddleware`
  `process_spider_input`(response, spider)

    This method is called for each response that goes through the spider middleware and into the spider, for processing.

    `process_spider_input()` should return `None` or raise and exception.

    If it returns `None`, Scrapy will continue processing this response, executing all other middlewares until, finally, the response is handled to the spider for processing.

    If it raises an exception, Scrapy won’t bother calling any other spider middleware `process_spider_input()` and will call the request errback. The output of the errback is chained back in the other direction for `process_spider_output()` to process it, or `process_spider_exception()` if it raised an exception.

    Parameters:

    - reponse – the response being processed
    - spider (`BaseSpider` object) – the spider for which this response is intended

  `process_spider_output`(response, result, spider)

    This method is called with the results returned from the Spider, after it has processed the response.

    `process_spider_output()` must return an iterable of `Request` or `Item` objects.

    Parameters:

    - response (class:~scrapy.http.Response object) – the response which generated this output from the spider
    - result (an iterable of `Request` or `Item` objects) – the result returned by the spider
    - spider (`BaseSpider` object) – the spider whose result is being processed

  `process_spider_exception`(response, exception, spider)

    This method is called when when a spider or `process_spider_input()` method (from other spider middleware) raises an exception.

    `process_spider_exception()` should return either `None` or an iterable of `Response` or `Item` objects.

    If it returns `None`, Scrapy will continue processing this exception, executing any other `process_spider_exception()` in the following middleware components, until no middleware components are left and the exception reaches the engine (where it’s logged and discarded).

    If it returns an iterable the `process_spider_output()` pipeline kicks in, and no other `process_spider_exception()` will be called.

    Parameters:

    - response (`Response` object) – the response being processed when the exception was raised
    - exception (Exception object) – the exception raised
    - spider (`scrapy.spider.BaseSpider` object) – the spider which raised the exception

## Built-in spider middleware reference

This page describes all spider middleware components that come with Scrapy. For information on how to use them and how to write your own spider middleware, see the spider middleware usage guide.

For a list of the components enabled by default (and their orders) see the `SPIDER_MIDDLEWARES_BASE` setting.

### DepthMiddleware

class `scrapy.contrib.spidermiddleware.depth.``DepthMiddleware`

  DepthMiddleware is a scrape middleware used for tracking the depth of each Request inside the site being scraped. It can be used to limit the maximum depth to scrape or things like that.

  The `DepthMiddleware` can be configured through the following settings (see the settings documentation for more info):

  > - `DEPTH_LIMIT` - The maximum depth that will be allowed to crawl for any site. If zero, no limit will be imposed.
  > - `DEPTH_STATS` - Whether to collect depth stats.
  > - `DEPTH_PRIORITY` - Whether to prioritize the requests based on their depth.

### HttpErrorMiddleware

class `scrapy.contrib.spidermiddleware.httperror.``HttpErrorMiddleware`

  Filter out unsuccessful (erroneous) HTTP responses so that spiders don’t have to deal with them, which (most of the time) imposes an overhead, consumes more resources, and makes the spider logic more complex.

According to the HTTP standard, successful responses are those whose status codes are in the 200-300 range.

If you still want to process response codes outside that range, you can specify which response codes the spider is able to handle using the `handle_httpstatus_list` spider attribute.

For example, if you want your spider to handle 404 responses you can do this:

    class MySpider(CrawlSpider):
        handle_httpstatus_list = [404]

The `handle_httpstatus_list` key of `Request.meta` can also be used to specify which response codes to allow on a per-request basis.

Keep in mind, however, that it’s usually a bad idea to handle non-200 responses, unless you really know what you’re doing.

For more information see: HTTP Status Code Definitions.

### OffsiteMiddleware

class `scrapy.contrib.spidermiddleware.offsite.``OffsiteMiddleware`

  Filters out Requests for URLs outside the domains covered by the spider.

  This middleware filters out every request whose host names aren’t in the spider’s `allowed_domains` attribute.

  When your spider returns a request for a domain not belonging to those covered by the spider, this middleware will log a debug message similar to this one:

      DEBUG: Filtered offsite request to 'www.othersite.com': <GET http://www.othersite.com/some/page.html>

  To avoid filling the log with too much noise, it will only print one of these messages for each new domain filtered. So, for example, if another request for `www.othersite.com` is filtered, no log message will be printed. But if a request for `someothersite.com` is filtered, a message will be printed (but only for the first request filtred).

  If the spider doesn’t define an `allowed_domains` attribute, or the attribute is empty, the offsite middleware will allow all requests.

  If the request has the `dont_filter` attribute set, the offsite middleware will allow the request even if its domain is not listed in allowed domains.

### RefererMiddleware

class `scrapy.contrib.spidermiddleware.referer.``RefererMiddleware`

  Populates Request referer field, based on the Response which originated it.

### UrlLengthMiddleware

class `scrapy.contrib.spidermiddleware.urllength.``UrlLengthMiddleware`

  Filters out requests with URLs longer than URLLENGTH_LIMIT

  The `UrlLengthMiddleware` can be configured through the following settings (see the settings documentation for more info):

  > - `URLLENGTH_LIMIT` - The maximum URL length to allow for crawled URLs.

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