5.4 KiB
Downloader Middleware
The downloader middleware is a framework of hooks into Scrapy's request/response processing. It's a light, low-level system for globally altering Scrapy's requests and responses.
Activating a downloader middleware
To activate a downloader middleware component, add it to the :setting:`DOWNLOADER_MIDDLEWARES` setting, which is a dict whose keys are the middleware class paths and their values are the middleware orders.
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Unknown interpreted text role "setting".Here's an example:
DOWNLOADER_MIDDLEWARES = {
'myproject.middlewares.CustomDownloaderMiddleware': 543,
}
The :setting:`DOWNLOADER_MIDDLEWARES` setting is merged with the :setting:`DOWNLOADER_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 downloader.
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Unknown interpreted text role "setting".System Message: ERROR/3 (<stdin>, line 26); backlink
Unknown interpreted text role "setting".To decide which order to assign to your middleware see the :setting:`DOWNLOADER_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.
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Unknown interpreted text role "setting".If you want to disable a builtin middleware (the ones defined in :setting:`DOWNLOADER_MIDDLEWARES_BASE` and enabled by default) you must define it in your project :setting:`DOWNLOADER_MIDDLEWARES` setting and assign None as its value. For example, if you want to disable the off-site middleware:
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Unknown interpreted text role "setting".System Message: ERROR/3 (<stdin>, line 38); backlink
Unknown interpreted text role "setting".DOWNLOADER_MIDDLEWARES = {
'myproject.middlewares.CustomDownloaderMiddleware': 543,
'scrapy.contrib.downloadermiddleware.useragent.UserAgentMiddleware': 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 downloader middleware
Writing your own downloader middleware is easy. Each middleware component is a single Python class that defines one or more of the following methods:
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Unknown directive type "method".
.. method:: process_request(request, spider)
request is a :class:`~scrapy.http.Request` object spider is a :class:`~scrapy.spider.BaseSpider` object
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Unknown interpreted text role "class".System Message: ERROR/3 (<stdin>, line 60); backlink
Unknown interpreted text role "class".This method is called for each request that goes through the download middleware.
process_request() should return either None, a :class:`~scrapy.http.Response` object, or a :class:`~scrapy.http.Request` object.
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Unknown interpreted text role "class".System Message: ERROR/3 (<stdin>, line 66); backlink
Unknown interpreted text role "class".If returns None, Scrapy will continue processing this request, executing all other middlewares until, finally, the appropriate downloader handler is called the request performed (and its response downloaded).
If returns a Response object, Scrapy won't bother calling ANY other request or exception middleware, or the appropriate download function; it'll return that Response. Response middleware is always called on every response.
If returns a :class:`~scrapy.http.Request` object, the returned request will be re-scheduled (in the Scheduler) to be downloaded in the future. The callback of the original request will always be called. If the new request has a callback it will be called with the response downloaded, and the output of that callback will then be passed to the original callback. If the new request doesn't have a callback, the response downloaded will be just passed to the original request callback.
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Unknown interpreted text role "class".If returns an :exception:`IgnoreRequest` exception, the entire request will be dropped completely and its callback never called.
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Unknown directive type "method".
.. method:: process_response(request, response, spider)
request is a :class:`~scrapy.http.Request` object response is a :class:`~scrapy.http.Response` object spider is a BaseSpider object
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Unknown interpreted text role "class".process_response() should return a Response object or raise a :exception:`IgnoreRequest` exception.
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Unknown interpreted text role "exception".If returns a Response (it could be the same given response, or a brand-new one) that response will continue to be processed with the process_response() of the next middleware in the pipeline.
If returns an :exception:`IgnoreRequest` exception, the response will be dropped completely and its callback never called.
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.. method:: process_download_exception(request, exception, spider)
request is a :class:`~scrapy.http.Request` object. exception is an Exception object spider is a BaseSpider object
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Unknown interpreted text role "class".Scrapy calls process_download_exception() when a download handler or a process_request() (from a downloader middleware) raises an exception.
process_download_exception() should return either None, :class:`~scrapy.http.Response` or :class:`~scrapy.http.Request` object.
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Unknown interpreted text role "class".If it returns None, Scrapy will continue processing this exception, executing any other exception middleware, until no middleware is left and the default exception handling kicks in.
If it returns a :class:`~scrapy.http.Response` object, the response middleware kicks in, and won't bother calling any other exception middleware.
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Unknown interpreted text role "class".If it returns a :class:`~scrapy.http.Request` object, returned request is used to instruct a immediate redirection. Redirection is handled inside middleware scope, and the original request won't finish until redirected request is completed. This stop process_download_exception() middleware as returning Response would do.
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Unknown interpreted text role "class".