downloader middleware的三个methods不同返回的情况

要激活一个meddleware, 要在设置里面添加。例如：

DOWNLOADER_MIDDLEWARES = {
    'myproject.middlewares.CustomDownloaderMiddleware': 543,
}

key是要激活的middleware的路径， value是它的value。其实scrapy本身就已经内置了很多middleware，所以在激活一个自己编写的middleware的时候，要在文档中查找默认的middleware的序号，以便把自己的middleware插入到正确的位置。

默认的middleware如下：

{
    'scrapy.downloadermiddlewares.robotstxt.RobotsTxtMiddleware': 100,
    'scrapy.downloadermiddlewares.httpauth.HttpAuthMiddleware': 300,
    'scrapy.downloadermiddlewares.downloadtimeout.DownloadTimeoutMiddleware': 350,
    'scrapy.downloadermiddlewares.defaultheaders.DefaultHeadersMiddleware': 400,
    'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware': 500,
    'scrapy.downloadermiddlewares.retry.RetryMiddleware': 550,
    'scrapy.downloadermiddlewares.ajaxcrawl.AjaxCrawlMiddleware': 560,
    'scrapy.downloadermiddlewares.redirect.MetaRefreshMiddleware': 580,
    'scrapy.downloadermiddlewares.httpcompression.HttpCompressionMiddleware': 590,
    'scrapy.downloadermiddlewares.redirect.RedirectMiddleware': 600,
    'scrapy.downloadermiddlewares.cookies.CookiesMiddleware': 700,
    'scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware': 750,
    'scrapy.downloadermiddlewares.stats.DownloaderStats': 850,
    'scrapy.downloadermiddlewares.httpcache.HttpCacheMiddleware': 900,
}

序号越小的middleware越接近engine,越大的越靠近downloader。

每一个downloader最多只能有四个methods。分别是：process_requests, process_response,process_exception和from_crawler。我们编写的downloader至少要有其中一个。

在engine发送requests给downloader这条路上，对于这个request，会依次调用所有middlware对它进行处理。（序号由小到大）

在downloader发送response给engine这条路上，对于这个response，会依次调用所有middleware对它进行处理。（序号由大到小）

下面是对这四个方法的介绍：

process_request(request, spider)

Parameters
request (Request object) – the request being processed

spider (Spider object) – the spider for which this request is intended

process_resquest可以 return None, return a Response object, return a Request object, or raise IgnoreRequest.

1. 返回None：把这个修改过的request继续传递下去。
2. 返回Response对象：会依次调用所有middleware的process_response对它进行处理。（序号由大到小）(不再进入downloader)
3. 返回Requests对象：把这个新的request放到调度队列的开头，即放到engine到downloader这条路的开头，会依次调用所有middlware的process_request对它进行处理。（序号由小到大）
4. 返回IgnoreRequest：会依次调用所有middleware的process_exception对它进行处理。（序号由大到小）。如果一个process_exception都没有，就会回调Request的errorback函数。如果又没有，这个错误就会被忽略。

process_response(request, response, spider)

Parameters
request (is a Request object) – the request that originated the response

response (Response object) – the response being processed

spider (Spider object) – the spider for which this response is intended

process_response可以 return a Response object, return a Request object or raise a IgnoreRequest exception.

1. 返回Response对象: 这个修改或没有修改过的response会依次被剩下的middleware的process_response调用，继续走向engine.
2. 返回Request对象：把这个request放到engine中执行调度，走向downloader。
3. 返回IgnoreRequest：回调Request的errorback函数。如果没有，这个错误就会被忽略。

process_exception(request, exception, spider)

Parameters
request (is a Request object) – the request that generated the exception

exception (an Exception object) – the raised exception

spider (Spider object) – the spider for which this request is intended

process_exception可以return: either None, a Response object, or a Request object.

1. 返回None: 继续在剩下的middleware中传递这个exception、执行process_exception方法。
2. 返回Response对象：把这个response放到downloader中执行调度，走向engine。
3. 返回Request对象：把这个request放到engine中执行调度，走向downloader。

from_crawler(cls, crawler)
If present, this classmethod is called to create a middleware instance from a Crawler. It must return a new instance of the middleware. Crawler object provides access to all Scrapy core components like settings and signals; it is a way for middleware to access them and hook its functionality into Scrapy.

Parameters
crawler (Crawler object) – crawler that uses this middleware