*****
## 下載中間件
使用方法：
- 編寫一個Download Middlewares和我們編寫一個pipeline一樣，定義一個類，然后再settings中開啟
Download Middlewares默認方法：
處理請求,處理響應,對應兩個方法
```
process_request(self,request,spider):
當每個request通過下載中間件時，該方法被調用
process_response(self,request,response,spider):
當下載器完成http請求，傳遞響應給引擎的時候調用
```
### 案例
```
在middlewares.py文件中
class RandomUserAgent(object):
def process_request(self,request,spider):
useragent = random.choice(spider.settings['USER_AGENTS'])
request.headers['User-Agent'] = useragent
class CheckUserAgent(object):
def process_response(self,request,response,spider):
print(request.headers['User-Agent'])
return response
```
### 隨機請求頭
USER_AGENTS = [ "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Win64; x64; Trident/5.0; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET CLR 2.0.50727; Media Center PC 6.0)", "Mozilla/5.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; .NET CLR 1.0.3705; .NET CLR 1.1.4322)", "Mozilla/4.0 (compatible; MSIE 7.0b; Windows NT 5.2; .NET CLR 1.1.4322; .NET CLR 2.0.50727; InfoPath.2; .NET CLR 3.0.04506.30)", "Mozilla/5.0 (Windows; U; Windows NT 5.1; zh-CN) AppleWebKit/523.15 (KHTML, like Gecko, Safari/419.3) Arora/0.3 (Change: 287 c9dfb30)", "Mozilla/5.0 (X11; U; Linux; en-US) AppleWebKit/527+ (KHTML, like Gecko, Safari/419.3) Arora/0.6", "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.2pre) Gecko/20070215 K-Ninja/2.1.1", "Mozilla/5.0 (Windows; U; Windows NT 5.1; zh-CN; rv:1.9) Gecko/20080705 Firefox/3.0 Kapiko/3.0", "Mozilla/5.0 (X11; Linux i686; U;) Gecko/20070322 Kazehakase/0.4.5" ]
