Scrapy提供了可自定義2種中間件，1個數(shù)據(jù)處理器

解釋：
用戶設(shè)置：是指custom_settings

可是它們繼承的父類竟然是Object…，每次都要查文檔。

正常來說應(yīng)該提供一個抽象函數(shù)作為接口，給使用者實現(xiàn)自己的具體功能，不知道為啥這么設(shè)計

通過幾段代碼及注釋，簡要說明三個中間件的功能

1、Spider

baidu_spider.py

from scrapy import Spider, cmdline

class BaiduSpider(Spider):
    name = "baidu_spider"

    start_urls = [
        "https://www.baidu.com/"
    ]

    custom_settings = {
        "SPIDER_DATA": "this is spider data",
        "DOWNLOADER_MIDDLEWARES": {
                "scrapys.mymiddleware.MyMiddleware": 100,
            },
        "ITEM_PIPELINES": {
            "scrapys.mypipeline.MyPipeline": 100,
        },
        "SPIDER_MIDDLEWARES":{
            "scrapys.myspidermiddleware.MySpiderMiddleware": 100,
        }
    }

    def parse(self, response):
        pass


if __name__ == '__main__':
    cmdline.execute("scrapy crawl baidu_spider".split())

2、Pipeline

mypipeline.py

class MyPipeline(object):
    def __init__(self, spider_data):
        self.spider_data = spider_data

    @classmethod
    def from_crawler(cls, crawler):
        """
        獲取spider的settings參數(shù),返回Pipeline實例對象
        """
        spider_data = crawler.settings.get("SPIDER_DATA")
        print("### pipeline get spider_data: {}".format(spider_data))

        return cls(spider_data)

    def process_item(self, item, spider):
        """
        return Item 繼續(xù)處理
        raise DropItem 丟棄
        """
        print("### call process_item")

        return item

    def open_spider(self, spider):
        """
        spider開啟時調(diào)用
        """
        print("### spdier open {}".format(spider.name))


    def close_spider(self, spider):
        """
        spider關(guān)閉時調(diào)用
        """
        print("### spdier close {}".format(spider.name))

3、Downloader-Middleware

mymiddleware.py

class MyMiddleware(object):
    def __init__(self, spider_data):
        self.spider_data = spider_data

    @classmethod
    def from_crawler(cls, crawler):
        """
        獲取spider的settings參數(shù),返回中間件實例對象
        """
        spider_data = crawler.settings.get("SPIDER_DATA")
        print("### middleware get spider_data: {}".format(spider_data))

        return cls(spider_data)

    def process_request(self, request, spider):
        """
        return
            None: 繼續(xù)處理Request
            Response: 返回Response
            Request: 重新調(diào)度
        raise IgnoreRequest:  process_exception -> Request.errback
        """
        print("### call process_request")

    def process_response(self, request, response, spider):
        """
        return
            Response: 繼續(xù)處理Response
            Request: 重新調(diào)度
        raise IgnoreRequest: Request.errback
        """
        print("### call process_response")
        return response

    def process_exception(self, request, exception, spider):
        """
        return
            None: 繼續(xù)處理異常
            Response: 返回Response
            Request: 重新調(diào)用
        """
        pass

4、Spider-Middleware

myspidermiddleware.py

class MySpiderMiddleware(object):
    def __init__(self, spider_data):
        self.spider_data = spider_data

    @classmethod
    def from_crawler(cls, crawler):
        """
        獲取spider的settings參數(shù),返回中間件實例對象
        """
        spider_data = crawler.settings.get("SPIDER_DATA")
        print("### spider middleware get spider_data: {}".format(spider_data))

        return cls(spider_data)

    def process_spider_input(self, response, spider):
        """
        url下載完成后執(zhí)行，交給parse處理response （parse之前執(zhí)行）
        return None  繼續(xù)處理response
        raise Exception
        """

        print("### call process_spider_input")

    def process_spider_output(self, response, result, spider):
        """
        response返回result時調(diào)用（result必須返回包含item 或者是 Request的可迭代對象）-----》yield item、yield Request(url)
        return
            iterable of Request、dict or Item
        """
        print("### call process_spider_output")

        for i in result:
            yield i

    def process_spider_exception(self, response, exception, spider):
        """
        return
            None
            iterable of Response, dict, or Item
        """
        pass

    def process_start_requests(self,start_requests,spider):

          """

	#爬蟲剛開始調(diào)用的時候執(zhí)行 （start_request 之前執(zhí)行）

	return: 包含Request對象的可迭代對象

	  """

	return start_requests

運行爬蟲后，查看日志

### middleware get spider_data: this is spider data
### spider middleware get spider_data: this is spider data
### pipeline get spider_data: this is spider data

### spdier open baidu_spider
### call process_request
### call process_response
### call process_spider_input
### call process_spider_output
### spdier close baidu_spider

中間件啟動順序
download middleware
spider middleware
pipeline
處理函數(shù)調(diào)用順序
spdier open
process_request
process_response
process_spider_input
process_spider_output
spdier close
--------------------- 

處理函數(shù)調(diào)用順序

參考

1. Item Pipeline
2. 下載器中間件(Downloader Middleware)
3. Spider中間件(Middleware)
4. Scrapy 1.5 documentation