信号量

from   scrapy  import   signals
engine_started = object()
engine_stopped = object()
spider_opened = object()
spider_idle = object()
爬虫被闲置的时候
spider_closed = object()
spider_error = object()
request_scheduled = object()
请求到调度器的时候
request_dropped = object()
请求被丢弃的时候
response_received = object()
请求被接受的时候
response_downloaded = object()
item_scraped = object()
item一个一个传递的时候
item_dropped = object()

'''
配置：EXTENSIONS={xxxxxxx}信号，开了几个，才可以多少个
使用框架预留的位置，帮助你自定义一些功能

'''

from   scrapy  import   signals
class  Singal():
    def __init__(self):
        pass

    @classmethod
    def   from_crawl(cls,crawler):
        self=cls()

        crawler.signals.connect(self.fun1,signal=signals.spider_opened)
        crawler.signals.connect(self.fun1,signal=signals.spider_closed)
##连接上，预留的信号，注册
        return  self

    def  fun1(self):
        pass

    def  fun2(self):
        pass