scrapy monitors spider startup and shutdown

In the pipeline, you can monitor the startup and shutdown of the spider, the code is as follows:
 

from scrapy.xlib.pydispatch import dispatcher
from scrapy import signals
from scrapy.exceptions import DropItem
class DuplicatesPipeline(object):
def __init__(self):
    self.duplicates = {}
    dispatcher.connect(self.spider_opened, signals.spider_opened)
    dispatcher.connect(self.spider_closed, signals.spider_closed)
def spider_opened(self, spider):
    self.duplicates[spider] = set()
def spider_closed(self, spider):
    del self.duplicates[spider]
def process_item(self, item, spider):
    if item[’id’] in self.duplicates[spider]:
        raise DropItem("Duplicate item found: %s" % item)
    else:
        self.duplicates[spider].add(item[’id’])
    return item