运行Begin.py报错：'pop from an empty set'

[voidhackett]

报错如下
2017-04-29 11:04:01 [scrapy.core.engine] ERROR: Error while obtaining start requests
Traceback (most recent call last):
File "/Library/Python/2.7/site-packages/scrapy/core/engine.py", line 127, in _next_request
request = next(slot.start_requests)
File "/Users/voidwalker/Downloads/爬虫和web程序/SinaSpider/Sina_spider1/Sina_spider1/spiders/spiders.py", line 64, in start_requests
ID = self.scrawl_ID.pop()
KeyError: 'pop from an empty set'