con:

from twisted.internet import reactor
from scrapy.crawler import CrawlerProcess

Siempre he ejecutado este proceso con éxito:

process = CrawlerProcess(get_project_settings())
process.crawl(*args)
# the script will block here until the crawling is finished
process.start() 

pero desde que moví este código a unweb_crawler(self) función, así:

def web_crawler(self):
    # set up a crawler
    process = CrawlerProcess(get_project_settings())
    process.crawl(*args)
    # the script will block here until the crawling is finished
    process.start() 

    # (...)

    return (result1, result2) 

y comencé a llamar al método usando instanciación de clase, como:

def __call__(self):
    results1 = test.web_crawler()[1]
    results2 = test.web_crawler()[0]

y corriendo:

test()

Estoy teniendo el siguiente error:

Traceback (most recent call last):
  File "test.py", line 573, in <module>
    print (test())
  File "test.py", line 530, in __call__
    artists = test.web_crawler()
  File "test.py", line 438, in web_crawler
    process.start() 
  File "/Library/Python/2.7/site-packages/scrapy/crawler.py", line 280, in start
    reactor.run(installSignalHandlers=False)  # blocking call
  File "/Library/Python/2.7/site-packages/twisted/internet/base.py", line 1194, in run
    self.startRunning(installSignalHandlers=installSignalHandlers)
  File "/Library/Python/2.7/site-packages/twisted/internet/base.py", line 1174, in startRunning
    ReactorBase.startRunning(self)
  File "/Library/Python/2.7/site-packages/twisted/internet/base.py", line 684, in startRunning
    raise error.ReactorNotRestartable()
twisted.internet.error.ReactorNotRestartable

¿qué está mal?