Running Scrapy tasks in Python

Question

My Scrapy script seems to work just fine when I run it in 'one off' scenarios from the command line, but if I try running the code twice in the same python session I get this error:

"ReactorNotRestartable"

Why?

The offending code (last line throws the error):

crawler = CrawlerProcess(settings)
crawler.install()
crawler.configure()

# schedule spider
#crawler.crawl(MySpider())
spider = MySpider()
crawler.queue.append_spider(spider)

# start engine scrapy/twisted
crawler.start()

Answer 1

Close to Joël's answer, but I want to elaborate a bit more than is possible in the comments. If you look at the Crawler source code, you see that the CrawlerProcess class has a start, but also a stop function. This stop function takes care of cleaning up the internals of the crawling so that the system ends up in a state from which it can start again.

So, if you want to restart the crawling without leaving your process, call crawler.stop() at the appropriate time. Later on, simply call crawler.start() again to resume operations.

Edit: in retrospect, this is not possible (due to the Twisted reactor, as mentioned in a different answer); the stop just takes care of a clean termination. Looking back at my code, I happened to have a wrapper for the Crawler processes. Below you can find some (redacted) code to make it work using Python's multiprocessing module. In this way you can more easily restart crawlers. (Note: I found the code online last month, but I didn't include the source... so if someone knows where it came from, I'll update the credits for the source.)

from scrapy import project, signals
from scrapy.conf import settings
from scrapy.crawler import CrawlerProcess
from scrapy.xlib.pydispatch import dispatcher
from multiprocessing.queues import Queue
from multiprocessing import Process

class CrawlerWorker(Process):
    def __init__(self, spider, results):
        Process.__init__(self)
        self.results = results

        self.crawler = CrawlerProcess(settings)
        if not hasattr(project, 'crawler'):
            self.crawler.install()
        self.crawler.configure()

        self.items = []
        self.spider = spider
        dispatcher.connect(self._item_passed, signals.item_passed)

    def _item_passed(self, item):
        self.items.append(item)

    def run(self):
        self.crawler.crawl(self.spider)
        self.crawler.start()
        self.crawler.stop()
        self.results.put(self.items)

# The part below can be called as often as you want
results = Queue()
crawler = CrawlerWorker(MySpider(myArgs), results)
crawler.start()
for item in results.get():
    pass # Do something with item

Answer 2

crawler.start() starts Twisted reactor. There can be only one reactor.

If you want to run more spiders - use

another_spider = MyAnotherSpider()
crawler.queue.append_spider(another_spider)