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Properly Unit Testing Scrapy Spiders

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Properly Unit Testing Scrapy Spiders

· DevOps Zone
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Scrapy, being based on Twisted, introduces an incredible host of obstacles to easily and efficiently writing self-contained unit tests:

1. You can't call reactor.run() multiple times
2. You can't stop the reactor multiple times, so you can't blindly call "crawler.signals.connect(reactor.stop, signal=signals.spider_closed)"
3. Reactor runs in its own thread, so your failed assertions won't make it to the main unittest thread, so test failures will be thrown as assertion errors but unittest doesn't know about them

To get around these hurdles, I created a BaseScrapyTestCase class that uses tl.testing's ThreadAwareTestCase and the following workarounds.

class BaseScrapyTestCase(ThreadAwareTestCase):
    in_suite = False
    def setUp(self):
        self.last_crawler = None
        self.settings = get_project_settings()

    def run_reactor(self, called_from_suite=False):
        if not called_from_suite and BaseScrapyTestCase.in_suite:
            return
        log.start()
        self.last_crawler.signals.connect(reactor.stop, signal=signals.spider_closed)
        reactor.run()

    def queue_spider(self, spider, callback):
        crawler = Crawler(self.settings)
        self.last_crawler = crawler
        crawler.signals.connect(callback, signal=signals.spider_closed)
        crawler.configure()
        crawler.crawl(spider)
        crawler.start()
        return crawler

    def wrap_asserts(self, fn):
        with ThreadJoiner(1):
            self.run_in_thread(fn)

You'll use it like so:

class SimpleScrapyTestCase(BaseScrapyTestCase):
    def test_suite(self):
        BaseScrapyTestCase.in_suite = True
        self.do_test_simple()
        self.run_reactor(True)
    def do_test_simple(self):
        spider = Spider("site.com")
        def _fn():
            def __fn():
                self.assertTrue(False)
            self.wrap_asserts(__fn)
        self.queue_spider(spider, _fn)
        self.run_reactor()
 

1. Call run_reactor() at the end of test method.
2. You have to place your assertions in its own function which gets called in a ThreadJoiner so that unittest knows about assertion failures.
3. If you're testing multiple spiders, just call queue_spider() for each, and run_reactor() at the end.
4. BaseScrapyTestCase keeps track of the crawlers created, and makes sure to only attach a reactor.stop signal to the last one.

Let me know if you come up with a better/more elegant way of testing scrapy spiders!


Find out more about how Scalyr built a proprietary database that does not use text indexing for their log management tool, allowing customers to search 1TB of data in under a second. 

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Published at DZone with permission of Kelvin Tan. See the original article here.

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