When you need to do some web scraping job in Python, an excellent choice is the Scrapy framework. Not only it takes care of most of the networking (HTTP, SSL, proxies, etc) but it also facilitates the process of extracting data from the web by providing things such as nifty xpath selectors.

Scrapy is built upon the Twisted networking engine. A limitation of its core component, the reactor, is that it cannot be restarted. This might cause us some troubles if we are trying to devise a mechanism to run Scrapy spiders independently from a Python script (and not from Scrapy shell). Say for example we want to implement a Python function that receives some parameters, performs a search/web scraping in some sites and returns a list of scrapped items. A naive solution such as this will not work, since in each of the function calls we need to have the Twisted reactor restarted, and this is unfortunately not possible.

A workaround for this is to run Scrapy on its own process. After doing a search, I could get no solution to work on latest Scrapy. However one of those used Multiprocessing and it came pretty close! Here is an updated version for Scrapy 0.13:

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
import multiprocessing

class CrawlerWorker(multiprocessing.Process):

    def __init__(self, spider, result_queue):
        multiprocessing.Process.__init__(self)
        self.result_queue = result_queue

        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.result_queue.put(self.items)

One way to invoke this, say inside a function, would be:

        result_queue = Queue()
        crawler = CrawlerWorker(MySpider(myArgs), result_queue)
        crawler.start()
        for item in result_queue.get():
            yield item

where MySpider is of course the class of the Spider you want to run, and myArgs are the arguments you wish to invoke the spider with.


Comments

  1. tre
    tre on 07/24/2012 1:07 a.m.
    Hey Alan, thank you for sharing this! (and for fixing the comment system)
  2. payala
    payala on 08/19/2012 6:11 p.m.
    I have tried this under windows but I never managed to make it work. I think the problem has to do with the limitations imposed by the multiprocessing module on windows platforms. I think this might be related: http://docs.python.org/library/multiprocessing.html#windows http://stackoverflow.com/questions/765129/hows-python-multiprocessing-implemented-on-windows
  3. akersof
    akersof on 09/15/2012 1:50 a.m.
    You do not set any environement variable? Just new in scrapy and still get an error . "crawler = CrawlerWorker(MySpider('url=http://www.example.com'), result_queue)" What should be MySpider? the class name? the project name? the name of of the crawler (name="myspider" in the class)? Regards,
  4. Serg
    Serg on 11/03/2012 1:55 p.m.
    It works only for one process running... When I run this code for two or more processes concurrently ... for spider in spiders: crawler = CrawlerWorker(spider(myArgs), result_queue) crawler.start() ... I have got errors with Twisted Unhandled Error Traceback (most recent call last): File "/usr/lib64/python2.7/site-packages/twisted/python/log.py", line 84, in callWithLogger return callWithContext({"system": lp}, func, *args, **kw) File "/usr/lib64/python2.7/site-packages/twisted/python/log.py", line 69, in callWithContext return context.call({ILogContext: newCtx}, func, *args, **kw) File "/usr/lib64/python2.7/site-packages/twisted/python/context.py", line 118, in callWithContext return self.currentContext().callWithContext(ctx, func, *args, **kw) File "/usr/lib64/python2.7/site-packages/twisted/python/context.py", line 81, in callWithContext return func(*args,**kw) --- <exception caught here> --- File "/usr/lib64/python2.7/site-packages/twisted/internet/posixbase.py", line 631, in _doReadOrWrite why = selectable.doWrite() File "/usr/lib64/python2.7/site-packages/twisted/internet/base.py", line 1094, in doWrite raise RuntimeError, "doWrite called on a %s" % reflect.qual(self.__class__) exceptions.RuntimeError: doWrite called on a twisted.internet.tcp.Port
  5. Serg
    Serg on 11/08/2012 7:13 a.m.
    Errors in Twisted in example above was eliminated by setting WEBSERVICE_ENABLED and TELNETCONSOLE_ENABLED to FALSE. So I can run any count of processes with own spider in process without errors
  6. sam
    sam on 03/17/2013 1:27 p.m.
    Does this technique work with scrapy 0,16?
  7. Alan Descoins
    Alan Descoins on 03/26/2013 10:12 p.m.
    I haven't used version 0.16, but I am almost sure the code will probably need some changes.
  8. Rajesh Lakshmanan
    Rajesh Lakshmanan on 10/03/2013 3:55 a.m.
    Hi Alan, I am learning scrapy and python basically I am a java developer, I am using Eclipse PyDev IDE for this development so i need to install scrapy in my eclipse, please help me out how to achieve it.
  9. Emiliano
    Emiliano on 10/20/2014 6:35 a.m.
    Hey Alan, thanks for the example! I'm building a spider for my site Spoots for crawl all pages and get social stats, but in Mac is not easy to install :'( On my server all goes well for luck. Thanks! Emiliano

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