def __init__(self, tag=None):
        super(myclass, self).__init__()

        if tag:
            self.logger.info('Scraping only {}.'.format(tag))
        else:
            self.logger.info('Scraping all tags.')

import scrapy
from scrapy.spiders import CrawlSpider, Rule
from scrapy.linkextractors import LinkExtractor

class DemoSpider(CrawlSpider):
   name = "demo"
   allowed_domains = ["www.demoexample.com"]
   start_urls = ["http://www.demoexample.com"]
      
   rules = ( 
      Rule(LinkExtractor(allow =(), restrict_xpaths = ("//div[@class = 'next']",)),
         callback = "parse_item", follow = True),
   )
   
   def parse_item(self, response):
   item = DemoItem()
   item["product_title"] = response.xpath("a/text()").extract()
   item["product_link"] = response.xpath("a/@href").extract()
   item["product_description"] = response.xpath("div[@class = 'desc']/text()").extract()
   return items

class MySpider:
    name = 'racers'
    allowed_domains = ['racing.hkjc.com']
    start_urls = []
    def __init__(self, subject, *args, **kwargs):
        super(MySpider, self).__init__(*args, **kwargs)
        self.start_urls = ['http://google.com/%s' % (subject)]
        self.subject = subject
        
        
        
obj = MySpider(subject='gogogo')
obj.subject
'gogogo'
obj.start_urls
['http://google.com/gogogo']