[FIXED] Within a Scrapy bot, I cannot increment a global variable (but can assign the same variable). Why?

Issue

I know that using global variables is not a good idea and I plan to do something different. But, while playing around I ran into a strange global variable issue within Scrapy. In pure python, I don't see this problem.

When I run this bot code:

import scrapy

from tutorial.items import DmozItem

class DmozSpider(scrapy.Spider):
    name = "dmoz"
    allowed_domains = ["lib-web.org"]
    start_urls = [
        "http://www.lib-web.org/united-states/public-libraries/michigan/"
    ]

    count = 0

    def parse(self, response):
        for sel in response.xpath('//div/div/div/ul/li'):
            item = DmozItem()
            item['title'] = sel.xpath('a/text()').extract()
            item['link'] = sel.xpath('a/@href').extract()
            item['desc'] = sel.xpath('p/text()').extract()

            global count;
            count += 1
            print count

            yield item

DmozItem:

import scrapy

class DmozItem(scrapy.Item):
    title = scrapy.Field()
    link = scrapy.Field()
    desc = scrapy.Field()

I get this error:

  File "/Users/Admin/scpy_projs/tutorial/tutorial/spiders/dmoz_spider.py", line 22, in parse
    count += 1
NameError: global name 'count' is not defined

But if I simply change 'count += 1' to just 'count = 1', it runs fine.

What's going on here? Why can I not increment the variable?

Again, if I run similar code outside of a Scrapy context, in pure Python, it runs fine. Here's the code:

count = 0

def doIt():
        global count
        for i in range(0, 10):
            count +=1
doIt()
doIt()
print count

Resulting in:

Admin$ python count_test.py
20

Solution

count is a class variable in your example, so you should access it using self.count. It solves the error, but maybe what you really need is an instance variable, because as a class variable, count is shared between all the instances of the class.

Assigning count = 1 inside the parse method works because it creates a new local variable called count, which is different from the class variable count.

Your pure Python example works because you did not define a class, but a function instead, and the variable count you created there has global scope, which is accessible from the function scope.

Answered By - Valdir Stumm Junior