Empty variable within instance of a class, despite specifically setting it

问题

When I run the following code:

import scrapy
from scrapy.crawler import CrawlerProcess

class QuotesSpider(scrapy.Spider):
    name = "quotes"
    search_url = ''

    def start_requests(self):
        print ('self.search_url is currently: ' + self.search_url)
        yield scrapy.Request(url=self.search_url, callback=self.parse)

    def parse(self, response):
        page = response.url.split("/")[-2]
        filename = 'quotes-%s.html' % page
        with open(filename, 'wb') as f:
            f.write(response.body)
        self.log('Saved file %s' % filename)

process = CrawlerProcess({
    'USER_AGENT': 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1)'
})

test_spider = QuotesSpider()
test_spider.search_url='http://quotes.toscrape.com/page/1/'

process.crawl(test_spider)
process.start() # the script will block here until the crawling is finished

I get the following error:

self.search_url is currently:
...
   ValueError('Missing scheme in request url: %s' % self._url)
ValueError: Missing scheme in request url:
...

It seems that within the function start_requests, self.search_url seems to be an empty variable, even though I have explicitly set its value to something before calling the function. I cannot seem to figure out why that is.

回答1:

The neatest way to do this, would be to use the constructor __init__(), but an easier(maybe just faster for what you want) is to move the definition of start_url inside the class. For example:

import scrapy
from scrapy.crawler import CrawlerProcess

class QuotesSpider(scrapy.Spider):

    name = "quotes"
    search_url = 'http://quotes.toscrape.com/page/1/'

    def start_requests(self):
        print ('search_url is currently: ' + self.search_url)
        yield scrapy.Request(url=self.search_url, callback=self.parse)

    def parse(self, response):
        page = response.url.split("/")[-2]
        filename = 'quotes-%s.html' % page
        with open(filename, 'wb') as f:
            f.write(response.body)
        self.log('Saved file %s' % filename)

process = CrawlerProcess({
    'USER_AGENT': 'Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1)'
})

test_spider = QuotesSpider()

process.crawl(test_spider)
process.start()

来源：https://stackoverflow.com/questions/56526926/empty-variable-within-instance-of-a-class-despite-specifically-setting-it

标签

python

python-2.7

scrapy