Python 类实例中的空变量，尽管专门设置了它

当我运行以下代码时：

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

我得到以下错误：

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

---

在函数start\u requests中，self.search\u url似乎是一个空变量，尽管我在调用函数之前已显式地将其值设置为某个值。我似乎不明白为什么会这样。

最简单的方法是使用构造函数

\uuu init\uuu（）

，但更简单的方法（可能只是更快）是在类中移动

start\u url

的定义。例如：

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()

---

是否有特定的原因要将

search\u url

声明为实例属性？你可以把它传给全班吗？