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when I set start_urls inside a Scrapy spider class, the fllowing code is OK:

class InfoSpider(scrapy.Spider):
    name = 'info'
    allowed_domains = ['isbn.szmesoft.com']
    isbns = list(set(pd.read_csv('E:/books.csv')['ISBN']))
    url = 'http://isbn.szmesoft.com/isbn/query?isbn='
    start_urls = [url + isbns[0]]
But then I got the error  Scrapy: NameError: name 'url' is not defined when I rewrite my code as follows:
class InfoSpider(scrapy.Spider):
    name = 'info'
    allowed_domains = ['isbn.szmesoft.com']
    isbns = list(set(pd.read_csv('E:/books.csv')['ISBN']))
    url = 'http://isbn.szmesoft.com/isbn/query?isbn='
    start_urls = [url + isbn for isbn in isbns[:3]]
Maybe I can solve this problem in other ways，but I want to know the reason for the ERROR
There are only four ranges in Python: LEGB, because the local scope of the class definition and the local extent of the list derivation are not nested functions, so they do not form the Enclosing scope.

Therefore, they are two separate local scopes that cannot be accessed from each other.
You need to pass string of it and try printing url so that you can also go and check it on browser if ut actually exists or not.
start_urls = [url + str(isbn) for isbn in isbns[:3]]
print(start_urls)
        self.name = 'info'
        self.allowed_domains = ['isbn.szmesoft.com']
        self.isbns = list(set(pd.read_csv('E:/books.csv')['ISBN']))
        self.url = 'http://isbn.szmesoft.com/isbn/query?isbn='
        self.start_urls = [url + isbn for isbn in isbns[:3]]
Then when you call it do self. before it
                the first code 'url' is defined but the second is not, the only difference is the last line
– MJ_0826
                Aug 10, 2018 at 4:35
        
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