按照文档中对于 priority的解释，这个属性是可以控制scheduler对于请求发起的顺序的，但实际测试下来并没有效果，请老师帮忙解答：

爬虫代码如下：

class TestSpider(scrapy.Spider):
    name = 'test'

    custom_settings = {
        'DOWNLOAD_DELAY': 5,
        'CONCURRENT_REQUESTS': 1
    }

    def start_requests(self):
        urls = {
            10: 'https://www.baidu.com/s?wd=1',
            20: 'https://www.baidu.com/s?wd=2',
            30: 'https://www.baidu.com/s?wd=3'
        }

        for index, url in urls.items():
            yield scrapy.Request(url,
                                 headers={
                                     'User-Agent': 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/103.0.0.0 Safari/537.36'
                                 },
                                 callback=self.parse,
                                 priority=index,
                                 meta={'priority': index},
                                 dont_filter=True)

    def parse(self, response, **kwargs):
        self.log(datetime.datetime.now().strftime('%H:%M:%S'))
        self.log(response.request.url)
        title = response.xpath('//title/text()').get()
        self.log(title)

执行日志：

2022-07-26 16:05:39 [scrapy.core.engine] DEBUG: Crawled (200) <GET https://www.baidu.com/s?wd=1> (referer: None)
2022-07-26 16:05:39 [test] DEBUG: 16:05:39
2022-07-26 16:05:39 [test] DEBUG: https://www.baidu.com/s?wd=1
2022-07-26 16:05:39 [test] DEBUG: 1_百度搜索
2022-07-26 16:05:45 [scrapy.core.engine] DEBUG: Crawled (200) <GET https://www.baidu.com/s?wd=2> (referer: None)
2022-07-26 16:05:45 [test] DEBUG: 16:05:45
2022-07-26 16:05:45 [test] DEBUG: https://www.baidu.com/s?wd=2
2022-07-26 16:05:45 [test] DEBUG: 2_百度搜索
2022-07-26 16:05:52 [scrapy.core.engine] DEBUG: Crawled (200) <GET https://www.baidu.com/s?wd=3> (referer: None)
2022-07-26 16:05:52 [test] DEBUG: 16:05:52
2022-07-26 16:05:52 [test] DEBUG: https://www.baidu.com/s?wd=3
2022-07-26 16:05:52 [test] DEBUG: 3_百度搜索
2022-07-26 16:05:52 [scrapy.core.engine] INFO: Closing spider (finished)

官方文档中解释：
priority (int) – the priority of this request (defaults to 0). The priority is used by the scheduler to define the order used to process requests. Requests with a higher priority value will execute earlier. Negative values are allowed in order to indicate relatively low-priority.

priority值越高，越优先执行，但是代码中 30 优先级的还是最后执行，测试了很多遍都是这样。