When using python script to run scrapy crawler, data is scraped successfully but the output file shows no data in it and is of 0 kb

#Scrapy News Crawler

#Importing Scrapy library
import scrapy


#Defining spider's url,headers
class DawnSpider(scrapy.Spider):
    name = 'dawn'
    allowed_domains = ['www.dawn.com']    #Channel link
    # start_urls = ['https://www.dawn.com/archive/2022-02-09']    
    # url = ['https://www.dawn.com']
    # page = 1

#defining function to set headers and setting Link from where to start scraping

    def start_requests(self):
        yield scrapy.Request(url='https://www.dawn.com/archive/2022-03-21', callback=self.parse, headers={'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64; rv:48.0) Gecko/20100101 Firefox/48.0'})

#Getting news healines and their links
    def parse(self, response):
        titles = response.xpath("//h2[@class = 'story__title      text-6  font-bold  font-merriweather      pt-1  pb-2  ']/a")    

        for title in titles:
            headline = title.xpath(".//text()").get()
            headline_link = title.xpath(".//@href").get()
#itrating News headline links

            yield response.follow(url=headline_link,  callback=self.parse_headline, meta={'heading': headline}, headers={'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64; rv:48.0) Gecko/20100101 Firefox/48.0'})

#COde for going to previous pages
            prev_page = response.xpath("//li[1]/a/@href").get()
            prev = 'https://www.dawn.com' + str(prev_page)

            yield scrapy.Request(url=prev, callback=self.parse, headers={'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64; rv:48.0) Gecko/20100101 Firefox/48.0'})
            

#Iterating headline links and getting healine details and date/time

    def parse_headline(self, response):
        headline = response.request.meta['heading']
        # logging.info(response.url)
        full_detail = response.xpath("//div[contains(@class , story__content)]/p[1]")
        date_and_time = response.xpath("//span[@class='timestamp--date']/text()").get()
        for detail in full_detail:
            data = detail.xpath(".//text()").get()
            yield {
                'headline': headline,
                'date_and_time': date_and_time,
                'details': data
            }

#Python script (Separate FIle )

from scrapy import cmdline

cmdline.execute("scrapy crawl dawn -o data.csv".split(" "))

Answer

1. Instead of running you spider with cmdline.execute you can run it with CrawlerProcess, read about common practices. You can see main.py as an example.
2. You can declare the headers once.
3. You’re getting a lot of 403, so you should add download delay to avoid getting banned.
4. You can use feeds export for the csv file.
5. It’s possible you’re interrupting the writing of the csv file, but it’s only a guess.

Here’s a working example (I checked it with 'CLOSESPIDER_ITEMCOUNT': 10, so give it some time when run run it).

spider.py:

#Importing Scrapy library
import scrapy


#Defining spider's url,headers
class DawnSpider(scrapy.Spider):
    name = 'dawn'
    allowed_domains = ['dawn.com']    #Channel link
    # start_urls = ['https://www.dawn.com/archive/2022-02-09']    
    # url = ['https://www.dawn.com']
    # page = 1

    custom_settings = {
        'DOWNLOAD_DELAY': 0.8,
        'FEEDS': {'data.csv': {'format': 'csv'}},
    }

    headers = {
        "Accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8",
        "Accept-Encoding": "gzip, deflate, br",
        "Accept-Language": "en-US,en;q=0.5",
        "Cache-Control": "no-cache",
        "Connection": "keep-alive",
        "Cookie": "scribe=true",
        "DNT": "1",
        "Host": "www.dawn.com",
        "Pragma": "no-cache",
        "Sec-Fetch-Dest": "document",
        "Sec-Fetch-Mode": "navigate",
        "Sec-Fetch-Site": "none",
        "Sec-Fetch-User": "?1",
        "Sec-GPC": "1",
        "TE": "trailers",
        "Upgrade-Insecure-Requests": "1",
        "User-Agent": "Mozilla/5.0 (X11; Linux x86_64; rv:48.0) Gecko/20100101 Firefox/48.0"
    }

    def start_requests(self):
        yield scrapy.Request(url='https://www.dawn.com/archive/2022-03-21', headers=self.headers)

        #Getting news healines and their links
    def parse(self, response):
        titles = response.xpath("//h2[@class = 'story__title      text-6  font-bold  font-merriweather      pt-1  pb-2  ']/a")

        for title in titles:
            headline = title.xpath(".//text()").get()
            headline_link = title.xpath(".//@href").get()
            #itrating News headline links

            yield response.follow(url=headline_link,  callback=self.parse_headline, cb_kwargs={'headline': headline}, headers=self.headers)

            #COde for going to previous pages
            prev_page = response.xpath("//li[1]/a/@href").get()
            if prev_page:
                prev = 'https://www.dawn.com' + str(prev_page)
                yield scrapy.Request(url=prev, callback=self.parse, headers=self.headers)

    def parse_headline(self, response, headline):
        # logging.info(response.url)
        full_detail = response.xpath("//div[contains(@class , story__content)]/p[1]")
        date_and_time = response.xpath("//span[@class='timestamp--date']/text()").get()
        for detail in full_detail:
            data = detail.xpath(".//text()").get()
            yield {
                'headline': headline,
                'date_and_time': date_and_time,
                'details': data
            }

main.py:

from scrapy.crawler import CrawlerProcess
from scrapy.utils.project import get_project_settings


if __name__ == "__main__":
    settings = get_project_settings()
    process = CrawlerProcess(settings)
    process.crawl('dawn')
    process.start()