How to build own middleware in Scrapy?

I'm just starting to learn Scrapy and I have such a question. for my "spider" I have to take a list of urls (start_urls) from the google sheets table and I have this code:

import gspread
from oauth2client.service_account import ServiceAccountCredentials

scope = ['https://spreadsheets.google.com/feeds','https://www.googleapis.com/auth/drive']
creds = ServiceAccountCredentials.from_json_keyfile_name('token.json', scope)


client = gspread.authorize(creds)
sheet = client.open('Sheet_1')
sheet_instance = sheet.get_worksheet(0)
records_data = sheet_instance.col_values(col=2)

for link in records_data:
    print(link)
    ........

How do I configure the middleware so that when the spider (scrappy crawl my_spider) is launched, links from this code are automatically substituted into start_urls? perhaps i need to create a class in middlewares.py? I will be grateful for any help, with examples. it is necessary that this rule applies to all new spiders, generating a list from a file in start_requests (for example start_urls = [l.strip() for an open string('urls.txt ').readline()]) is not convenient...

Solution

Read this

spider.py:

import scrapy


class ExampleSpider(scrapy.Spider):
    name = 'example'

    custom_settings = {
        'SPIDER_MIDDLEWARES': {
            'tempbuffer.middlewares.ExampleMiddleware': 543,
        }
    }
    
    def parse(self, response):
        print(response.url)

middlewares.py:

class ExampleMiddleware(object):
    def process_start_requests(self, start_requests, spider):
        # change this to your needs:
        with open('urls.txt', 'r') as f:
            for url in f:
                yield scrapy.Request(url=url)

urls.txt:

https://example.com
https://example1.com
https://example2.org

output:

[scrapy.core.engine] DEBUG: Crawled (200) <GET https://example2.org> (referer: None)
[scrapy.core.engine] DEBUG: Crawled (200) <GET https://example.com> (referer: None)
[scrapy.core.engine] DEBUG: Crawled (200) <GET https://example1.com> (referer: None)
https://example2.org
https://example.com
https://example1.com