scrapy怎么实现自定爬取深度？

Question

我想实现对一个网址的图片进行下载，然后对该网址里面所有a标签的链接页面的图片进行下载，以此类推，这是我写的代码，求教大佬怎么做到在spider里用循环实现自定义深度啊

import scrapy
from ..items import ImgspiderItem

full_img_list = []

class TestSpiderSpider(scrapy.Spider):
    name = 'test_spider'
    url = input("请输入要爬取的网址：")
    start_urls = [url]

    def parse(self, response):
        img_list = response.xpath('//img/@src').extract()
        a_list = response.xpath('//a/@href').extract()

        if img_list:
            item = ImgspiderItem()
            for img in img_list:
                if img is not None:
                    if img[0:4]!='http':
                        img = 'https:'+img
                        full_img_list.append(img)
                    elif img[0:5]!='https':
                        img = 'https:'+img.split(':',1)[1]
                        full_img_list.append(img)
                    else:
                        full_img_list.append(img)
            item['image_urls'] = full_img_list
            yield item
        for a in a_list:
            if a is not None:
                if a[0:4]!='http':
                    a = 'https:'+a
                elif a[0:5]!='https':
                    a = 'https:'+a.split(':',1)[1]
            yield scrapy.Request(
                a,
                callback=self.parse
            )