Scrapy使用自定义继承ImagesPipeline的Pipeline类后爬虫无法下载图片

使用Python 3.7环境和Scrapy爬虫框架爬取网页上的图片并下载，使用内置的ImagesPipeline可以正常下载，但是使用自定义的Pipeline类后只在命令行输出图片链接地址，而无法下载图片到本地

相关代码

items.py

class ImageItem(scrapy.Item):
    #文件名
    #image_names = scrapy.Field()
    #文件夹名
    #fold = scrapy.Field()
    #图片路径
    #image_paths = scrapy.Field()
    #图片链接地址
    image_urls = scrapy.Field()

pipelines.py

class PicPipeline(ImagesPipeline):
    def process_item(self, item, spider):
        return item

    def get_media_requests(self, item, info):
        for image_url in item['image_urls']:
            yield scrapy.Request(image_url)

    def item_completed(self, results, item, info):
        image_paths = [x['path'] for ok, x in results if ok]
        if not image_paths:
            raise DropItem("Item contains no images")
        item['image_paths'] = image_paths
        return item

settings.py

ITEM_PIPELINES = {
    'Pic.pipelines.PicPipeline': 300
}
IMAGE_STORES= 'D:\Pic'
IMAGES_URLS_FIELD = 'image_urls'

PicSpider.py

class PicSpider(scrapy.Spider):
    name = 'picspider'
    allowed_domains = ['guokr.com']
    start_urls = ['https://www.guokr.com/']

    def parse(self, response):
        images = response.xpath('//img/@src').extract()
        item = ImageItem()
        item['image_urls'] = images
        yield item

就只是想写个小Demo练习一下爬取图片，使用内置的ImagesPipeline能够正常下载图片，但是自定义的Pipeline就无法下载，在命令行输出图片链接地址后就没有了，请各位指教。

命令行输出如下

2019-02-19 12:11:06 [scrapy.core.engine] INFO: Spider opened
2019-02-19 12:11:06 [scrapy.extensions.logstats] INFO: Crawled 0 pages (at 0 pages/min), scraped 0 items (at 0 items/min)
2019-02-19 12:11:06 [scrapy.extensions.telnet] INFO: Telnet console listening on 127.0.0.1:6023
2019-02-19 12:11:06 [scrapy.core.engine] DEBUG: Crawled (200) <GET https://www.guokr.com/robots.txt>; (referer: None)
2019-02-19 12:11:06 [scrapy.core.engine] DEBUG: Crawled (200) <GET https://www.guokr.com/>; (referer: None)
2019-02-19 12:11:06 [scrapy.core.scraper] DEBUG: Scraped from <200 https://www.guokr.com/>;
{'image_urls': ['https://3-im.guokr.com/vXVhDq_6nindVo2LqIloosK-2bHrkYpU8DEXP75DpnZKAQAA6wAAAEpQ.jpg',
                'https://2-im.guokr.com/hD7RoVC8IpQGnc2humofXMGyex-iSZH1VDaWLq2VWCE2BAAA7gMAAEpQ.jpg?imageView2/1/w/330/h/235',
                'https://2-im.guokr.com/AU-Q8pTYY_OffTqWyKfXTC5NV0RmarK_QJ9m6A6_7qhKAQAA6wAAAEpQ.jpg',
                'https://1-im.guokr.com/IIlEodManGB8jos3eP7KcrMhu3l8dtG6F5nrJczcrTiAAwAAUwIAAEpQ.jpg?imageView2/1/w/330/h/235',
                'https://1-im.guokr.com/klfXUFzwXV_jz42yk497oZ-RkLAJEc03spAKMg9AeIw4BAAADQMAAEpQ.jpg?imageView2/1/w/330/h/235',
                'https://1-im.guokr.com/BZ7R7bpcrwjOyFJ5kajc0tVHlOF8BUyEs3IpWB0l6Q4sAgAA2AEAAEpQ.jpg?imageView2/1/w/135/h/90',
                'https://1-im.guokr.com/1CJgQkib1ePSCpLBARUhOyMdf6THL2BGrkDj6WDc5eiGAQAABAEAAEpQ.jpg?imageView2/1/w/135/h/90',
                'https://1-im.guokr.com/4prMeIXxsaF2y6OTfpCB2IiI7udvwK8f_lsTcqbFcaeHAAAAWgAAAEpQ.jpg',
                'https://1-im.guokr.com/WPrAHjwbKwXNYqiYZgkaYEyh9i2R8zm9noog_AxfpHiaAgAAvAEAAEpQ.jpg?imageView2/1/w/135/h/90',
                'https://2-im.guokr.com/TNpsKxaaNGuIDTJWTpy2P5wfji_oG66rHUWGa8L7zFhKAQAAtQAAAFBO.png?imageView2/1/w/135/h/90',
                'https://2-im.guokr.com/gLbC7ix6NWlx3bz6ihFyOxsl_fWqwtB554NswEOmACFKAQAA8AAAAEpQ.jpg?imageView2/1/w/135/h/90',
                'https://2-im.guokr.com/Rx9MyfI6hndQBTyoGWvfOyb469BZ7ruf0w0k7V0aJ1pKAQAA6wAAAEpQ.jpg?imageView2/1/w/135/h/90',
                'https://2-im.guokr.com/-OmYOzUa0Nhm9vKimCFn2c2ZR9pHmgxqMiMxijD5KwkLAQAAngAAAFBO.png?imageView2/1/w/135/h/90',
                'https://1-im.guokr.com/mysULQspmaLPEMu-MQFZGHwaccTPPs9msjtLrYoDtGcsAQAAagAAAEpQ.jpg',
                'https://2-im.guokr.com/fSvqlLJ6wcRv8cCCc5Ehm5pgqZWg7TyiLZdEba34NTKgAAAAoAAAAEpQ.jpg?imageView2/1/w/48/h/48',
                'https://3-im.guokr.com/F9IifzSeB9OoKKIP-_2i3SnWHnUceIpmGyOMuwgRvgGgAAAAoAAAAEpQ.jpg?imageView2/1/w/48/h/48',
                'https://sslstatic.guokr.com/skin/imgs/dimensions-code.jpg?v=unknown',
                'https://3-im.guokr.com/0Al5wQUv5IAuo87evbERy190Y83ENmP9OpIs8Stm2lMUAAAAFAAAAFBO.png']}
2019-02-19 12:11:06 [scrapy.core.engine] INFO: Closing spider (finished)
2019-02-19 12:11:06 [scrapy.statscollectors] INFO: Dumping Scrapy stats:
{'downloader/request_bytes': 434,
 'downloader/request_count': 2,
 'downloader/request_method_count/GET': 2,
 'downloader/response_bytes': 12316,
 'downloader/response_count': 2,
 'downloader/response_status_count/200': 2,
 'finish_reason': 'finished',
 'finish_time': datetime.datetime(2019, 2, 19, 4, 11, 6, 755334),
 'item_scraped_count': 1,
 'log_count/DEBUG': 3,
 'log_count/INFO': 9,
 'response_received_count': 2,
 'robotstxt/request_count': 1,
 'robotstxt/response_count': 1,
 'robotstxt/response_status_count/200': 1,
 'scheduler/dequeued': 1,
 'scheduler/dequeued/memory': 1,
 'scheduler/enqueued': 1,
 'scheduler/enqueued/memory': 1,
 'start_time': datetime.datetime(2019, 2, 19, 4, 11, 6, 142378)}
2019-02-19 12:11:06 [scrapy.core.engine] INFO: Spider closed (finished)