Python：Scrapy中重写ImagePipeline组件的file_path函数，但没被调用

环境

Python：2.7.6(64位)
Scrapy：0.22.2(64位)
操作系统：Windows7(64位)

问题需求

默认情况下，使用ImagePipeline组件下载图片的时候，图片名称是以图片URL的SHA1值进行保存的。
如：
图片URL:http://www.example.com/image.jpg
SHA1结果：3afec3b4765f8f0a07b78f98c07b83f013567a0a
则图片名称：3afec3b4765f8f0a07b78f98c07b83f013567a0a.jpg
但是，我想要以原来的图片名称进行保存，比如上面例子中的图片保存到本地的话，图片名称就应该是：image.jpg
stackoverflow上说是可以重写image_key函数，不过我试了下，结果发现不行，重写的image_key函数没被调用。后面查看了下ImagePipeline的源码：

class ImagesPipeline(FilesPipeline):
    """Abstract pipeline that implement the image thumbnail generation logic

    """

    MEDIA_NAME = 'image'
    MIN_WIDTH = 0
    MIN_HEIGHT = 0
    THUMBS = {}
    DEFAULT_IMAGES_URLS_FIELD = 'image_urls'
    DEFAULT_IMAGES_RESULT_FIELD = 'images'

...省略

def file_path(self, request, response=None, info=None):
        ## start of deprecation warning block (can be removed in the future)
        def _warn():
            from scrapy.exceptions import ScrapyDeprecationWarning
            import warnings
            warnings.warn('ImagesPipeline.image_key(url) and file_key(url) methods are deprecated, '
                          'please use file_path(request, response=None, info=None) instead',
                          category=ScrapyDeprecationWarning, stacklevel=1)

        # check if called from image_key or file_key with url as first argument
        if not isinstance(request, Request):
            _warn()
            url = request
        else:
            url = request.url

        # detect if file_key() or image_key() methods have been overridden
        if not hasattr(self.file_key, '_base'):
            _warn()
            return self.file_key(url)
        elif not hasattr(self.image_key, '_base'):
            _warn()
            return self.image_key(url)
        ## end of deprecation warning block

        image_guid = hashlib.sha1(url).hexdigest()  # change to request.url after deprecation
        return 'full/%s.jpg' % (image_guid)
    # deprecated
    def image_key(self, url):
        return self.file_path(url)
    image_key._base = True

...省略

其中，有这么一句话：
ImagesPipeline.image_key(url) and file_key(url) methods are deprecated, please use file_path(request, response=None, info=None) instead
也就是说，在最新版本的Scrapy中（0.22.2），使用file_path代替image_key函数。
因此，我在自定义的ImagePipeline类中，重写了file_path函数，但是结果运行的时候，发现也没法被调用。
代码如下：

from scrapy.contrib.pipeline.images import ImagesPipeline
from scrapy.exceptions import DropItem
from scrapy.http import Request
import os
class DownPhotosPipeline(ImagesPipeline):

    def file_path(self, request):
        print "~~~~~~~~~~~~~~~~~~~~~~"
        print "~~~~~~~"+request.url+"~~~~~~~"
        print "~~~~~~~~~~~~~~~~~~~~~~"
        image_guid = request.url.split('/')[-1]
        return 'full/%s' % (image_guid)

    def get_media_requests(self, item, info):
        for image_url in item['images']:
            yield 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

DOWNLOAD_DELAY = 2
IMAGES_STORE = 'budejie_photos'
DOWNLOAD_TIMEOUT = 1200
ITEM_PIPELINES = ['scrapy.contrib.pipeline.images.ImagesPipeline'
]