code: def get_thumb(self, path, key, format):
          
          thumbpath = self.get_thumbpath(path, key, format)
          fullpath = os.path.join(self.out_path, thumbpath)
          if os.path.isfile(fullpath):
              url = self.get_url(thumbpath)
              return Thumb(url, key)
          return Thumb()
docs: Get the stored thumbnail if exists.
  
          path:
              path of the source image
          key:
              key of the thumbnail
          format:
              thumbnail's file extension