code: def get_async(filename):
      
  
      if os.path.isfile(os.path.join(config.static_folder, filename)):
          return flask.redirect(flask.url_for('static', filename=filename))
  
      retry_count = int(flask.request.args.get('retry_count', 0))
      if retry_count < 10:
          time.sleep(0.25)  # ghastly hack to get the client to backoff a bit
          return flask.redirect(flask.url_for('async',
                                              filename=filename,
                                              cb=random.randint(0, 2**48),
                                              retry_count=retry_count + 1))
  
      # the image isn't available yet; generate a placeholder and let the
      # client attempt to re-fetch periodically, maybe
      vals = [int(b) for b in hashlib.md5(
          filename.encode('utf-8')).digest()[0:12]]
      placeholder = PIL.Image.new('RGB', (2, 2))
      placeholder.putdata(list(zip(vals[0::3], vals[1::3], vals[2::3])))
      outbytes = io.BytesIO()
      placeholder.save(outbytes, "PNG")
      outbytes.seek(0)
  
      response = flask.make_response(
          flask.send_file(outbytes, mimetype='image/png'))
      response.headers['Refresh'] = 5
      return response
docs: Asynchronously fetch an image