diff --git a/02.recognize_digits/client/client.py b/02.recognize_digits/client/client.py
index 45b338d6de402100e0ea20bd4361c70dc1bdb80a..bda7cec91677c9fdc9dd2846a02acb7bee1d0b01 100644
--- a/02.recognize_digits/client/client.py
+++ b/02.recognize_digits/client/client.py
@@ -3,6 +3,9 @@ from PIL import Image
 import numpy as np
 import os
 
+# this client is used by Paddle serve: https://github.com/PaddlePaddle/book/tree/develop/serve
+# please do not use it directly
+
 
 def load_image(file):
     im = Image.open(file).convert('L')