fix bug to avoid warning once import paddle.fluid

Author: JiabinYang

python/paddle/dataset/image.py

@@ -36,11 +36,6 @@
 try:
     import cv2
 except ImportError:
-    import sys
-    sys.stderr.write(
-        '''Warning with paddle image module: opencv-python should be imported,
-    or paddle image module could NOT work; please install opencv-python first.'''
-    )
     cv2 = None
 import os
 import tarfile
@@ -53,6 +48,18 @@
 ]
 
 
+def _check_cv2():
+    if cv2 is None:
+        import sys
+        sys.stderr.write(
+            '''Warning with paddle image module: opencv-python should be imported,
+         or paddle image module could NOT work; please install opencv-python first.'''
+        )
+        return False
+    else:
+        return True
+
+
 def batch_images_from_tar(data_file,
                           dataset_name,
                           img2label,
@@ -134,7 +141,7 @@ def load_image_bytes(bytes, is_color=True):
                      load and return a gray image.
     :type is_color: bool
     """
-    assert cv2 is not None
+    assert _check_cv2() is True
 
     flag = 1 if is_color else 0
     file_bytes = np.asarray(bytearray(bytes), dtype=np.uint8)
@@ -159,7 +166,7 @@ def load_image(file, is_color=True):
                      load and return a gray image.
     :type is_color: bool
     """
-    assert cv2 is not None
+    assert _check_cv2() is True
 
     # cv2.IMAGE_COLOR for OpenCV3
     # cv2.CV_LOAD_IMAGE_COLOR for older OpenCV Version
@@ -188,7 +195,7 @@ def resize_short(im, size):
     :param size: the shorter edge size of image after resizing.
     :type size: int
     """
-    assert cv2 is not None
+    assert _check_cv2() is True
 
     h, w = im.shape[:2]
     h_new, w_new = size, size