diff --git a/tests/qemu-iotests/iotests.py b/tests/qemu-iotests/iotests.py
index 82dd096c6e2affc89e3a97c0414d5d7ca69d22bc..52fc77563c73eb4e2cf15685321cb3d38d6b36e1 100644
--- a/tests/qemu-iotests/iotests.py
+++ b/tests/qemu-iotests/iotests.py
@@ -236,6 +236,12 @@ def image_size(img):
     r = qemu_img_pipe('info', '--output=json', '-f', imgfmt, img)
     return json.loads(r)['virtual-size']
 
+def is_str(val):
+    if sys.version_info.major >= 3:
+        return isinstance(val, str)
+    else:
+        return isinstance(val, str) or isinstance(val, unicode)
+
 test_dir_re = re.compile(r"%s" % test_dir)
 def filter_test_dir(msg):
     return test_dir_re.sub("TEST_DIR", msg)