diff --git a/zoopt/dimension.py b/zoopt/dimension.py
index 69832c2..b613558 100644
--- a/zoopt/dimension.py
+++ b/zoopt/dimension.py
@@ -14,7 +14,7 @@
 import copy
 
 
-class Dimension:
+class Dimension(object):
     """
     This class describes the dimension information of the search space.
     """
@@ -223,7 +223,7 @@ class ValueType(enumerate):
     DISCRETE = 0
 
 
-class Dimension2:
+class Dimension2(object):
     """
     This class describes the dimension information of the search space.
     `Dimension2` has the same function as `Dimension` class, but the format of parameters is different.
diff --git a/zoopt/utils/tool_function.py b/zoopt/utils/tool_function.py
index 2f3b0a3..09fd01f 100644
--- a/zoopt/utils/tool_function.py
+++ b/zoopt/utils/tool_function.py
@@ -4,7 +4,7 @@
 Author:
     Yu-Ren Liu, Yang Yu
 """
-
+from __future__ import print_function
 import pickle