diff --git a/pgl/utils/mp_reader.py b/pgl/utils/mp_reader.py
index fd3b97295e1373942b97afd69ab4310277b4aa4b..55c8b640fc5d460a24e87baeae66dfe1f537bd93 100644
--- a/pgl/utils/mp_reader.py
+++ b/pgl/utils/mp_reader.py
@@ -38,17 +38,20 @@ def serialize_data(data):
         return None
     return numpy_serialize_data(data)  #, ensure_ascii=False)
 
+def index_iter(data):
+    """return indexing iter"""
+    if isinstance(data, list):
+        return range(len(data))
+    elif isinstance(data, dict):
+        return data.keys()
+
 
 def numpy_serialize_data(data):
     """serialize_data"""
     ret_data = copy.deepcopy(data)
-    if isinstance(ret_data, list):
-        for key, value in enumerate(ret_data):
-            if isinstance(ret_data[key], np.ndarray):
-                ret_data[key] = _np_serialized_data(value=ret_data[key].tobytes(),
-                                shape=list(ret_data[key].shape), dtype="%s" % ret_data[key].dtype)
-    elif isinstance(ret_data, dict):
-        for key in ret_data:
+
+    if isinstance(ret_data, (dict, list)):
+        for key in index_iter(ret_data):
             if isinstance(ret_data[key], np.ndarray):
                 ret_data[key] = _np_serialized_data(value=ret_data[key].tobytes(),
                                 shape=list(ret_data[key].shape), dtype="%s" % ret_data[key].dtype)
@@ -60,14 +63,8 @@ def numpy_deserialize_data(data):
     if data is None:
         return None
 
-    if isinstance(data, list):
-        for key, value in enumerate(data):
-            if isinstance(value, _np_serialized_data):
-                data[key] = np.frombuffer(buffer=data[key].value,
-                                dtype=data[key].dtype).reshape(data[key].shape)
-
-    elif isinstance(data, dict):
-        for key in data:
+    if isinstance(data, (dict, list)):
+        for key in index_iter(data):
             if isinstance(data[key], _np_serialized_data):
                 data[key] = np.frombuffer(buffer=data[key].value,
                                 dtype=data[key].dtype).reshape(data[key].shape)