diff --git a/goblin/fileio/graphson.py b/goblin/fileio/graphson.py
index d531017d616af6993bd51db12ecd0cbe3610eaf5..c9932427f281bad1a847f2a30cc55de8f12b3c12 100644
--- a/goblin/fileio/graphson.py
+++ b/goblin/fileio/graphson.py
@@ -17,9 +17,9 @@ AdjList = collections.namedtuple("AdjList", "vertex inE outE")
 vp_id = 10
 
 
-def dump(fpath, *adj_lists):
+def dump(fpath, *adj_lists, mode="w"):
     """Convert Goblin elements to GraphSON"""
-    with open(fpath, "w") as f:
+    with open(fpath, mode) as f:
         for adj_list in adj_lists:
             dumped = dumps(adj_list)
             f.write(dumped + '\n')