[graph-tool] Cannot pickle graph

Hi everyone, The following code snippet gives me a TypeError and I am wondering if you have any suggestion. ------------------------------------------------ import graph_tool as gt import pickle g = gt.Graph() pickle.dump(g, open('test_file', 'w’)) ------------------------------------------------ The error: TypeError: coercing to Unicode: need string or buffer, _io.BytesIO found I am working on Yosemite OS with graph_tool 2.2.35 (installed via macports). Thank you very much and happy new year!! Helen