Python json.dumps 時含有時間物件

TypeError: datetime.datetime(2014, 9, 16, 10, 24, 26, 755110) is not JSON serializable

class DateEncoder(json.JSONEncoder):
    def default(self, obj):
        if hasattr(obj, 'isoformat'):
            return obj.isoformat()
        else:
            return str(obj)
        return json.JSONEncoder.default(self, obj)


json.dumps(my_variable, cls=DateEncoder)

References :
python - Cannot serialize datetime as JSON from Cherrypy - Stack Overflow