Encode object as json str.

def _encode_json(obj):
    '''
    Encode object as json str.
    '''
    def _dump_obj(obj):
        if isinstance(obj, dict):
            return obj
        d = dict()
        for k in dir(obj):
            if not k.startswith('_'):
                d[k] = getattr(obj, k)
        return d
    return json.dumps(obj, default=_dump_obj)