def modify_tuple(obj, positions, new_values):
    """
    Modify the tuple object at certain positions by certain value
    
    >>> modify_tuple((1,2,3), [0,1,2], [3,2,1])
    (3, 2, 1)
    """
    assert isinstance(obj, tuple)
    assert len(positions) == len(new_values)

    alt = list(obj)
    for pos, val in zip(positions, new_values):
        alt[pos] = val
        
    return tuple(alt)