def identity(mask):
    """
    This is identity function that can be used as an argument of functions :func:`set_encoder` and :func:`set_decoder`.
    
    :param mask: input mask
    :type mask: numpy.ndarray
    :return: the same mask
    :rtype: numpy.ndarray
    
    Example:

    .. code-block:: python
       
        ACDC.set_encoder(ACDC.identity)
        ACDC.set_decoder(ACDC.identity)
    """
    return mask