@classmethod
def channels_axis(cls, data_format='channels_last'):
    """ Return the channels axis for the tensor

        Parameters
        ----------
        data_format : str {'channels_last', 'channels_first', 'N***'} or None

        Returns
        -------
        int
        """
    return 1 if data_format == 'channels_first' or data_format.startswith('NC'
        ) else -1
