def generate_anisotropy_str(anisotropy):
    """Generate a string from anisotropy specification parameters.

    Parameters
    ----------
    anisotropy : None or tuple of values

    Returns
    -------
    anisotropy_str : string

    """
    if anisotropy is None:
        anisotropy_str = 'none'
    else:
        anisotropy_str = '_'.join(str(param) for param in anisotropy)
    return anisotropy_str