How to use the tern.utils.constants function in tern

def get_os_release():
    '''Given the base layer object, determine if an os-release file exists and
    return the PRETTY_NAME string from it. If no release file exists,
    return an empty string. Assume that the layer filesystem is mounted'''
    # os-release may exist under /etc/ or /usr/lib. We should first check
    # for the preferred /etc/os-release and fall back on /usr/lib/os-release
    # if it does not exist under /etc
    etc_path = os.path.join(rootfs.get_working_dir(), constants.mergedir,
                            constants.etc_release_path)
    lib_path = os.path.join(rootfs.get_working_dir(), constants.mergedir,
                            constants.lib_release_path)
    if not os.path.exists(etc_path):
        if not os.path.exists(lib_path):
            return ''
        etc_path = lib_path
    # file exists at this point, try to read it
    with open(etc_path, 'r') as f:
        lines = f.readlines()
    pretty_name = ''
    # iterate through os-release file to find OS
    for l in lines:
        key, val = l.rstrip().split('=', 1)
        if key == "PRETTY_NAME":
            pretty_name = val
            break
    return pretty_name.strip('"')

def set_up():
    '''Create required directories'''
    workdir_path = os.path.join(constants.temp_folder, constants.workdir)
    mergedir_path = os.path.join(constants.temp_folder, constants.mergedir)
    if not os.path.isdir(workdir_path):
        os.mkdir(workdir_path)
    if not os.path.isdir(mergedir_path):
        os.mkdir(mergedir_path)