How to use the tern.command_lib.command_lib.get_pkg_attr_list function in tern

def collate_list_metadata(shell, listing):
    '''Given the shell and the listing for the package manager, collect
    metadata that gets returned as a list'''
    pkg_dict = {}
    msgs = ''
    warnings = ''
    for item in command_lib.base_keys:
        if item in listing.keys():
            items, msg = command_lib.get_pkg_attr_list(shell, listing[item])
            msgs = msgs + msg
            pkg_dict.update({item: items})
        else:
            warnings = warnings + errors.no_listing_for_base_key.format(
                listing_key=item)
    return pkg_dict, msgs, warnings

rootfs.prep_rootfs(target)
        # invoke commands in chroot
        # if we're looking up the snippets library
        # we should see 'snippets' in the keys
        if 'snippets' in args.keys and 'packages' in args.keys:
            # get the package info that corresponds to the package name
            # or get the default
            last = args.keys.pop()
            info_list = look_up_lib(args.keys)
            info_dict = command_lib.check_for_unique_package(
                info_list, args.package)[last]
        else:
            info_dict = look_up_lib(args.keys)
        # try to invoke the commands
        try:
            result = command_lib.get_pkg_attr_list(
                args.shell, info_dict, args.package)
            print('Output list: ' + ' '.join(result[0]))
            print('Error messages: ' + result[1])
            print('Number of elements: ' + str(len(result[0])))
        except subprocess.CalledProcessError as error:
            print(error.output)
        # undo the mounts
        rootfs.undo_mount()
        rootfs.unmount_rootfs()
    else:
        print("Something when wrong in loading the image")
    report.teardown()
    report.clean_image_tars(image_obj)
    report.clean_working_dir(False)

if version_listing:
        version_list, invoke_msg = command_lib.get_pkg_attr_list(
            shell, version_listing, package_name=pkg_obj.name)
        if version_list:
            pkg_obj.version = version_list[0]
        else:
            pkg_obj.origins.add_notice_to_origins(
                origin_str, Notice(invoke_msg, 'error'))
    else:
        pkg_obj.origins.add_notice_to_origins(
            origin_str, Notice(listing_msg, 'warning'))
    # license
    license_listing, listing_msg = command_lib.check_library_key(
        pkg_listing, 'license')
    if license_listing:
        license_list, invoke_msg = command_lib.get_pkg_attr_list(
            shell, license_listing, package_name=pkg_obj.name)
        if license_list:
            pkg_obj.license = license_list[0]
        else:
            pkg_obj.origins.add_notice_to_origins(
                origin_str, Notice(invoke_msg, 'error'))
    else:
        pkg_obj.origins.add_notice_to_origins(
            origin_str, Notice(listing_msg, 'warning'))
    # proj_urls
    url_listing, listing_msg = command_lib.check_library_key(
        pkg_listing, 'proj_url')
    if url_listing:
        url_list, invoke_msg = command_lib.get_pkg_attr_list(
            shell, url_listing, package_name=pkg_obj.name)
        if url_list:

if license_listing:
        license_list, invoke_msg = command_lib.get_pkg_attr_list(
            shell, license_listing, package_name=pkg_obj.name)
        if license_list:
            pkg_obj.license = license_list[0]
        else:
            pkg_obj.origins.add_notice_to_origins(
                origin_str, Notice(invoke_msg, 'error'))
    else:
        pkg_obj.origins.add_notice_to_origins(
            origin_str, Notice(listing_msg, 'warning'))
    # proj_urls
    url_listing, listing_msg = command_lib.check_library_key(
        pkg_listing, 'proj_url')
    if url_listing:
        url_list, invoke_msg = command_lib.get_pkg_attr_list(
            shell, url_listing, package_name=pkg_obj.name)
        if url_list:
            pkg_obj.proj_url = url_list[0]
        else:
            pkg_obj.origins.add_notice_to_origins(
                origin_str, Notice(invoke_msg, 'error'))
    else:
        pkg_obj.origins.add_notice_to_origins(
            origin_str, Notice(listing_msg, 'warning'))

def get_package_dependencies(package_listing, package_name, shell):
    '''The package listing is the result of looking up the command name in the
    command library. Given this listing, the package name and the shell
    return a list of package dependency names'''
    deps_listing, deps_msg = command_lib.check_library_key(
        package_listing, 'deps')
    if deps_listing:
        deps_list, invoke_msg = command_lib.get_pkg_attr_list(
            shell, deps_listing, package_name=package_name)
        if deps_list:
            return list(set(deps_list)), ''
        return [], invoke_msg
    return [], deps_msg