Some potential alternatives to consider:

1.

    classList = ['highlighted', 'primary']
        .concatif(discount, 'on-sale')
        .join(' ')

2.

    classList = ' '.join(['highlighted', 'primary'] + (['on-sale'] if discount else []))

3.

    mut classList = ['highlighted', 'primary']
    if discount:
        classList.append('on-sale')
    classList = ' '.join(classList)

    freeze classList

4.

    def get_class_list(discount):
        mut classList = ['highlighted', 'primary']
        if discount:
            classList.append('on-sale')
        classList = ' '.join(classList)
        return classList

    classList = get_class_list(discount)