ex2.py

def generate_tag_string(post_id, tags=[], new=False):
    """
    ...
    """
    if new:
        tags.append('new')

    post = get_post(post_id)
    
    post_tags = [t.decode("utf8", "ignore") for t in post.get('tags', [])]
    additional_tags = [t.decode("utf8", "ignore") for t in tags]
    final_tags = set(post_tags + additional_tags) - set([''])

    return '{}: {}'.format(post.get('author'), ', '.join(final_tags))