Random username picker

import praw
import random

r = praw.Reddit('Comment Scraper 1.0 by u/_Daimon_ see '
                'https://praw.readthedocs.org/en/latest/'
                'pages/comment_parsing.html')
submission = r.get_submission(submission_id='3gqwl5')

submission.replace_more_comments(limit=None, threshold=0)
allComments = submission.comments
flatComments = praw.helpers.flatten_tree(allComments)

usernames = set()
for comment in flatComments:
    if comment.author is not None and comment.author.name not in usernames:
            usernames.add(comment.author.name)

print(len(usernames))

print( random.sample(usernames, 1) )