import praw
import random

#Using the user agent from the example, because I'm lazy
r = praw.Reddit('Comment Scraper 1.0 by u/_Daimon_ see'
                'https://praw.readthedocs.org/en/latest/'
                'pages/comment_parsing.html')

#Pick the submission to choose a comment from, using the post's UUID (/r/subreddit/comments/[Submission ID]
submission = r.get_submission(submission_id='3jfwb1')

#Put all the comments in the submission in the list, "allComments"
submission.replace_more_comments(limit=None, threshold=0)
submissionComments = submission.comments
allComments = praw.helpers.flatten_tree(submissionComments)

#If the user's name is not already in the set "usernames", add it
usernames = set()
for comment in allComments:
    if comment.author is not None and comment.author.name not in usernames:
            usernames.add(comment.author.name)

#Print the number of unique usernames that commented
print(len(usernames))

#Get one random username from the usernames
print( random.sample(usernames, 1) )