This is a very inneficient implementation of a YouTube comment scrapper.
Also, it does not work properly for all videos (some comments are not retrieved). For example, https://www.youtube.com/watch?v=PUzzx-Kb3WE has 772 as of October 12th 2016, but the script only finds 669.
I opened a question on StackOverflow about that and answers/suggestions are welcome: http://stackoverflow.com/questions/34606055/how-to-get-comments-from-videos-using-youtube-api-v3-and-python