code: def find_username_from_user_id(session, user_id):
      
      comments_page = session.session.get(u'http://myanimelist.net/comments.php?' + urllib.urlencode({'id': int(user_id)})).text
      comments_page = bs4.BeautifulSoup(comments_page)
      username_elt = comments_page.find('h1')
      if "'s Comments" not in username_elt.text:
        raise InvalidUserError(user_id, message="Invalid user ID given when looking up username")
      return username_elt.text.replace("'s Comments", "")
docs: Look up a MAL username's user ID.
  
      :type session: :class:`myanimelist.session.Session`
      :param session: A valid MAL session.
  
      :type user_id: int
      :param user_id: The user ID for which we want to look up a username.
  
      :raises: :class:`.InvalidUserError`
  
      :rtype: str
      :return: The given user's username.