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.

def find_username_from_user_id(session, user_id):
    """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.
    """
    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", "")