def _get_comments_request(post, sort_mode, max_depth, max_breadth):
    """
    Used to build the request string used by :func:`get_comments`.
    
    
    :param str post: The unique id of a Post from which Comments will be returned.
    :param str sort_mode: The order that the Posts will be sorted by. Options are: "top" (ranked by upvotes minus downvotes), "best" (similar to top, except that it uses a more complicated algorithm to have good posts jump to the top and stay there, and bad comments to work their way down, see http://blog.reddit.com/2009/10/reddits-new-comment-sorting-system.html), "hot" (similar to "top", but weighted by time so that recent, popular posts are put near the top), "new" (posts will be sorted by creation time).
    :returns: str
    """
    return "http://www.reddit.com/r/all/comments/{}/{}.json?max_depth={}&max_breadth={}".format(post, sort_mode, max_depth, max_breadth)