def rect_at(point, size, size_y=-1):
    """Returns a rectangle centred at `point`. If only `size` is provided then
    the rectangle will be a square. The dimensions will be `size` * 2.
    """

    if size_y == -1:
        size_y = size

    return (point[0] - size, point[1] - size_y,
            point[0] + size, point[1] + size_y)