def parse_vertices(vertices_str):
    """
    Parse vertices stored as a string.

    :param vertices: "x1,y1,x2,y2,...,xn,yn"
    :param return: [(x1,y1), (x1, y2), ... (xn, yn)]
    """
    s = [float(t) for t in vertices_str.split(',')]
    return zip(s[::2], s[1::2])