load a polygon from a file

def polygon_load(filename):
    '''load a polygon from a file'''
    ret = []
    f = open(filename)
    for line in f:
        if line.startswith('#'):
            continue
        line = line.strip()
        if not line:
            continue
        a = line.split()
        if len(a) != 2:
            raise RuntimeError("invalid polygon line: %s" % line)
        ret.append((float(a[0]), float(a[1])))
    f.close()
    return ret