sur = rs.GetObject("select a object")
dnu = rs.SurfaceDomain(sur, 0)
dnv = rs.SurfaceDomain(sur, 1)
u = float(dnu[0]+dnu[1])/2
v = float(dnv[0]+dnv[1])/2
po = rs.EvaluateSurface(sur, u, v)
pa = rs.SurfaceClosestPoint(sur, po)
cur = rs.ExtractIsoCurve(sur, pa, 1)