How to call ltm function using rpy package in python

#1
I am trying the following code--

from rpy import *
r.library("ltm")

dat= #some data frame or matrix
r.ltm(r('dat~z1'))

error coming is---
RPy_RException: Error in eval(expr, envir, enclos) : object 'dat' not found

Please tell me the right way to call ltm function using rpy library