Python os.forkpty为什么我不能让它工作

import pty
import os
import sys
import time

pid, fd = os.forkpty()

if pid == 0:
    # Slave
    os.execlp("su","su","MYUSERNAME","-c","id")

# Master
print os.read(fd, 1000)
os.write(fd,"MYPASSWORD\n")
time.sleep(1)
print os.read(fd, 1000)
os.waitpid(pid,0)
print "Why have I not seen any output from id?"
import pty
import os
import sys
import time

pi