Python yield

def sayhi():
   seek = 0
   while True :
     with open('test.txt','r') as f :
         f.seek(seek)
         data = f.readline()
         if data:
             seek = f.tell()
             #yield data
             yield  data
         else:
             return
rf = sayhi()
print rf.next()
print "---------"
print rf.next()