[Discuss-gnuradio] data from flow graph is not updated

[lishan_wh]

Hi, all
  I met a problem when I tried to read data from USRP. I hope someone can help me. Thanks.
  Here is my flow graph:
  source_c --> keep_one_in_n --> g.head --> gr.complex_to_mag --> gr.vector_sink_f

if __name__ == '__main__':
    tb = top_block()
        try:
                tb.start()              # start executing flow graph in another thread...
                time.sleep(1)
                main_loop(tb)        
        except KeyboardInterrupt:
                pass

  In main_loop function, I read data from vector_sink_f.
def main(tb):
    while(True):
        time.sleep(1)  
        data = "">
But when I print data, the result shows that data doesn't change at all. I don't why. In my opinion, data should be updated as the flow graph runs. And that changing data with time is what I wanted. Does anybody know the reason? Thanks!
             Ally