non-blocking socket,error is always

sock.setblocking(0)
try:
    data = sock.recv(1024)
 except socket.error, e:
    if e.args[0] == errno.EWOULDBLOCK: 
          print 'EWOULDBLOCK'
else:            
   if not data:   #recv over
      sock.close()
      print 'close================='       
   else:
      print 'recv ---data---------'
      poem += data

all above code is in a loop.using non-blocking socket(just want to test 'non-blocking socket') to get data. But always print 'EWOULDBLOCK',i don't know why?