python 中的 @ 是什么意思???

CODE:

from twisted.internet.threads import deferToThread
  deferred = deferToThread.__get__

  ## example code

  def print_(result):
    print result
  
  def running():
      "Prints a few dots on stdout while the reactor is running."
      sys.stdout.write("."); sys.stdout.flush()
      reactor.callLater(.1, running)

@deferred
  def sleep(sec):
    "A blocking function magically converted in a non-blocking one."
    print 'start sleep %s' % sec
    time.sleep(sec)
    print '\nend sleep %s' % sec
    return "ok"
  
  if __name__ == "__main__":
     sleep(2).addBoth(print_)
     reactor.callLater(.1, running)
     reactor.callLater(3, reactor.stop)
     reactor.run()