如何優雅地處理SIGTERM信號?

不中斷當前的執行，而是在一個單獨的線程中處理signal，以便mainloop()有機會優雅地停止?

import signal
import time

class GracefulKiller:
  kill_now = False
  def __init__(self):
    signal.signal(signal.SIGINT, self.exit_gracefully)
    signal.signal(signal.SIGTERM, self.exit_gracefully)

  def exit_gracefully(self,signum, frame):
    self.kill_now = True

if __name__ == '__main__':
  killer = GracefulKiller()
  while True:
    time.sleep(1)
    print("doing something in a loop ...")
    if killer.kill_now:
      break

  print "End of the program. I was killed gracefully :)"