code: def port_has_listener(address, port):
      
      cmd = ['nc', '-z', address, str(port)]
      result = subprocess.call(cmd)
      return not(bool(result))
docs: Returns True if the address:port is open and being listened to,
      else False.
  
      @param address: an IP address or hostname
      @param port: integer port
  
      Note calls 'zc' via a subprocess shell