python中类继承的类似虚函数的问题--一个开源代码的疑问（补充）?报错

我的上一个问题http://www.oschina.net/question/437227_126231是我看到这个开源的代码模拟的一个小例子，但没有达到同样的效果，而且我好像也没有表达清楚，现在直接引用原来的代码：

有一个player.py（https://github.com/remap/ndnvideo/blob/master/videostreaming/player.py），里面有一个类GstPlayer：

class GstPlayer(gobject.GObject):
	__pipeline = None

	def __init__(self, videowidget, cmd_args):
		self.player = gst.parse_launch(self.__pipeline)

class GstPlayer(player.GstPlayer):
	__pipeline = """
		multiqueue name=mqueue use-buffering=true
		identity name=video_input ! ffdec_h264 ! mqueue. mqueue. ! %s \
		identity name=audio_input ! decodebin ! mqueue. mqueue. ! %s
	""" % (utils.video_sink, utils.audio_sink)

当我把play.py中的__pipeline注释掉后，运行报错如下：

Traceback (most recent call last):
  File "./play.py", line 76, in <module>
    sys.exit(main())
  File "./play.py", line 68, in main
    w = player_gui.PlayerWindow(GstPlayer, cmd_args)
  File "/home/kl/ndn/ndnvideo/videostreaming/player_gui.py", line 42, in __init__
    self.player = gst_player(self.videowidget, cmd_args)
  File "/home/kl/ndn/ndnvideo/videostreaming/player.py", line 35, in __init__
    self.player = gst.parse_launch(self.__pipeline)
TypeError: parse_launch() argument 1 must be string, not None