old_contents: class PowerSocketInterface(object):
      def active(self):
          raise NotImplementedError("PowerSocket should active")
  
  class EUSocket(PowerSocketInterface):
      def __init__(self, eu_plug):
          self.plug = eu_plug
  
      def active(self):
          self.plug.eu_connect()
  
  
  class EUPlug(object):
      def __init__(self):
          self.message = "EU plug connected\n"
  
      def eu_connect(self):
          print self.message
  
  
  class CNPlug(object):
      def __init__(self):
          self.message = 'CN plug connected\n'
  
      def cn_connect(self):
          print self.message
  
  class CNEUAdapter(EUPlug):
      def __init__(self, cn_plug):
          self.message = "CNEU adapter in use"
  
      def eu_connect(self):
          print self.message
          return cn_plug.cn_connect()
  
  print "plug a eu_plug direct to eu socket"
  eu_plug = EUPlug()
  socket = EUSocket(eu_plug)
  socket.active()
  
  print "plug a cn_plug through a cn_eu_adapter to eu socket"
  cn_plug = CNPlug()
  adapter = CNEUAdapter(cn_plug)
  socket = EUSocket(adapter)
  socket.active()
  
new_contents: class PowerSocketInterface(object):
      def active(self):
          raise NotImplementedError("active not implemented")
  
  class EUSocket(PowerSocketInterface):
      def __init__(self, eu_plug):
          self.plug = eu_plug
  
      def active(self):
          self.plug.eu_connect()
  
  
  class EUPlug(object):
      def __init__(self):
          self.message = "EU plug connected\n"
  
      def eu_connect(self):
          print self.message
  
  
  class CNPlug(object):
      def __init__(self):
          self.message = 'CN plug connected\n'
  
      def cn_connect(self):
          print self.message
  
  class CNEUAdapter(EUPlug):
      def __init__(self, cn_plug):
          self.message = "CNEU adapter in use"
  
      def eu_connect(self):
          print self.message
          return cn_plug.cn_connect()
  
  print "plug a eu_plug direct to eu socket"
  eu_plug = EUPlug()
  socket = EUSocket(eu_plug)
  socket.active()
  
  print "plug a cn_plug through a cn_eu_adapter to eu socket"
  cn_plug = CNPlug()
  adapter = CNEUAdapter(cn_plug)
  socket = EUSocket(adapter)
  socket.active()
  
subject: Change error string