Python superclass and subclass interface consistency

Let Bar inherit from Foo and define x against both classes explicitly. Then define Foo.y, Foo.z equal to None but derive Bar.y, Bar.z as properties.

This doesn't work.

   class Foo(object):
      def __init__(self, x):
         self.x = x
         self.y = None
         self.z = None

   class Bar(Foo):
      def __init__(self, x):
         Foo.__init__(self, x)

      @property
      def y(self):
         return self.x * 100

      @property
      def z(self):
         return self.x * 1000

   >>> foo = Foo(7)
   >>> foo.x
   7
   >>> foo.y
   >>> foo.z

   >>> bar = Bar(8)
   Traceback (most recent call last):
     File "", line 1, in 
     File "test.py", line 9, in __init__
       Foo.__init__(self, x)
     File "test.py", line 4, in __init__
       self.y = None
   AttributeError: can't set attribute

But this does.

   class Foo(object):
      def __init__(self, x):
         self.x = x

      @property
      def y(self):
         return None

      @property
      def z(self):
         return None

   class Bar(Foo):
      def __init__(self, x):
         Foo.__init__(self, x)

      @property
      def y(self):
         return self.x * 100

      @property
      def z(self):
         return self.x * 1000

   >>> foo = Foo(7)
   >>> foo.x
   7
   >>> foo.y
   >>> foo.z

   >>> bar = Bar(8)
   >>> bar.x
   8
   >>> bar.y
   800
   >>> bar.z
   8000

A property in the subclass must implement as a property in the superclass. The pattern here ensures the interface consistency of both.