Jan Eden wrote:
>  My actual code looks like this:
>  
>  class Base:
>      def GetOwnType(self):
>          try: return self._own_type
>          except: return self.child_type
>      
>      def SetOwnType(self, value):
>          self._own_type = value
>          
>      own_type = property(GetOwnType, SetOwnType)
>  
>  For some of the subclasses of Base, the attribute own_type is
>  defined, the others should use child_type.
>  
>  For both groups of subclasses, this works fine - if own_type has not
>  been set somewhere else, self.child_type is returned when calling
>  self.own_type.
>  
>  When checking Data.Base.__mro__, I get an error, so it is not a
>  new-style class by itself.
>  
>  On the other hand, every time I use the own_type attribute, I do so
>  via instances of new-style classes (Show.Page, Show.Author etc).

That is the key

>  
>  Could it be that the nature of these classes makes the code in
>  Data.Base behave according to the new-style rules?> 

Yes, I tried to show that in my example. Any class that has a new-style class as one of its 
base classes will be a new-style class. The attribute lookup is implemented in the metaclass
.

For an instance of Data.Base, the metaclass (the class of its class) is types.ClassType, whi
ch implements old-style attribute access. So if you create a Data.Base directly the property
 access will be broken.

On the other hand if you create an instance of Show.Page, the meta class is type, which impl
ements new-style attribute access and the properties will behave correctly.

You have to keep in mind, this is Python, everything is dynamic. When you define Data.Base y
ou don't define its behaviour irrevocably. Attribute lookup happens at runtime and is affect
ed by the current state of the object.

Kent

>  Thanks,
>  
>  Jan

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