(Newbie) Restricting inherited methods to operate on element from same subclass

[Duncan Booth]

andy2o wrote:

> But I want to raise an exception if my code finds:
> 
> f = A1()
> g = A2()
> f.join(g) #should raise exception, as cannot join an 
>           #instance of A1 to an instance of A2.
> 
> How can I verify in a method defined in class A that the subclasses I
> am joining are exactly the same? Or is there a design flaw here I
> haven't spotted that makes this a bad idea? Or do I need to code N
> join methods?

Assuming that A is a new-style class then if they have to be exactly the 
same type compare the types:

   def join(self, other):
      if type(self) != type(other):
          raise whatever

if the 'other' value can be a subclass of self:

   def join(self, other):
       if not isinstance(other, type(self)):
           raise whatever

If A is an old-style class then you would have to compare the __class__ 
attribute: self.__class__ != other.__class__