Want - but cannot get - a nested class to inherit from outer class

Jeff jeffober at gmail.com
Fri Mar 7 22:12:18 CET 2008


Use metaclasses?

DBak wrote:
> I want - but cannot get - a nested class to inherit from an outer
> class.  (I searched the newsgroup and the web for this, couldn't find
> anything - if I missed an answer to this please let me know!)
>
> I would like to build a class for a data structure such that nodes of
> the data structure - of interest only to the data structure
> implementation itself and not to the consumer - are instances of one
> of two class types.  I thought to encapsulate the nodes' classes like
> this:
>
> class Tree(object):
> ...class _MT(Tree):
> ......def isEmpty(self): return True
> ......def insert(self, X): return Tree._Node(X)
> ...class _Node(Tree):
> ......def isEmpty(self): return False
> ......def insert(self, X): return _Node(X, self, Tree._MT())
> ...def merge(self, T):
> ......def __init__(): return _MT()
> ......<code for merging tree T into self>
>
> In other words, some methods would be implemented on instances'
> classes (like isEmpty and insert) and some on the outer class (like
> merge).  Users of the data structure never need to know about the
> nodes, much less the nodes' classes, so I wanted to encapsulate them
>
> However I can't do this, because, of course, the name Tree isn't
> available at the time that the classes _MT and _Node are defined, so
> _MT and _Node can't inherit from Tree.
>
> What is the Pythonic thing I should be doing instead?
>
> (Easy answer:  Put this code in a module, exposing only a factory
> function.  I could do that, but wanted to know if I could encapsulate
> it as described so I could actually put several similar data
> structures into one module.)
>
> Thanks! -- David



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