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 -- http://mail.python.org/mailman/listinfo/python-list
