"spir" <[email protected]> wrote
Say I have a Tree type that may be based on 2 kinds of Node-s.
Both conceptually and practically, a tree itself is a node, namely the
top/root one.
Thats one way to do it...
But a tree also has some additional tree-level behaviour, which is
independant
of the kind of node.
In which case its possibly not a node because it may be breaking the
Liskov Substitution Principle. In that case a Tree has its own identity
and behaviour and attributes, one of which is a heirarchy of nodes.
So you must decide first of all; Is a tree really a node - ie is it fully
interchangable with a node?
Say, the node type performs all the underlying tree mechanics. Thus the
Tree type looks like:
class Tree(_Node):
def __init__(self, args):
_Node.__init__(self)
...
... tree-level methods ...
The issue is the actual kind of node is not only initialised
through "_Node.__init__(self)": it must first feature as super class
in "class Tree(Node)".
Surely the different types of node are all subclasses of the
(possibly abstract) Node class? ie you have 3 subclasses
of Node?
I tried to play with __new__ in Tree and Node types, but
this error seems unavoidable. Maybe I did not do it right.
I'm not totally sure what you did to get the error?
I can indeed have __new__ return an instance of either
Node type, but then for any reason it "forgets" it is also
a tree (so that calling any tree method fails).
I there a way to do this right?
Provided the tree is not dependant on the node types
there should not be a problem. The Tree treats all nodes
as Nodes. Polymorphism should do the rest.
Workaround ideas:
* Make the root node an attribute of tree: unsatisfying.
Why is it unsatisfying? Its a perfectly legitimate model.
It is the one used by most GUI frameworks for the widget
containment tree
* Create a common tree type and 2 specialised ones:
class TreeN(Tree,NodeN)
def __init__(self, args):
_NodeN.__init__(self)
Tree.__init__(self, args)
# ... e basta ...
Or a Common Node type and 3 subclasses, Tree,, Node1 and Node2
HTH,
--
Alan Gauld
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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