delete and circular references

[Paul-Michael Agapow]

I can't find an answer to this in the documentation so: what's the best
way to handle circular references with an object that is about to be
deleted? It is unclear to me exactly what __del__ () does, whether it is
a deletion or pre-deletion handler.

For example: I have a class that is a tree data structure. As I wish to
traverse up and down the branches fast, the nodes contain references to
their parent and their child nodes. Hence the circularity:

class node:
   def __init__ (self):
      parent = None
      children = []
   # etc. etc.

class tree:
   def __init__ (self, treename):
      theNodes = []
      theName = treename
      theRoot = None
   # etc. etc.

When the tree (or any individual node) is being deleted, the circular
references must be broken. So do I break the links in __del__()?

   def __del__ (self)
      for node in theNodes:
         node.parent = node.children = None

If so, do I then have to write as well

      del theName
      del theRoot
      del theNodes

or is __del__ just for pre-deletion manipulations (the system handling
the second part of the function itself)?
   

-- 
Paul-Michael Agapow (p.agapow at ic.ac.uk), Biology, Imperial College
"Pikachu's special power is that he is monophyletic with lagomorphs ..."