__del__ doesn't work

[Graham Dumpleton]

PeterAbel at gmx.net (Peter Abel) wrote in message news:<21064255.0402151141.51a38fed at posting.google.com>...
> I have an application, which is an instance of a class
> with a deeply nested object hierarchy. Among others one
> method will be executed as a thread, which can be stopped.
> Everything works fine except that when deleting the main
> instance -  after the thread has been stopped -
> the __del__ method will not be carried out.

Does your deeply nested object hierarchy contain references back
onto the top level class instance itself? For example, run the following:

print "start"

class A:
  def __init__(self):
    print "init"
  def __del__(self):
    print "del"

a = A()
del a

class B:
  def __init__(self):
    print "init"
    self.a = self
  def __del__(self):
    print "del"

b = B()
del b

print "finish"

And you get:

start
init
del
init
finish

Ie., you don't see "del" for class B. This is because B had a
member variable which looped back onto the object itself.

Same thing can happen where a class holds a reference back to
a method of the class, ie., a callback.

I don't believe that Python can cope with this and the object
will never actually be deleted. Please correct me if I am wrong.

In certain circumstances where I have had this problem and I
needed to know the object would be destroyed as soon as
possible, I have had to add a method to the class which could
be called to undo any references the class held which pointed
back onto itself.

I have looked at weak references, but have not been satisfied
that it could be used in the situation where I had to overcome the
problem.