Problem with garbage collection (sort of)

[Frank Millman]

frank at chagford.com (Frank Millman) wrote:

<snip>

>y is an instance of class c, which creates an instance of class b,
which creates an instance of class a. When y goes out of scope and is
deleted, I want the instances of class b and class a to be deleted as
well.

>The problem is that class a keeps a reference to class b (self.b) and
class b keeps a reference to class a (self.p), so the reference counts
do not go down to zero without some additional action.

Thanks for all the replies - I have learned a lot. It seems that this
whole thing is not a problem at all. In other words, my instances
*are* being deleted by the cyclic garbage collector, and my only
problem was that I could not confirm this positively.

As Michael says, the very act of creating a __del__ method in my
classes prevents them from being deleted! However, as I only added the
__del__ method to try to confirm the deletion, this is not a problem
in practice.

I replaced my __del__ methods with the DelWatcher class suggested by
Tim. At first, nothing changed. Then I added 'import gc; gc.collect()'
at the end, and lo and behold, I could see all my instances being
deleted.

Being a bit of a sceptic, I still did not regard this as positive
confirmation that it will work in practice, as I do not have a
gc.collect() in my live application, so I added the DelWatcher class
there to see if I got the 'deleted' messages. I understand that
gc.collect() runs automatically from time to time, so I waited a
while, and did not get any messages. Then as I did some more work in
the application, the messages started appearing for the older items.
Re-reading the documentation on the gc module confirms that
gc.collect() is only triggered when the number of 'dirty' objects
exceeds a threshold.

For the record, I tried Tim's suggestion of using weakrefs, and it
worked perfectly. I did some timing tests and it seems to have very
little overhead. However, as Tim says, it is better to stick to the
cyclic garbage collector now that I have confidence that it is working
correctly.

Many thanks to all.

Frank Millman