Problems when destroy object which is referenced member objects

[Terry Reedy]

"Minkyu Kim" <levites at lycos.co.kr> wrote in message
news:a99ed18b.0402190557.51e501ed at posting.google.com...

> class TestA:
>    def __init__(self):
>       print "init TestA"
>    def __del__(self):
>       print "del TestA"
>    def SetEvent(self, event):
>       self.event = event
>
> class TestB:
>    def __init__(self):
>       print "init TestB"
>       self.testA = TestA()
>       self.testA.SetEvent(self.Test)
>    def __del__(self):
>       print "del TestB"
>    def Test(self):
>       pass
>
> testB = TestB()
> del testB
> ---------------------
>
> I want to run this code that destroy testB and testB's member object
> testA. But no object destroy. I know testA increase testB's reference
> count, but why wouldn't python decrease testB's reference count? Had I
> do wrong something?

You created a circular reference loop of objects with __del__ methods.
Without adding 'del testB.testA' to break the loop, before the testB
deletion, the reference counts never go to 0.

I believe the gc garbage collector will also leave the loop alone because
of the __del__ methods.  Trying to watch gc in action stops it (!) because
__del__ methods can do (and have done) things that mess up the gc process,
so gc no longer tries when they are present.  So break the loop as
indicated above.

Terry J. Reedy