[Python-Dev] __del__ unexpectedly being called twice

[Duncan Booth]

There's a thread on comp.lang.python at the moment under the subject "It is 
__del__ calling twice for some instances?" which seems to show that when 
releasing a long chain of old-style classes every 50th approximately has 
its finaliser called twice. I've verified that this happens on both Python 
1.4 and 1.5.


My guess is that there's a bug in the trashcan mechanism: calling the 
__del__ method means creating a descriptor, and if that descriptor gets 
queued in the trashcan then releasing it calls the __del__ method a second 
time. I'm not sure if there is going to be a particularly easy fix for 
that.

Would someone who knows this code (instance_dealloc in classobject.c) like 
to have a look at it, should I just submit a bug report, or isn't it worth 
bothering about?


The code which exhibits the problem:

#!/usr/local/bin/python -d
# -*- coding: koi8-u -*-
import sys

class foo:
    def __init__(self, other):
        self.other = other
        self._deleted = False

        global ini_cnt
        ini_cnt +=1

    def __del__(self):
        if self._deleted:
            print "aargh!"
        self._deleted = True
        global del_cnt
        del_cnt +=1
        print "del",del_cnt,"at",id(self)

def stat():
    print "-"*20
    print "ini_cnt = %d" % ini_cnt
    print "del_cnt = %d" % del_cnt
    print "difference = %d" % (ini_cnt-del_cnt)

ini_cnt = 0
del_cnt = 0
loop_cnt = 55

a = foo(None)

for i in xrange(loop_cnt):
    a = foo(a)

stat()
a = None
stat()


The original thread is at:
http://groups.google.com/group/comp.lang.python/browse_thread/thread/293acf433a39583b/bfd4af9c6008a34e