[ python-Bugs-1273504 ] Cycle containing a Set is not GC'd [leak]

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Bugs item #1273504, was opened at 2005-08-25 22:20
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Category: Python Library
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Pierre-Frédéric Caillaud (peufeu)
Assigned to: Nobody/Anonymous (nobody)
Summary: Cycle containing a Set is not GC'd [leak]

Initial Comment:
Summary
-------------------------
Python's GC detects cycles except if there is a set somewhere in the 
cycle.

For instance.

>>> class Survivor( object ):
...     def __init__( self ):
...             self.s = set( (self,) )
...
>>> Survivor()
<__main__.Survivor object at 0xb7ce68ac>
>>> Survivor()
<__main__.Survivor object at 0xb79b0b0c>
>>> len( [ x for x in gc.get_objects() if isinstance( x, Survivor ) ] )
2
>>> gc.collect()
0
>>> gc.collect()
0
>>> len( [ x for x in gc.get_objects() if isinstance( x, Survivor ) ] )
2
>>> [ x for x in gc.get_objects() if isinstance( x, Survivor ) ]
[<__main__.Survivor object at 0xb7ce68ac>, <__main__.Survivor 
object at 0xb79b0b0c>]

So, the Survivors survive gc.collect() !

See more complete test case below.

System infos
-------------------------
$ python
Python 2.4.1 (#1, May 14 2005, 18:44:52)
[GCC 3.3.5  (Gentoo Linux 3.3.5-r1, ssp-3.3.2-3, pie-8.7.7.1)] on linux2
>>> import gc
>>> gc.get_threshold()
(700, 10, 10)

OK, gc is compiled in, proceed...


Test Case
-------------------------

The attached program creates a new-style class and an old-style class 
(of appropriate names). We create instances of these objects, each 
having only a member, being a set, or a dict, or a list. Then we create 
a cycle by putting the object itself inside this list, dict, or set, collect() 
a few times ; cycles involving lists and dicts are gone, cycles involving 
sets stay.

This was to check if it had anything to do with something else than 
sets ; apparently sets are the cause.

The source is self-explanatory.

Enjoy !




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