garbage collection / cyclic references

Aaron Brady castironpi at
Sat Mar 21 09:26:28 CET 2009

On Mar 20, 8:12 pm, "andrew cooke" <and... at> wrote:
> Aaron Brady wrote:
> [...]
> > caveats and fragilities?  If free software can do it, why isn't it all
> > over the industry?  What disqualifies it from solved-problem status?
> the two dominant virtual machines - .net and the jvm both handle circular
> references with no problem whatever.  this is standard in modern garbage
> collection - go read a book on the subject (personally i like grune et
> al's modern compiler design).  it *is* a solved problem.  if anything,
> python is behind the curve, not ahead of it, but this may change with the
> next generation of python implementations (pypy targets a variety of vms,
> i think).
> as for the extra methods you suggest - why do you want to expose
> implementation details in an api?  that is not the normal aim of good
> design.
> andrew

"Circular references ...can only be cleaned up if there are no Python-
level __del__() methods involved."  __del__ doc.

"Python doesn’t collect ... cycles automatically because, in general,
it isn’t possible for Python to guess a safe order in which to run the
__del__() methods."  gc.garbage doc.

"Errors should never pass silently." -The Zen of Python

I advance that cyclic objects should be notified when their external
references go to zero, but their usual '__del__' is inappropriate.  If
objects implement a __del__ method, they can choose to implement a
'__gc_cycle__' method, and then just drop the specified attribute.  It
needn't be called on every object in the cycle, either; once it's
called on one object, another object's normal __del__ may be safely
called.  Output, unproduced:

>>> del x
In X.__gc_cycle__, 'other' attribute.  Deleting...
In Y.__del__.
In X.__del__.

The actual backend of CPython requires garbage-collected container
types to implement tp_inquiry and tp_clear methods, but user-defined
types apparently aren't required to conform.

Supporting Cyclic Garbage Collection

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