[Python-Dev] Linus on garbage collection

Mark Shannon marks at dcs.gla.ac.uk
Fri May 6 19:46:37 CEST 2011

Michael Foord wrote:
> On 06/05/2011 18:26, Mark Shannon wrote:
>> Michael Foord wrote:
>>> On 06/05/2011 17:51, Stefan Behnel wrote:
>>>> Mark Shannon, 06.05.2011 18:33:
>>>>> skip at pobox.com wrote:
>>>>>> Antoine> Since we're sharing links, here's Matt Mackall's take:
>>>>>> Antoine>
>>>>>> http://www.selenic.com/pipermail/mercurial-devel/2011-May/031055.html
>>>>>>> From that note:
>>>>>> 1: You can't have meaningful destructors, because when destruction
>>>>>> happens is undefined. And going-out-of-scope destructors are 
>>>>>> extremely
>>>>>> useful. Python is already a rather broken in this regard, so feel 
>>>>>> free
>>>>>> to ignore this point.
>>>>>> Given the presence of cyclic data I don't see how reference 
>>>>>> counting or
>>>>>> garbage collection win. Ignoring the fact that in a pure reference 
>>>>>> counted
>>>>>> system you won't even consider cycles for reclmation, would both 
>>>>>> RC and GC
>>>>>> have to punt because they can't tell which object's destructor to 
>>>>>> call
>>>>>> first?
>>>>> It doesn't matter which is called first.
>>>> May I quote you on that one the next time my software crashes?
>>> Arbitrarily breaking cycles *could* cause a problem if a destructor 
>>> attempts to access an already collected object. Not breaking cycles 
>>> *definitely* leaks memory and definitely doesn't call finalizers.
>> You don't need to break the cycles to call the finalizers. Just call 
>> them, then collect the whole cycle (assuming it is still unreachable).
>> The GC will *never* reclaim a reachable object. Objects awaiting 
>> finalization are reachable, by definition.
> Well it was sloppily worded, so replace it with:
>      if a finalizer attempts to access an already finalized object.

A finalized object will still be a valid object.
Python code cannot make an object unsafe.
Obviously C code can make it unsafe, but that's true of C code anywhere.

For example, a file object will close itself during finalization,
but its still a valid object, just a closed file rather than an open one.
> Michael
>>> Michael
>>>> It may not make a difference for the runtime, but the difference for 
>>>> user software may be "dead" or "alive".
>>>> Stefan
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