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Re: REMOVE ME

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  Re: REMOVE ME John Pote <johnhpote@o2.co.uk> - 2016-04-11 23:44 +0100
    Re: REMOVE ME Steven D'Aprano <steve@pearwood.info> - 2016-04-12 09:10 +1000
      Re: REMOVE ME Chris Angelico <rosuav@gmail.com> - 2016-04-12 10:44 +1000

#106890 — Re: REMOVE ME

On 10/04/2016 04:52, Chris Angelico wrote:
> On Sun, Apr 10, 2016 at 1:46 PM, fan nie <niefan68@gmail.com> wrote:
>> --
>> https://mail.python.org/mailman/listinfo/python-list
> Sure. I presume you mean something like this:
>
> class Thing:
>      things = []
>      def __init__(self):
>          self.things.append(self)
>      def __del__(self):
>          # Remove me when I'm dead
>          self.things.remove(self)
>
> This ingenious technique guarantees that you never have dead objects
> in your list, by having each object remove itself when it dies.
>
> ChrisA
I'm not quite sure how tongue in cheek ChrisA's reply and the Thing 
class was but it did make me think and wonder if my understanding of 
Python lore was quite right. To my mind it looks like a Zombie or even 
Revenant class.

If all external references to an instance of Thing are deleted there is 
still the reference from the class list 'things []'. In which case will 
not this prevent the instance from being garbage collected and __del__() 
never called on the dead instance and its memory never released. But a 
memory dump could reveal the ghost of the instance. On the other hand a 
code wizard with the right lore could resurect the instance by getting 
the reference to it and bring it back to life -
     revenant = Thing.things[x]

But then I notice 'things' is used as an instance attribute rather than 
a class attribute. All seems to be shrouded in a web of mystery

Regards,
John

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#106892

On Tue, 12 Apr 2016 08:44 am, John Pote wrote:

> On 10/04/2016 04:52, Chris Angelico wrote:
>> On Sun, Apr 10, 2016 at 1:46 PM, fan nie <niefan68@gmail.com> wrote:
>>> --
>>> https://mail.python.org/mailman/listinfo/python-list
>> Sure. I presume you mean something like this:
>>
>> class Thing:
>>      things = []
>>      def __init__(self):
>>          self.things.append(self)
>>      def __del__(self):
>>          # Remove me when I'm dead
>>          self.things.remove(self)
>>
>> This ingenious technique guarantees that you never have dead objects
>> in your list, by having each object remove itself when it dies.
>>
>> ChrisA
> I'm not quite sure how tongue in cheek ChrisA's reply and the Thing
> class was 

I'm sure it was extremely tongue in cheek.


> but it did make me think and wonder if my understanding of 
> Python lore was quite right. To my mind it looks like a Zombie or even
> Revenant class.
> 
> If all external references to an instance of Thing are deleted there is
> still the reference from the class list 'things []'. In which case will
> not this prevent the instance from being garbage collected and __del__()
> never called on the dead instance and its memory never released. 

Correct.


> But a 
> memory dump could reveal the ghost of the instance. On the other hand a
> code wizard with the right lore could resurect the instance by getting
> the reference to it and bring it back to life -
>      revenant = Thing.things[x]
> 
> But then I notice 'things' is used as an instance attribute rather than
> a class attribute. All seems to be shrouded in a web of mystery

Your first analysis was correct. "self.things" is actually a class
attribute, despite being accessed from "self". The reason being, attribute
lookups look for:

- instance attributes;
- class attributes;
- superclass attributes;

in that order. (This is actually an extremely simplified view, but close
enough for what we're discussing.) Hence "self.things" will return the same
list each time, the one defined as a class attribute, regardless of
which "self" does the lookup.

If a method were to assign to the attribute, for example "self.things = []",
that would create an instance attribute, but that doesn't happen.



-- 
Steven

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#106897

On Tue, Apr 12, 2016 at 9:10 AM, Steven D'Aprano <steve@pearwood.info> wrote:
> Hence "self.things" will return the same
> list each time, the one defined as a class attribute, regardless of
> which "self" does the lookup.
>
> If a method were to assign to the attribute, for example "self.things = []",
> that would create an instance attribute, but that doesn't happen.

Indeed. And the __del__ method is guaranteed always to raise an
exception; by the time __del__ gets called, there can't be any
references to the object anywhere, so it can't be in that list any
more...

ChrisA

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