Re: does python have useless destructors?

From: Ype Kingma (ykingma_at_accessforall.nl)
Date: 06/15/04 

Date: Tue, 15 Jun 2004 23:20:15 +0200

David Turner wrote:

> "Delaney, Timothy C (Timothy)" <tdelaney@avaya.com> wrote in message
news:<mailman.957.1087256687.6949.python-list@python.org>...
>> David Turner wrote:
>>
>> > This will be a pain for the Jython implementers. However, it is
>> > doable. Note that I never said the objects couldn't be garbage
>> > collected, just that del had to be called at certain well-defined
>> > times. What this will involve is the Jython compiler inserting a lot
>> > of implicit try/finally constructs.
>> >
>> > Can anyone see a reason why this scheme wouldn't work?
>>
>> Yes - Jython cannot know about references to objects created *in Java
>> code*. It is therefore impossible for Jython to maintain reference
>> counts when an object is passed to Java (i.e. non-Python) code.
>>
>
> Thank you, Tim, that's the first substantial objection I've heard :-).
>
> So, if Java code makes use of a Python object with __del__, all bets
> are off. This is certainly a limitation, but I'm not sure that it's a
> very serious one, as RAII objects tend to be used in localized
> contexts.

It's not that all bets are off. In Jython the __del__() method is called
from java.lang.Object.finalize(), so you can place your bets there:
http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html#finalize()

This javadoc begins with:
"Called by the garbage collector on an object when garbage collection
determines that there are no more references to the object."

The closest thing to a guarantee is to explicitly call the garbage
collector:
http://java.sun.com/j2se/1.4.2/docs/api/java/lang/System.html#gc()

>From Jython:

import java
java.lang.System.gc()

I wouldn't bet against this, ie. the last time I tried the first call
to this gc() called all __del__() methods instantly on non referenced
Python objects.

>
> I think the only available behaviour is simply to ignore the
> possibility of an object being referenced externally. Let Java code
> use Java idioms (try/finally).

For the moment, try/finally should also be a CPython idiom because
the call to __del__() is not guaranteed there either.

Kind regards,
Ype
  

-- 
email at xs4all.nl

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