[Python-Dev] reference leaks, __del__, and annotations

[Thomas Wouters]

On 4/1/06, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

> I'm becoming more and more convinced that we
> desperately need something better than __del__
> methods to do finalisation. A garbage collector
> that can't be relied upon to collect garbage
> is simply not acceptable.


Sure. I don't believe it's too hard, it just means violating some of the
premises people have been writing __del__ methods under. For instance, to
clean up cycles nicely we might have to set some attributes to None before
calling __del__, so you can't rely on attributes being meaningful anymore.
However, this is already the case for global names; I've seen many people
wonder about their __del__ method raising warnings (since exceptions are
ignored) going, say, 'NoneType has no attribute 'registry'' when they try to
un-register their class but the global registry has been cleaned up already.
While we're at it, I would like for the new __del__ (which would probably
have to be a new method) to disallow reviving self, just because it makes it
unnecessarily complicated and it's rarely needed. Allowing a partially
deleted object (or an object part of a partially deleted reference-cycle) to
revive itself is not terribly useful, and there's no way to restore the rest
of the cycle. I suggested a __dealloc__ method earlier in the thread to do
this. I didn't think of allowing attributes to be cleared before calling the
method, but I do believe that is necessary to allow, now that I've thought
more about it.

An alternative would be to make GC check for a 'cleanup-cycle' method on any
of the objects in the cycle, and just feed it the complete cycle of objects,
asking it to clean it up itself (or maybe reconnect one of the objects
itself.) That would also make debugging uncollectable cycles a lot easier
;-) But I'm not sure whether that will improve things. The generator
example, the trigger for this discussion, could solve its cycle by just
closing itself, after which the cycle is either broken or reconnected, but I
don't know if other typical cycles could be resolved that easily.

--
Thomas Wouters <thomas at python.org>

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