[Python-ideas] Why not break cycles with one __del__?

[Guido van Rossum]

On Mon, Sep 13, 2010 at 8:04 PM, Tim Peters <tim.peters at gmail.com> wrote:
> [Nick Coghlan]
>>> Alternatively, when building finalizers, build two
>>> lists: one for objects with __del__ methods and one for objects that
>>> are reachable from objects with __del__ methods.
>
> [Greg Ewing]
>> But since it's a cycle, isn't *everything* in the cycle
>> going to be reachable from everything else?
>
> Note that I was sloppy in saying that CPython's cyclic gc only sees
> trash objects in cycles.  More accurately, it sees trash objects in
> cycles, and objects (which may or may not be in cycles) reachable only
> from trash objects in cycles.  For example, if objects A and B point
> to each other, that's a cycle.  If A also happens to point to D, where
> D has a __del__ method, and nothing else points to D, then that's a
> case where D is not in a cycle, but is nevertheless trash if A and B
> are trash.  And if A and B lack finalizers, then CPython's cyclic gc
> will reclaim D, despite that it does have a __del__.
>
> That pattern is exploitable too.  If, e.g., you have some resource R
> that needs to be cleaned up, owned by an object A that may participate
> in cycles, it's often possible to put R in a different, very simple
> object with a __del__ method, and have A point to that latter object
> instead.

Yeah, I think we even recommended this pattern at some point. ISTR we
designed the new io library to exploit it.

-- 
--Guido van Rossum (python.org/~guido)