[Python-Dev] finalization again

[M.-A. Lemburg]

> [Guido]
> The question remains, what to do with trash cycles?  I find having a
> separate __cleanup__ protocol cumbersome.  I think that the "finalizer
> only called once by magic" rule is reasonable.  I believe that the
> ordering problems will be much less than in Java, because we use
> topsort whenever we can.

Note that the __cleanup__ protocol is intended to break cycles
*before* calling the garbage collector. After those cycles are broken,
ordering is not a problem anymore and because __cleanup__ can
do its task on a per-object basis all magic is left in the hands
of the programmer.

The __cleanup__ protocol as I use it is designed to be called
in situations where the system knows that all references into
a cycle are about to be dropped (I typically use small cyclish
object systems in my application, e.g. ones that create and
reference namespaces which include a reference to the hosting
object itself). In my application that is done by using mxProxies
at places where I know these cyclic object subsystems are being
referenced. In Python the same could be done whenever the
interpreter knows that a certain object is about to be
deleted, e.g. during shutdown (important for embedding Python
in other applications such as Apache) or some other major
subsystem finalization, e.g. unload of a module or killing
of a thread (yes, I know these are nonos, but they could
be useful, esp. the thread kill operation in multi-threaded
servers).

After __cleanup__ has done its thing, the finalizer can either
choose to leave all remaining cycles in memory (and leak) or
apply its own magic to complete the task. In any case, __del__
should be called when the refcount reaches 0. (I find it somewhat
strange that people are argueing to keep external resources
alive even though there is a chance of freeing them.)

-- 
Marc-Andre Lemburg
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