[Python-Dev] Combining the best of PEP 288 and PEP 325: generator exceptions and cleanup

[Tim Peters]

[Phillip J. Eby]
> Now you've shaken my faith in Uncle Timmy.  :)

Now, now, a mere technical matter is no cause for soul-damning heresy!

>  Seriously, he did *say*:
> 
> """For example, it doesn't know the difference between an object
> that's in a trash cycle, and an object that's not in a trash cycle but
> is reachable only from trash cycles."""
>
> So now I wonder what he *did* mean.

What I said, of course ;-)  I hope my later email clarified it.  gc
knows which trash objects have __del__ methods, and which don't. 
That's all it needs to know so that a __del__ method on an object
that's not in a trash cycle but is reachable only from trash cycles
will get reclaimed (provided that no __del__ method on a predecessor
object that's not in a trash cycle but is reachable only from trash
cycles resurrects it).  gc doesn't know whether the set of objects it
_intends_ to call tp_clear on are or aren't in cycles, but all objects
directly in __del__ free cycles are included in that set.  That's
enough to ensure that trash "hanging off of them" sees its refcounts
fall to 0 as gc applies tp_clear to the objects in that set.  Note
that this set can mutate as gc goes along:  calling tp_clear on one
object can (also as a side effect of refcounts falling to 0) remove
any number of other objects from that set (that's why I said "intends"
above:  there's no guarantee that gc will end up calling tp_clear on
any object other than "the first" one in the set, where "the first" is
utterly arbitrary now).

If an object in a trash cycle has a __del__ method, this is why the
cycle won't be reclaimed:  all trash objects with __del__ methods, and
everything transitively reachable from them, are moved to the
"finalizers" list early on.  If that happens to include a trash cycle
C, then all of C ends up in the "finalizers" list, and no amount of
tp_clear'ing on the objects that remain can cause the refcount on any
object in C to fall to 0.  gc has no direct knowledge of cycles in
this case either.