[Python-Dev] Re: More fun with Python shutdown

[Greg Ewing]

Tim:

> - weakref callbacks are easier to live with than __del__ methods in
>   one (and maybe only one) respect:  when the death of X triggers
>   a weakref callback C, C isn't passed X, but X.__del__ is.  So a
>   weakref callback can't resurrect X, but X.__del__ can.

The object causing trouble doesn't need to be the one that died,
e.g. doing a tp_clear on X causes Y to die which triggers a weakref
callback which references X by some route. Resurrection of X isn't an
issue, because it's not dead yet -- it is, however, in the process of
being indiscriminately torn apart by the GC, messing up who-knows-what
invariant that the callback might be relying on.

So I can't see that the lack of possibility of resurrection helps
much at all.

> e.g., if it were always true that refcount-dropped-to-0 implies
> can't-be-touched-again-by-Python-code, then what is it that gets
> passed to x.__del__()?  x does -- but we cheat

But (I hope, at least!) it's guaranteed that the x.__del__()
call is completed before any of the C-level deallocation code
for x is begun...

Greg Ewing, Computer Science Dept, +--------------------------------------+
University of Canterbury,	   | A citizen of NewZealandCorp, a	  |
Christchurch, New Zealand	   | wholly-owned subsidiary of USA Inc.  |
greg at cosc.canterbury.ac.nz	   +--------------------------------------+