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Thu Nov 4 19:38:38 CET 2004 · https://mail.python.org/pipermail/python-dev/2004-November/049781.html

[Jim Fulton]
> Treating weakrefs with callbacks as reachable is a *big* change, though
> in subtle ways.

It sure appears to have "more consequences" than I guessed it would.

> I agree that this is too big for 2.4, but I *do* think it should be
> considered for 2.5.

Me too.  It ensures that a callback is never ignored unless a referent
definitely outlives a weakref with that callback, even at the cost of
resurrecting trash weakrefs (to make their callbacks safe to call).
"Definitely outlives" is usually easy to talk about in CPython because of
refcount lifetime semantics, but hard to make sense of operationally without
refcount semantics.  When they're both in cyclic trash, a weakref and its
referent both become unreachable truly at the same instant.  The approach so
far has then been to say (in effect):

    Fine, then it's legitimate to impose any linear order on death, and
    if we say the weakref dies first, then there's no issue remaining --
    just as in non-cyclic trash, when a weakref dies before its
    referent, the wr's callback will never be invoked.

That's defensible but not compelling.  I'm still not sure any other approach
is compelling either.  patch-finalizer effectly reverses the current
approach, saying:

    Fine, then it's legitimate to impose any linear order on death, and
    if we say the referent dies first, then there's no issue remaining --
    just as in non-cyclic trash, when the referent dies before its
    weakref, the wr's callback will be invoked(*).

    (*) Except we have to resurrect the weakref to make this possible, and
        that may also end up resurrecting the referent, and then neither
        actually goes away, and so the callback doesn't actually get
        invoked either (although it may get invoked later, if the user
        fiddles with the stuff in gc.garbage enough to make that
        possible).

All I can say is that the callback better be damned important to justify all
the extra pain <wink>.

still-seeking-use-cases-ly y'rs  - tim

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[Python-Dev] RE: patch-finalizer vs Zope3