[Python-Dev] finalization again

Tim_Peters@Dragonsys.com Tim_Peters@Dragonsys.com
Tue, 7 Mar 2000 17:30:16 -0500

> Tim tells Guido again that he finds the Java rules bad, slinging some
> mud at Guy Steele, but without explaining what the problem with them
> is ...

Slinging mud?  Let's back off here.  You've read the Java spec and were
impressed.  That's fine -- it is impressive <wink>.  But go on from
there and see where it leads in practice.  That Java's GC model did a
masterful job but includes a finalization model users dislike is really
just conventional wisdom in the Java world.  My sketch of Guy Steele's
involvement was an attempt to explain why both halves of that are valid.

I didn't think "explaining the problem" was necessary, as it's been
covered in depth multiple times in c.l.py threads, by Java programmers
as well as by me.  Searching the web for articles about this turns up
many; the first one I hit is typical:


eventually concludes

    Consequently we recommend that [Java] programmers support but do
    not rely on finalization. That is, place all finalization semantics
    in finalize() methods, but call those methods explicitly and in the
    order required.  The points below provide more detail.

That's par for the Java course:  advice to write finalizers to survive
being called multiple times, call them explicitly, and do all you can
to ensure that the "by magic" call is a nop.  The lack of ordering
rules in the language forces people to "do it by hand" (as the Java
spec acknowledges: "It is straightforward to implement a Java class
that will cause a set of finalizer-like methods to be invoked in a
specified order for a set of objects when all the objects become
unreachable. Defining such a class is left as an exercise for the
reader."  But from what I've seen, that exercise is beyond the
imagination of most Java programmers!  The perceived need for ordering
is not.).

It's fine that you want to restrict finalizers to "simple" cases; it's
not so fine if the language can't ensure that simple cases are the only
ones the user can write, & can neither detect & complain at runtime
about cases it didn't intend to support.  The Java spec is unhelpful
here too:

   Therefore, we recommend that the design of finalize methods be kept
   simple and that they be programmed defensively, so that they will
   work in all cases.

Mom and apple pie, but what does it mean, exactly?  The spec realizes
that you're going to be tempted to try things that won't work, but
can't really explain what those are in terms simpler than the full set
of implementation consequences.  As a result, users hate it -- but
don't take my word for that!  If you look & don't find that Java's
finalization rules are widely viewed as "a problem to be wormed around"
by serious Java programmers, fine -- then you've got a much better
search engine than mine <wink>.

As for why I claim following topsort rules is very likely to work out
better, they follow from the nature of the problem, and can be
explained as such, independent of implementation details.  See the
Boehm reference for more about topsort.

will-personally-use-python-regardless-ly y'rs  - tim