[Python-Dev] Finalizers considered questionable ;)

[Jim Fulton]

Here's my $0.02.

I agree with the sentiments that use of finalizers 
should be discouraged.  They are extremely helpful
in cases like tempfile.TemporaryFileWrapper, so I
think that they should be supported. I do think that
the language should not promise a high level of service.

Some observations:

  - I spent a little bit of time on the ANSI 
    Smalltalk committee, where I naively advocated
    adding finalizers to the language. I was resoundingly
    told no. :)

  - Most of the Python objects I deal with these days
    are persistent. Their lifetimes are a lot more complicated
    that most Python objects.  They get created once, but they
    get loaded into and out of memory many times.  In fact, they
    can be in memory many times simultaneously. :) A couple
    of years ago I realized that it only made sense to call
    __init__ when an object was first created, not when it is
    subsequently (re)loaded into memory.  This led to a 
    change in Python pickling semantics and the deprecation
    of the loathsome __getinitargs__ protocol. :)

    For me, a similar case can be made against use of __del__
    for persistent objects.  For persistent objects, a __del__
    method should only be used for cleaning up the most volatile
    of resources. A persistent object __del__ should not perform
    any semantically meaningful operations because __del__ has 
    no semantic meaning.

  - Zope has a few uses of __del__. These are all for
    non-persistent objects. Interesting, in grepping for __del__,
    I found a lot of cases where __del__ was used and then commented 
    out.  Finalizers seem to be the sort of thing that people
    want initially and then get over.

I'm inclined to essentially keep the current rules and
simply not promise that __del__ will be able to run correctly.
That is, Python should call __del__ and ignore exceptions raised
(or provide some *optional* logging or other debugging facility).
There is no reason for __del__ to fail unless it depends on
cyclicly-related objects, which should be viewed as a design
mistake.

OTOH, __del__ should never fail because module globals go away. 
IMO, the current circular references involving module globals are
unnecessary, but that's a different topic. ;)

Jim

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