[Python-Dev] Marking GC details as CPython-only

[Maciej Fijalkowski]

On Wed, Feb 13, 2013 at 9:09 PM, Xavier Morel <python-dev at masklinn.net> wrote:
> On 2013-02-13, at 19:48 , Maciej Fijalkowski wrote:
>
>> Hi
>>
>> I've tried (and failed) to find what GC details (especially finalizer
>> semantics) are CPython only and which ones are not. The best I could
>> find was the documentation of __del__ here:
>> http://docs.python.org/2/reference/datamodel.html
>>
>> Things were pypy differs:
>>
>> * finalizers in pypy will be called only once, even if the object is
>> resurrected. I'm not sure if this is detail or we're just plain
>> incompatible.
>>
>> * pypy breaks cycles and runs finalizers in random order (but
>> topologically correct), hence gc.garbage is always empty. I *think*
>> this part is really just an implementation detail
>>
>> * we're discussing right now about running multiple finalizers. We
>> want to run them in order, but if there is a link a -> b and a becomes
>> unreachable, we want to reserve the right to call finalizer a then
>> finalizer b, even if a.__del__ resurrects a. What do you think?
>>
>> Overall, the __del__ is baaad.
>>
>> Cheers,
>> fijal
>
> There may be one more, although I'm not sure whether it's a GC artifact
> or something completely unspecified: if a context manager is part of a
> suspended stack (because it's in a generator) when the program
> terminates, cpython will run __exit__ but pypy will not
>
> --
> # -*- encoding: utf-8 -*-
> class C(object):
>     def __enter__(self):
>         print ("entering")
>     def __exit__(self, *args):
>         print ("exiting")
>
> def gen():
>     with C():
>         yield
>
> r = gen()
> next(r)
> --
> $ python2 test.py
> entering
> exiting
> $ python3 test.py
> entering
> exiting
> $ pypy test.py
> entering
> $
> --
>
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I think it's well documented you should not rely on stuff like that
being run at the exit of the interpreter. I think we'll try harder to
run finalizers at the end of the interpreter (right now we only flush
files). File the issue.