[Python-Dev] [Python-checkins] cpython: Add reference implementation for PEP 443

[PJ Eby]

On Fri, Jun 7, 2013 at 5:16 PM, Łukasz Langa <lukasz at langa.pl> wrote:
> On 7 cze 2013, at 22:50, PJ Eby <pje at telecommunity.com> wrote:
>
>> On Fri, Jun 7, 2013 at 10:27 AM, Thomas Wouters <thomas at python.org> wrote:
>>> This isn't a new bug, but it's exposed by always importing weakref and
>>> atexit during interpreter startup. I'm wondering if that's really necessary
>>> :)
>>
>> In short, the overall answer right now is, "maybe", and the answer
>> later is "rather likely".  ;-)
>
> I would rather say that it's "rather certain".
>
> functools is necessary for setup.py to work while bootstrapping, whereas
> pkgutil is used in runpy.py which is always imported in Modules/main.c.
>
> So we're left with having to fix atexit to support subinterpreters. I wonder
> how difficult that will be.

If the problem really has to do with interpreter startup, then there
actually is a workaround possible, at the cost of slightly hairier
code.

If dispatch() looked in the registry *first* and avoided the cache in
that case, and lazily created the cache (including the weakref
import), then interpreter startup would not trigger an import of
weakref in the default case.

(Of course, depending on whether site/sitecustomize results in the use
of importer subclasses and such, this might not help.  It might be
necessary to take the even more complex tack of avoiding the use of a
cache entirely until an ABC is registered, and walking mro's.)

Anyway, it remains to be seen whether these workarounds are easier or
more difficult than fixing the atexit problem.  ;-)

Hmm...  actually, there are a couple other ways around this.
singledispatch doesn't use finalize(), so it doesn't really need
atexit.  It doesn't even do much with WeakKeyDictionary, so it could
actually just "from _weakref import ref", and inline the relevant
operations.

Or, WeakKeyDictionary could be pulled out into a separate module,
where singledispatch could pull it from without importing finalize.

Or, weakref.finalize could be fixed so that the atexit import and
register() are deferred until actual use.

(Of all of these, that last one actually sounds like the least
invasive workaround, with fewest lines of code likely to be changed.)