[Python-Dev] PEP 487: Simpler customization of class creation

[Tim Delaney]

On 21 June 2016 at 06:12, Guido van Rossum <guido at python.org> wrote:

> OK, basically you're arguing that knowing the definition order of class
> attributes is often useful when (ab)using Python for things like schema or
> form definitions. There are a few ways to go about it:
>
> 1. A hack using a global creation counter
> <https://github.com/GoogleCloudPlatform/datastore-ndb-python/blob/master/ndb/model.py#L888>
> 2. Metaclass with __prepare__
> <https://docs.python.org/3/reference/datamodel.html#prepare>
> 3. PEP 520 <https://www.python.org/dev/peps/pep-0520/>
> 4a. Make all dicts OrderedDicts in CPython
> <http://bugs.python.org/issue27350>
> 4b. Ditto in the language standard
>
> If we can make the jump to (4b) soon enough I think we should skip PEP
> 520; if not, I am still hemming and hawing about whether PEP 520 has enough
> benefits over (2) to bother.
>
> Sorry Eric for making this so hard. The better is so often the enemy of
> the good. I am currently somewhere between -0 and +0 on PEP 520. I'm not
> sure if the work on (4a) is going to bear fruit in time for the 3.6
> feature freeze <https://www.python.org/dev/peps/pep-0494/#schedule>; if
> it goes well I think we should have a separate conversation (maybe even a
> PEP?) about (4b). Maybe we should ask for feedback from the Jython
> developers? (PyPy already has this IIUC, and IronPython
> <https://github.com/IronLanguages/main> seems moribund?)
>

Although not a Jython developer, I've looked into the code a few times.

The major stumbling block as I understand it will be that Jython uses a
ConcurrentHashMap as the underlying structure for a dictionary. This would
need to change to a concurrent LinkedHashMap, but there's no such thing in
the standard library. The best option would appear to be
https://github.com/ben-manes/concurrentlinkedhashmap.

There are also plenty of other places that use maps and all of them would
need to be looked at. In a lot of cases they're things like IdentityHashMap
which may also need an ordered equivalent.

There is a repo for Jython 3.5 development:
https://github.com/jython/jython3 but it doesn't seem to be very active -
only 11 commits in the last year (OTOH that's also in the last 3 months).

Tim Delaney
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