[Python-Dev] Preserving the definition order of class namespaces.

Eric Snow ericsnowcurrently at gmail.com
Sun May 24 22:36:25 CEST 2015

On May 24, 2015 3:35 AM, "Nick Coghlan" <ncoghlan at gmail.com> wrote:
> Is it specifically necessary to save the order by default? Metaclasses
> would be able to access the ordered namespace in their __new__ method
> regardless, and for 3.6, I still like the __init_subclass__ hook idea
> proposed in PEP 487, which includes passing the original namespace to
> the new hook.
> So while I'm sold on the value of making class execution namespaces
> ordered by default, I'm not yet sold on the idea of *remembering* that
> order without opting in to doing so in the metaclass.
> If we leave __definition_order__ out for the time being then, for the
> vast majority of code, the fact that the ephemeral namespace used to
> evaluate the class body switched from being a basic dictionary to an
> ordered one would be a hidden implementation detail, rather than
> making all type objects a little bigger.

It's too late for 3.5 to negotiate much so I'll try to make my case here
for __definition_order__ one last time.   If that's not sufficient then
I'll defer further discussion to 3.6.

My premise for storing the definition order on the class is that Guido was
okay with using OrderedDict for cls.__dict__, which is a bigger change.
Regardless, there are two reasons why it makes sense:

* If it makes sense to use OrderedDict by default for class definition then
it makes sense to preserve the extra information OrderedDict provides.
* As I noted at the beginning of the thread, you could still preserve that
info manually, but that makes it less convenient for library authors.

If you still think that's not enough justification then we can table
__definition_order__ for now.

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