[Python-Dev] PEP 520: Ordered Class Definition Namespace

Nikita Nemkin nikita at nemkin.ru
Thu Jun 16 07:11:33 EDT 2016

I'll reformulate my argument:

Ordered class namespaces are a minority use case that's already covered
by existing language features (custom metaclasses) and doesn't warrant
the extension of the language (i.e. making OrderedDict a builtin type).
This is about Python-the-Language, not CPython-the-runtime.

If you disagree with this premise, there's no point arguing about
the alternatives. That being said, below are the answers to your objections
to specific alternatives.

On Thu, Jun 16, 2016 at 1:30 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 14 June 2016 at 02:41, Nikita Nemkin <nikita at nemkin.ru> wrote:
> Adding metaclasses to an existing class can break compatibility with
> third party subclasses, so making it possible for people to avoid that
> while still gaining the ability to implicitly expose attribute
> ordering to class decorators and other potentially interested parties
> is a recurring theme behind this PEP and also PEPs 422 and 487.

The simple answer is "don't do that", i.e. don't pile an ordered metaclass
on top of another metaclass. Such use case is hypothetical anyway.

Also, namespace argument to the default metaclass doesn't cause conflicts.

>> 3. Making compiler fill in __definition_order__ for every class
>>     (just like __qualname__) without touching the runtime.
>> ?
> Class scopes support conditionals and loops, so we can't necessarily
> be sure what names will be assigned without running the code. It's
> also possible to make attribute assignments via locals() that are
> entirely opaque to the compiler, but visible to the interpreter at
> runtime.

All explicit assignments in the class body can be detected statically.
Implicit assignments via locals(), sys._frame() etc. can't be detected,
BUT they are unlikely to have a meaningful order!
It's reasonable to exclude them from __definition_order__.

This also applies to documentation tools. If there really was a need,
they could have easily extracted static order, solving 99.9999% of
the problem.

> The rationale for "Why not make this configurable, rather than
> switching it unilaterally?" is that it's actually *simpler* overall to
> just make it the default - we can then change the documentation to say
> "class bodies are evaluated in a collections.OrderedDict instance by
> default" and record the consequences of that, rather than having to
> document yet another class customisation mechanism.

It would have been a "simpler" default if it was the core dict that
became ordered. Instead, it brings in a 3rd party (OrderedDict).

Documenting an extra metaclass or an extra type kward would hardly
take more space. And it's NOT yet another mechanism. It's the good old
metaclass mechanism.

> It also eliminates boilerplate from class decorator usage
> instructions, where people have to write "to use this class decorator,
> you must also specify 'namespace=collections.OrderedDict' in your
> class header"

Statically inferred __definition_order__ would work here.
Order-dependent decorators don't seem to be important enough
to worry about their usability.

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