> From: Eric Snow
> Sent: Tuesday, June 07, 2016 1:52 PM
> To: Python-Dev
> Subject: [Python-Dev] PEP: Ordered Class Definition Namespace
>
>
> Currently the namespace used during execution of a class body defaults
> to dict. If the metaclass defines ``__prepare__()`` then the result of
> calling it is used. Thus, before this PEP, if you needed your class
> definition namespace to be ``OrderedDict`` you had to use a metaclass.
Formatting nit: ``dict``
> Specification
> =============
>
> * the default class *definition* namespace is now ``OrderdDict``
> * the order in which class attributes are defined is preserved in the
> new ``__definition_order__`` attribute on each class
> * "dunder" attributes (e.g. ``__init__``, ``__module__``) are ignored
What does this imply? If I define some __dunder__ methods, will they simply
not be present in __definition_order__? What if I want to keep the order of
those? While keeping the order of these might be meaningless in most cases,
I don't think there's really a huge problem in doing so. Maybe I'm
overthinking it.
> * ``__definition_order__`` is a tuple
> * ``__definition_order__`` is a read-only attribute
> * ``__definition_order__`` is always set:
>
> * if ``__definition_order__`` is defined in the class body then it
> is used
> * types that do not have a class definition (e.g. builtins) have
> their ``__definition_order__`` set to ``None``
> * types for which `__prepare__()`` returned something other than
> ``OrderedDict`` (or a subclass) have their ``__definition_order__``
> set to ``None``
I would probably like a ``type.definition_order`` method, for which the
return value is bound to __definition_order__ when the class is created
(much like the link between ``type.mro`` and ``cls.__mro__``. Additionally
I'm not sure if setting the attribute to None is a good idea; I'd have it as
an empty tuple. Then again I tend to overthink a lot.
> The following code demonstrates roughly equivalent semantics::
>
> class Meta(type):
> def __prepare__(cls, *args, **kwargs):
> return OrderedDict()
>
> class Spam(metaclass=Meta):
> ham = None
> eggs = 5
> __definition_order__ = tuple(k for k in locals()
> if (!k.startswith('__') or
> !k.endswith('__')))
Mixing up C and Python syntax here.
> However, using ``OrderedDict`` for ``type,__dict__`` would obscure the
> relationship with the definition namespace, making it less useful.
> Additionally, doing this would require significant changes to the
> semantics of the concrete dict C-API.
Formatting nit: ``dict``
I'm +1 on the whole idea (one of my common uses of metaclasses was to keep
the definition order *somewhere*). Thank you for doing that!
-Emanuel
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