[Python-Dev] PEP: Ordered Class Definition Namespace

Eric Snow ericsnowcurrently at gmail.com
Tue Jun 7 14:51:34 EDT 2016

On Tue, Jun 7, 2016 at 11:36 AM, Émanuel Barry <vgr255 at live.ca> wrote:
>> From: Eric Snow
>> * "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.

"dunder" names (not just methods) will not be present in
__definition_order__.  I'll add an explanation to the PEP.  The gist
of it is that they are reserved for use by the interpreter and will
always clutter up __definition_order__.  Since needing dunder names
included in __definition_order__ would be rather exceptional, and
there are other options available, leaving them out by default is a
matter of practicality.

>> * ``__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__``.

What is the value of type.definition_order()?  If you need a mutable
copy then pass __definition_order__ to list().

> 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.

None indicates that there is no order.  An empty tuple indicates that
there were no attributes.

>>        __definition_order__ = tuple(k for k in locals()
>>                                     if (!k.startswith('__') or
>>                                         !k.endswith('__')))
> Mixing up C and Python syntax here.

nice catch :)

> 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!



More information about the Python-Dev mailing list