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

Ethan Furman ethan at stoneleaf.us
Tue Jun 7 21:20:38 EDT 2016

On 06/07/2016 05:50 PM, Eric Snow wrote:

Overall +1.  Some nits below.

> Specification
> =============

>    3. types for which `__prepare__()`` returned something other than
>       ``OrderedDict`` (or a subclass) have their ``__definition_order__``
>       set to ``None``

        (unless ``__definition_order__`` is present in
        the class dict either by virtue of being in the class body or
        because the metaclass inserted it before calling

>         __definition_order__ = tuple(k for k in locals()
>                                      if (!k.startswith('__') or
>                                          !k.endswith('__')))

Still mixing C and Python!  ;)

> Why a tuple?
> ------------
> Use of a tuple reflects the fact that we are exposing the order in
> which attributes on the class were *defined*.  Since the definition
> is already complete by the time ``definition_order__`` is set, the
> content and order of the value won't be changing.  Thus we use a type
> that communicates that state of immutability.

> Why a read-only attribute?
> --------------------------
> As with the use of tuple, making ``__definition_order__`` a read-only
> attribute communicates the fact that the information it represents is
> complete.  Since it represents the state of a particular one-time event
> (execution of the class definition body), allowing the value to be
> replaced would reduce confidence that the attribute corresponds to the
> original class body.
> If a use case for a writable (or mutable) ``__definition_order__``
> arises, the restriction may be loosened later.  Presently this seems
> unlikely and furthermore it is usually best to go immutable-by-default.

If __definition_order__ is supposed to be immutable as well as read-only
then we should convert non-tuples to tuples.  No point in letting that
user bug slip through.

> Why ignore "dunder" names?
> --------------------------
> Names starting and ending with "__" are reserved for use by the
> interpreter.  In practice they should not be relevant to the users of
> ``__definition_order__``.  Instead, for early everyone they would only


> Why is __definition_order__ even necessary?
> -------------------------------------------
> Since the definition order is not preserved in ``__dict__``, it would be
> lost once class definition execution completes.  Classes *could*
> explicitly set the attribute as the last thing in the body.  However,
> then independent decorators could only make use of classes that had done
> so.  Instead, ``__definition_order__`` preserves this one bit of info
> from the class body so that it is universally available.

s/would be/is


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