Focus PEP 520 on __definition_order__. (#27)

Author: ericsnowcurrently

pep-0520.txt

@@ -1,5 +1,5 @@
 PEP: 520
-Title: Ordered Class Definition Namespace
+Title: Preserving Class Attribute Definition Order
 Version: $Revision$
 Last-Modified: $Date$
 Author: Eric Snow <[email protected]>
@@ -8,7 +8,7 @@ Type: Standards Track
 Content-Type: text/x-rst
 Created: 7-Jun-2016
 Python-Version: 3.6
-Post-History: 7-Jun-2016, 11-Jun-2016
+Post-History: 7-Jun-2016, 11-Jun-2016, 20-Jun-2016
 
 
 Abstract
@@ -20,44 +20,50 @@ namespace is copied into new ``dict`` and the original definition
 namespace is discarded.  The new copy is stored away as the class's
 namespace and is exposed as ``__dict__`` through a read-only proxy.
 
-This PEP changes the default class definition namespace to ``OrderedDict``.
-The long-lived class namespace (``__dict__``) will remain a ``dict``.
-Furthermore, the order in which the attributes are defined in each class
-body will now be preserved in the ``__definition_order__`` attribute of
-the class.  This allows introspection of the original definition order,
+This PEP preserves the order in which the attributes in the definition
+namespace were added to it, before that namespace is discarded.  This
+means it reflects the definition order of the class body.  That order
+will now be preserved in the ``__definition_order__`` attribute of the
+class.  This allows introspection of the original definition order,
 e.g. by class decorators.
 
+Additionally, this PEP changes the default class definition namespace
+to ``OrderedDict``.  The long-lived class namespace (``__dict__``) will
+remain a ``dict``.
+
 
 Motivation
 ==========
 
-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.
-
-Metaclasses introduce an extra level of complexity to code and in some
-cases (e.g. conflicts) are a problem.  So reducing the need for them is
-worth doing when the opportunity presents itself.  Given that we now have
-a C implementation of ``OrderedDict`` and that ``OrderedDict`` is the
-common use case for ``__prepare__()``, we have such an opportunity by
-defaulting to ``OrderedDict``.
-
-The usefulness of ``OrderedDict``-by-default is greatly increased if the
-definition order is directly introspectable on classes afterward,
-particularly by code that is independent of the original class definition.
+Currently Python does not preserve the order in which attributes are
+added to the class definition namespace. 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, to access your class definition namespace you must
+use ``OrderedDict`` along with a metaclass. Then you must preserve the
+definition order (from the ``OrderedDict``) yourself.  This has a
+couple of problems.
+
+First, it requires the use of a metaclass.  Metaclasses introduce an
+extra level of complexity to code and in some cases (e.g. conflicts)
+are a problem.  So reducing the need for them is worth doing when the
+opportunity presents itself.  PEP 422 and PEP 487 discuss this at
+length.  Given that we now have a C implementation of ``OrderedDict``
+and that ``OrderedDict`` is the common use case for ``__prepare__()``,
+we have such an opportunity by defaulting to ``OrderedDict``.
+
+Second, only classes that opt in to using the ``OrderedDict``-based
+metaclass will have access to the definition order. This is problematic
+for cases where universal access to the definition order is important.
 One of the original motivating use cases for this PEP is generic class
 decorators that make use of the definition order.
 
-Changing the default class definition namespace has been discussed a
-number of times, including on the mailing lists and in PEP 422 and
-PEP 487 (see the References section below).
-
 
 Specification
 =============
 
-* the default class *definition* namespace is now ``OrderdDict``
+Part 1:
+
 * 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
@@ -74,6 +80,10 @@ Specification
      ``OrderedDict`` (or a subclass) have their ``__definition_order__``
      set to ``None`` (except where #1 applies)
 
+Part 2:
+
+* the default class *definition* namespace is now ``OrderdDict``
+
 The following code demonstrates roughly equivalent semantics for the
 default behavior::