Update PEP 520 per python-dev feedback.

Author: ericsnowcurrently

pep-0520.txt

@@ -78,6 +78,7 @@ The following code demonstrates roughly equivalent semantics for the
 default behavior::
 
    class Meta(type):
+       @classmethod
        def __prepare__(cls, *args, **kwargs):
            return OrderedDict()
 
@@ -96,7 +97,7 @@ 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
+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.
 
@@ -156,6 +157,11 @@ so that consumers of ``__definition_order__`` may have a consistent
 expectation for the value.  That helps maximize the feature's
 usefulness.
 
+We could also also allow an arbitrary iterable for a manually set
+``__definition_order__`` and convert it into a tuple.  However, not
+all iterables infer a definition order (e.g. ``set``).  So we opt in
+favor of requiring a tuple.
+
 Why is __definition_order__ even necessary?
 -------------------------------------------
 
@@ -172,7 +178,8 @@ Compatibility
 
 This PEP does not break backward compatibility, except in the case that
 someone relies *strictly* on ``dict`` as the class definition namespace.
-This shouldn't be a problem.
+This shouldn't be a problem since ``issubclass(OrderedDict, dict)`` is
+true.
 
 
 Changes
@@ -203,7 +210,7 @@ The implementation is found in the tracker. [impl_]
 Alternatives
 ============
 
-<class>.__dict__ as OrderedDict
+cls.__dict__ as OrderedDict
 -------------------------------
 
 Instead of storing the definition order in ``__definition_order__``,
@@ -223,6 +230,34 @@ PEP 422 introduced a new "namespace" keyword arg to class definitions
 that effectively replaces the need to ``__prepare__()``. [pep422_]
 However, the proposal was withdrawn in favor of the simpler PEP 487.
 
+A stdlib Metaclass that Implements __prepare__() with OrderedDict
+-----------------------------------------------------------------
+
+This has all the same problems as writing your own metaclass.  The
+only advantage is that you don't have to actually write this
+metaclass.  So it doesn't offer any benefit in the context of this
+PEP.
+
+Set __definition_order__ at Compile-time
+----------------------------------------
+
+Each class's ``__qualname__`` is determined at compile-time.
+This same concept could be applied to ``__definition_order__``.
+The result of composing ``__definition_order__`` at compile-time
+would be nearly the same as doing so at run-time.
+
+Comparative implementation difficulty aside, the key difference
+would be that at compile-time it would not be practical to
+preserve definition order for attributes that are set dynamically
+in the class body (e.g. ``locals()[name] = value``).  However,
+they should still be reflected in the definition order.  One
+posible resolution would be to require class authors to manually
+set ``__definition_order__`` if they define any class attributes
+dynamically.
+
+Ultimately, the use of ``OrderedDict`` at run-time or compile-time
+discovery is almost entirely an implementation detail.
+
 
 References
 ==========