Update 422 based on python-dev feedback

pep-0422.txt

@@ -44,7 +44,7 @@ the metaclass hint that may be provided as part of the class definition.
 While in many cases these two meanings end up referring to one and the same
 object, there are two situations where that is not the case:
 
-* If the metaclass hint refers to an instance of ``type``, then it is
+* If the metaclass hint refers to a subclass of ``type``, then it is
   considered as a candidate metaclass along with the metaclasses of all of
   the parents of the class being defined. If a more appropriate metaclass is
   found amongst the candidates, then it will be used instead of the one
@@ -72,6 +72,16 @@ attribute, and the class creation process would extract that value from the
 class namespace to use as the metaclass hint. There is `published code`_ that
 makes use of this feature.
 
+Another new feature in Python 3 is the zero-argument form of the ``super()``
+builtin, introduced by PEP 3135. This feature uses an implicit ``__class__``
+reference to the class being defined to replace the "by name" references
+required in Python 2. Just as code invoked during execution of a Python 2
+metaclass could not call methods that referenced the class by name (as the
+name had not yet been bound in the containing scope), similarly, Python 3
+metaclasses cannot call methods that rely on the implicit ``__class__``
+reference (as it is not populated until after the metaclass has returned
+control to the class creation machiner).
+
 
 Proposal
 ========
@@ -90,10 +100,10 @@ following criteria:
 4. Can be added to an existing base class without a significant risk of
    introducing backwards compatibility problems
 
-One mechanism that would achieve this goal is to add a new class
+One mechanism that can achieve this goal is to add a new class
 initialisation hook, modelled directly on the existing instance
-initialisation hook. However, the signature would be constrained to ensure
-that correctly supporting multiple inheritance is kept as simple as possible.
+initialisation hook, but with the signature constrained to match that
+of an ordinary class decorator.
 
 Specifically, it is proposed that class definitions be able to provide a
 class initialisation hook as follows::
@@ -110,51 +120,57 @@ class initialisation hook as follows::
 If present on the created object, this new hook will be called by the class
 creation machinery *after* the ``__class__`` reference has been initialised.
 For ``types.new_class()``, it will be called as the last step before
-returning the created class object. Calling the hook automatically from
-``type.__init__`` unfortunately doesn't work, as it would mean the
-``__init_class__`` method would be unable to call any methods that relied
-on the ``__class__`` reference (or used the zero-argument form of
-``super()``).
+returning the created class object.
 
 If a metaclass wishes to block class initialisation for some reason, it
 must arrange for ``cls.__init_class__`` to trigger ``AttributeError``.
 
-This general proposal is not a new idea (it was first suggested `more than
-10 years ago`_), but I believe the situation has changed sufficiently in
-that time that the idea is worth reconsidering.
+This general proposal is not a new idea (it was first suggested for
+inclusion in the language definition `more than 10 years ago`_, and a
+similar mechanism has long been supported by `Zope's ExtensionClass`_),
+but I believe the situation has changed sufficiently in recent years that
+the idea is worth reconsidering.
 
 
 Key Benefits
 ============
 
 
-Replaces dynamic setting of ``__metaclass__``
----------------------------------------------
+Replaces many use cases for dynamic setting of ``__metaclass__``
+-----------------------------------------------------------------
 
-For use cases that didn't involve completely replacing the defined class,
+For use cases that don't involve completely replacing the defined class,
 Python 2 code that dynamically set ``__metaclass__`` can now dynamically
 set ``__init_class__`` instead. For more advanced use cases, introduction of
-an explicit metaclass will still be necessary in order to support Python 3.
+an explicit metaclass (possibly made available as a required base class) will
+still be necessary in order to support Python 3.
 
 
 Easier inheritance of definition time behaviour
 -----------------------------------------------
 
-Understanding Python's metaclass system requires a deep understanding of
+Understanding Python's metaclasses requires a deep understanding of
 the type system and the class construction process. This is legitimately
-seen as confusing, due to the need to keep multiple moving parts (the code,
+seen as challenging, due to the need to keep multiple moving parts (the code,
 the metaclass hint, the actual metaclass, the class object, instances of the
-class object) clearly distinct in your mind.
+class object) clearly distinct in your mind. Even when you know the rules,
+it's still easy to make a mistake if you're not being extremely careful.
+An earlier version of this PEP actually included such a mistake: it
+stated "instance of type" for a constraint that is actually "subclass of
+type".
 
-Understanding the proposed class initialisation hook requires understanding
-decorators and ordinary method inheritance, which is a much simpler prospect.
+Understanding the proposed class initialisation hook only requires
+understanding decorators and ordinary method inheritance, which isn't
+quite as daunting a task. The new hook provides a more gradual path
+towards understanding all of the phases involved in the class definition
+process.
 
 
 Reduced chance of metaclass conflicts
 -------------------------------------
 
 One of the big issues that makes library authors reluctant to use metaclasses
-(even when it would be appropriate) is the risk of metaclass conflicts.
+(even when they would be appropriate) is the risk of metaclass conflicts.
 These occur whenever two unrelated metaclasses are used by the desired
 parents of a class definition. This risk also makes it very difficult to
 *add* a metaclass to a class that has previously been published without one.
@@ -164,12 +180,12 @@ a similar level of risk to adding an ``__init__`` method: technically, there
 is a risk of breaking poorly implemented subclasses, but when that occurs,
 it is recognised as a bug in the subclass rather than the library author
 breaching backwards compatibility guarantees. In fact, due to the constrained
-signature, the risk in this case is actually even lower than in the case of
-``__init__``.
+signature of ``__init_class__``, the risk in this case is actually even
+lower than in the case of ``__init__``.
 
 
-Integrates cleanly with PEP 3135
---------------------------------
+Integrates cleanly with \PEP 3135
+---------------------------------
 
 Unlike code that runs as part of the metaclass, code that runs as part of
 the new hook will be able to freely invoke class methods that rely on the
@@ -280,6 +296,35 @@ just go away, as that kind of thing is taken care of through the use of an
 ordinary class method invocation.
 
 
+Automatic metaclass derivation
+------------------------------
+
+When no appropriate metaclass is found, it's theoretically possible to
+automatically derive a metaclass for a new type based on the metaclass hint
+and the metaclasses of the bases.
+
+While adding such a mechanism would reduce the risk of spurious metaclass
+conflicts, it would do nothing to improve integration with PEP 3135, would
+not help with porting Python 2 code that set ``__metaclass__`` dynamically
+and would not provide a more straightforward inherited mechanism for invoking
+additional operations after the class invocation is complete.
+
+In addition, there would still be a risk of metaclass conflicts in cases
+where the base metaclasses were not written with multiple inheritance in
+mind. In such situations, there's a chance of introducing latent defects
+if one or more metaclasses are not invoked correctly.
+
+
+Calling the new hook from ``type.__init__``
+-------------------------------------------
+
+Calling the new hook automatically from ``type.__init__``, would achieve most
+of the goals of this PEP. However, using that approach would mean that
+``__init_class__`` implementations would be unable to call any methods that
+relied on the ``__class__`` reference (or used the zero-argument form of
+``super()``), and could not make use of those features themselves.
+
+
 References
 ==========
 
@@ -289,6 +334,8 @@ References
 .. _more than 10 years ago:
    http://mail.python.org/pipermail/python-dev/2001-November/018651.html
 
+.. _Zope's ExtensionClass:
+   http://docs.zope.org/zope_secrets/extensionclass.html
 
 Copyright
 =========