More good stuff. Consider this just a checkpoint.

pep-0253.txt

@@ -77,6 +77,21 @@ Introduction
     checking of this flag bit.  This should be fixed before the final
     release.)
 
+    In current Python, a distinction is made between types and
+    classes.  This PEP together with pep-0254 will remove that
+    distinction.  However, for backwards compatibility there will
+    probably remain a bit of a distinction for years to come, and
+    without pep-0254, the distinction is still large: types ultimately
+    have a built-in type as a base class, while classes ultimately
+    derive from a user-defined class.  Therefore, in the rest of this
+    PEP, I will use the word type whenever I can -- including base
+    type or supertype, derived type or subtype, and metatype.
+    However, sometimes the terminology necessarily blends, e.g. an
+    object's type is given by its __class__ attribute, and subtyping
+    in Python is spelled with a class statement.  If further
+    distinction is necessary, user-defined classes can be referred to
+    as "classic" classes.
+
 
 About metatypes
 
@@ -258,6 +273,9 @@ Requirements for a type to allow subtyping
                     PyObject *args,
                     PyObject *kwds)
 
+    [XXX We'll have to rename tp_alloc to something else, because in
+    debug mode there's already a tp_alloc field.]
+
     The arguments for tp_alloc are the same as for tp_new, described
     above.  The arguments for tp_init are the same except that the
     first argument is replaced with the instance to be initialized.
@@ -270,9 +288,9 @@ Requirements for a type to allow subtyping
     cause a core dump or memory leakage this way.
 
     Because tp_init is in a sense optional, tp_alloc is required to do
-    *some* initialization of the object.  It is required to initialize
-    ob_refcnt to 1 and ob_type to its type argument.  To be safe, it
-    should probably zero out the rest of the object.
+    *some* initialization of the object.  It must initialize ob_refcnt
+    to 1 and ob_type to its type argument.  It should zero out the
+    rest of the object.
 
     The constructor arguments are passed to tp_alloc so that for
     variable-size objects (like tuples and strings) it knows to
@@ -488,20 +506,64 @@ Subtyping in Python
     to be provided for the creation of C is: its name (in this example
     the string "C"); the list of base classes (a singleton tuple
     containing B); and the results of executing the class body, in the
-    form of a dictionary (e.g. {"var1": 1, "method1": <function...>,
-    ...}).
+    form of a dictionary (e.g. {"var1": 1, "method1": <function
+    method1 at ...>, ...}).
 
-    According to the Don Beaudry hook, the following call is made:
+    I propose to rig the class statement to make the following call:
 
-    C = M("C", (B,), dict)
+        C = M("C", (B,), dict)
 
     (where dict is the dictionary resulting from execution of the
-    class body).  In other words, the metatype (M) is called.  Note
-    that even though we currently require there to be exactly one base
-    class, we still pass in a (singleton) sequence of base classes;
-    this makes it possible to support multiple inheritance later (or
-    for types with a different metaclass!) without changing this
-    interface.
+    class body).  In other words, the metatype (M) is called.
+
+    Note that even though we currently require there to be exactly one
+    base class, we still pass in a (singleton) sequence of base
+    classes; this makes it possible to support multiple inheritance
+    later (or for types with a different metaclass!) without changing
+    this interface.
+
+    In current Python, this is called the "Don Beaudry hook" after its
+    inventor; it is an exceptional case that is only invoked when a
+    base class is not a regular class.  For a regular base class (or
+    when no base class is specified), current Python calls
+    PyClass_New(), the C level factory function for classes, directly.
+    I propose to change this so that Python *always* determines a
+    metaclass and calls it as given above.  When one or more bases are
+    given, the type of the first base is used as the metatype; 
+    when no base class is given, a default metaclass is chosen.  By
+    setting the default metaclass to PyClass_Type, the metatype of
+    "classic" classes, the classic behavior of the class statement is
+    retained.
+
+    There are two further refinements here.  First, a useful feature
+    is to be able to specify a metatype directly.  If the class
+    statement defines a variable __metaclass__, that is the metatype
+    to call.
+
+    Second, with multiple bases, not all bases need to have the same
+    metatype.  This is called a metaclass conflict [1].  Some
+    metaclass conflicts can be resolved by searching through the set
+    of bases for a metatype that derives from all other given
+    metatypes.  If such a metatype cannot be found, an exception is
+    raised and the class statement fails.
+
+    This conflict resultion can be implemented in the metatypes
+    itself: the class statement just calls the metatype of the first
+    base, and this metatype's constructor looks for the most derived
+    metatype.  If that is itself, it proceeds; otherwise, it calls
+    that metatype's constructor.  (Ultimate flexibility: another
+    metatype might choose to require that all bases have the same
+    metatype, or that there's only one base class, or whatever.)
+
+    (Theoretically, it might be possible to automatically derive a new
+    metatype that is a subtype of all given metatypes; but since it is
+    questionable how conflicting method definitions of the various
+    metatypes should be merged, I don't think this is useful or
+    feasible.  Should the need arise, the user can derive such a
+    metatype and specify it using the __metaclass__ variable.  It is
+    also possible to have a new metatype that does this.)
+
+    HIRO
 
     Note that calling M requires that M itself has a type: the
     meta-metatype.  In the current implementation, I have introduced a