Update PEP 487 (#53)

* Update PEP 487

This adds some changes proposed by Guido.

* removed outdated comment

* rename __set_owner__ to __set_name__

pep-0487.txt

@@ -58,15 +58,15 @@ into the class creation:
 
 1. An ``__init_subclass__`` hook that initializes
    all subclasses of a given class.
-2. upon class creation, a ``__set_owner__`` hook is called on all the
+2. upon class creation, a ``__set_name__`` hook is called on all the
    attribute (descriptors) defined in the class, and
 
-The third category is the topic of another PEP 520.
+The third category is the topic of another PEP, PEP 520.
 
 As an example, the first use case looks as follows::
 
    >>> class QuestBase:
-   ...    # this is implicitly a @classmethod
+   ...    # this is implicitly a @classmethod (see below for motivation)
    ...    def __init_subclass__(cls, swallow, **kwargs):
    ...        cls.swallow = swallow
    ...        super().__init_subclass__(**kwargs)
@@ -89,7 +89,7 @@ similar mechanism has long been supported by `Zope's ExtensionClass`_),
 but the situation has changed sufficiently in recent years that
 the idea is worth reconsidering for inclusion.
 
-The second part of the proposal adds an ``__set_owner__``
+The second part of the proposal adds an ``__set_name__``
 initializer for class attributes, especially if they are descriptors.
 Descriptors are defined in the body of a
 class, but they do not know anything about that class, they do not
@@ -116,9 +116,23 @@ referenced values::
             instance.__dict__[self.name] = weakref.ref(value)
 
         # this is the new initializer:
-        def __set_owner__(self, owner, name):
+        def __set_name__(self, owner, name):
             self.name = name
             
+Such a ``WeakAttribute`` may, for example, be used in a tree structure
+where one wants to avoid cyclic references via the parent::
+
+    class TreeNode:
+        parent = WeakAttribute()
+        
+        def __init__(self, parent):
+            self.parent = parent
+            
+Note that the ``parent`` attribute is used like a normal attribute,
+yet the tree contains no cyclic references and can thus be easily
+garbage collected when out of use. The ``parent`` attribute magically
+becomes ``None`` once the parent ceases existing.
+
 While this example looks very trivial, it should be noted that until
 now such an attribute cannot be defined without the use of a metaclass.
 And given that such a metaclass can make life very hard, this kind of
@@ -128,7 +142,7 @@ Initializing descriptors could simply be done in the
 ``__init_subclass__`` hook. But this would mean that descriptors can
 only be used in classes that have the proper hook, the generic version
 like in the example would not work generally. One could also call
-``__set_owner__`` from whithin the base implementation of
+``__set_name__`` from whithin the base implementation of
 ``object.__init_subclass__``. But given that it is a common mistake
 to forget to call ``super()``, it would happen too often that suddenly
 descriptors are not initialized.
@@ -179,7 +193,7 @@ Subclass registration
 Especially when writing a plugin system, one likes to register new
 subclasses of a plugin baseclass. This can be done as follows::
 
-   class PluginBase(Object):
+   class PluginBase:
        subclasses = []
 
        def __init_subclass__(cls, **kwargs):
@@ -212,7 +226,7 @@ PEP::
            else:
                raise ValueError("value not in range")
 
-       def __set_owner__(self, owner, name):
+       def __set_name__(self, owner, name):
            self.key = name
 
 Implementation Details
@@ -234,7 +248,7 @@ and ``type`` defined here inherit from the usual ones::
                 ns['__init_subclass__'] = classmethod(init)
             self = super().__new__(cls, name, bases, ns)
             for k, v in self.__dict__.items():
-                func = getattr(v, '__set_owner__', None)
+                func = getattr(v, '__set_name__', None)
                 if func is not None:
                     func(self, k)
             super(self, self).__init_subclass__(**kwargs)
@@ -251,14 +265,14 @@ and ``type`` defined here inherit from the usual ones::
     class object(object, metaclass=type):
         pass
 
-In this code, first the ``__set_owner__`` are called on the descriptors, and
+In this code, first the ``__set_name__`` are called on the descriptors, and
 then the ``__init_subclass__``. This means that subclass initializers already
 see the fully initialized descriptors. This way, ``__init_subclass__`` users
 can fix all descriptors again if this is needed.
 
-Another option would have been to call ``__set_owner__`` in the base
+Another option would have been to call ``__set_name__`` in the base
 implementation of ``object.__init_subclass__``. This way it would be possible
-event to prevent ``__set_owner__`` from being called. Most of the times,
+even to prevent ``__set_name__`` from being called. Most of the times,
 however, such a prevention would be accidental, as it often happens that a call
 to ``super()`` is forgotten.
 
@@ -309,14 +323,9 @@ The original proposal also made major changes in the class
 initialization process, rendering it impossible to back-port the
 proposal to older Python versions.
 
-More importantly, having a pure Python implementation allows us to
-take two preliminary steps before before we actually change the
-interpreter, giving us the chance to iron out all possible wrinkles
-in the API.
 
-
-Other variants of calling the hook
-----------------------------------
+Other variants of calling the hooks
+-----------------------------------
 
 Other names for the hook were presented, namely ``__decorate__`` or
 ``__autodecorate__``. This proposal opts for ``__init_subclass__`` as
@@ -324,6 +333,9 @@ it is very close to the ``__init__`` method, just for the subclass,
 while it is not very close to decorators, as it does not return the
 class.
 
+For the ``__set_name__`` hook other names have been proposed as well,
+``__set_owner__``, ``__set_ownership__`` and ``__init_descriptor__``.
+
 
 Requiring an explicit decorator on ``__init_subclass__``
 --------------------------------------------------------