Furhter updates

pep-0447.txt

@@ -56,7 +56,8 @@ Rationale
 It is currently not possible to influence how the `super class`_ looks
 up attributes (that is, ``super.__getattribute__`` unconditionally
 peeks in the class ``__dict__``), and that can be problematic for
-dynamic classes that can grow new methods on demand.
+dynamic classes that can grow new methods on demand, for example dynamic
+proxy classes.
 
 The ``__getdescriptor__`` method makes it possible to dynamically add
 attributes even when looking them up using the `super class`_.
@@ -113,7 +114,8 @@ The attribute resolution proces as implemented by ``object.__getattribute__``
 straightforward, but not entirely so without reading C code.
 
 The current CPython implementation of object.__getattribute__ is basicly
-equivalent to the following (pseudo-) Python code (excluding some house keeping and speed tricks)::
+equivalent to the following (pseudo-) Python code (excluding some house
+keeping and speed tricks)::
 
 
     def _PyType_Lookup(tp, name):
@@ -269,6 +271,9 @@ this.
 In C code
 ---------
 
+A new type flag ``Py_TPFLAGS_GETDESCRIPTOR`` with value ``(1UL << 11)`` that
+indicates that the new slot is present and to be used.
+
 A new slot ``tp_getdescriptor`` is added to the ``PyTypeObject`` struct, this
 slot corresponds to the ``__getdescriptor__`` method on `type`_.
 
@@ -281,6 +286,9 @@ superclasses, and should not invoke descriptors. The method returns ``NULL``
 without setting an exception when the *name* cannot be found, and returns a
 new reference otherwise (not a borrowed reference).
 
+Classes with a ``tp_getdescriptor`` slot must add ``Py_TPFLAGS_GETDESCRIPTOR``
+to ``tp_flags`` to indicate that new slot must be used.
+
 Use of this hook by the interpreter
 -----------------------------------
 
@@ -300,12 +308,16 @@ Because of this ``_PyType_Lookup`` will be renamed to ``_PyType_LookupName``,
 this will cause compile-time errors for all out-of-tree users of this
 private API.
 
+For the same reason ``_PyType_LookupId`` is renamed to ``_PyType_LookupId2``.
+A number of other functions in typeobject.c with the same issue do not get
+an updated name because they are private to that file.
+
 The attribute lookup cache in ``Objects/typeobject.c`` is disabled for classes
 that have a metaclass that overrides ``__getdescriptor__``, because using the
 cache might not be valid for such classes.
 
 Impact of this PEP on introspection
------------------------------------
+===================================
 
 Use of the method introduced in this PEP can affect introspection of classes
 with a metaclass that uses a custom ``__getdescriptor__`` method. This section
@@ -334,33 +346,46 @@ changes to the visible behaviour of the ``object.__getattribute__``.
   be ignored and is another way in which the result of ``inspect.getattr_static``
   can be different from that of ``builtin.getattr``.
 
-* ``inspect.getmembers`` and ``inspect.get_class_attrs``
+* ``inspect.getmembers`` and ``inspect.classify_class_attrs``
 
   Both of these functions directly access the class __dict__ of classes along
   the MRO, and hence can be affected by a custom ``__getdescriptor__`` method.
 
-  **TODO**: I haven't fully worked out what the impact of this is, and if there
-  are mitigations for those using either updates to these functions, or
-  additional methods that users should implement to be fully compatible with
-  these functions.
+  Code with a custom ``__getdescriptor__`` method that want to play nice with
+  these methods also needs to ensure that the ``__dict__`` is set up correctly
+  when that is accessed directly by Python code.
 
-  One possible mitigation is to have a custom ``__getattribute__`` for these
-  classes that fills ``__dict__`` before returning and and defers to the
-  default implementation for other attributes.
+  Note that ``inspect.getmembers`` is used by ``pydoc`` and hence this can
+  affect runtime documentation introspection.
 
 * Direct introspection of the class ``__dict__``
 
   Any code that directly access the class ``__dict__`` for introspection
-  can be affected by a custom ``__getdescriptor__`` method.
+  can be affected by a custom ``__getdescriptor__`` method, see the previous
+  item.
 
 
 Performance impact
-------------------
+==================
 
 **WARNING**: The benchmark results in this section are old, and will be updated
 when I've ported the patch to the current trunk. I don't expect significant
 changes to the results in this section.
 
+Micro benchmarks
+----------------
+
+`Issue 18181`_ has a micro benchmark as one of its attachments
+(`pep447-micro-bench.py`_) that specifically tests the speed of attribute
+lookup, both directly and through super.
+
+Note that attribute lookup with deep class hierarchies is significantly slower
+when using a custom ``__getdescriptor__`` method. This is because the
+attribute lookup cache for CPython cannot be used when having this method.
+
+Pybench
+-------
+
 The pybench output below compares an implementation of this PEP with the
 regular source tree, both based on changeset a5681f50bae2, run on an idle
 machine an Core i7 processor running Centos 6.4.
@@ -579,10 +604,10 @@ the performance impact is minimal::
 
 
 Alternative proposals
----------------------
+=====================
 
 ``__getattribute_super__``
-..........................
+--------------------------
 
 An earlier version of this PEP used the following static method on classes::
 
@@ -593,7 +618,7 @@ necessarily limited to working only with ``super.__getattribute__``.
 
 
 Reuse ``tp_getattro``
-.....................
+---------------------
 
 It would be nice to avoid adding a new slot, thus keeping the API simpler and
 easier to understand.  A comment on `Issue 18181`_ asked about reusing the
@@ -605,16 +630,44 @@ That won't work because ``tp_getattro`` will look in the instance
 This would mean that using ``tp_getattro`` instead of peeking the class
 dictionaries changes the semantics of the `super class`_.
 
-Alternate placement of the new method
-.....................................
+Alternative placement of the new method
+---------------------------------------
 
 This PEP proposes to add ``__getdescriptor__`` as a method on the metaclass.
 An alternative would be to add it as a class method on the class itself
 (simular to how ``__new__`` is a `staticmethod`_ of the class and not a method
 of the metaclass).
 
-The two are functionally equivalent, and there's something to be said about
-not requiring the use of a meta class.
+The advantage of using a method on the metaclass is that will give an error
+when two classes on the MRO have different metaclasses that may have different
+behaviors for ``__getdescriptor__``. With a normal classmethod that problem
+would pass undetected while it might cause subtle errors when running the code.
+
+History
+=======
+
+* 23-Jul-2015: Added type flag ``Py_TPFLAGS_GETDESCRIPTOR`` after talking
+  with Guido.
+
+  The new flag is primarily useful to avoid crashing when loading an extension
+  for an older version of CPython and could have positive speed implications
+  as well.
+
+* Jul-2014: renamed slot to ``__getdescriptor__``, the old name didn't
+  match the naming style of other slots and was less descriptive.
+
+Discussion threads
+==================
+
+* The initial version of the PEP was send with
+  Message-ID `<75030FAC-6918-4E94-95DA-67A88D53E6F5@mac.com>`_
+
+* Further discusion starting at a message with
+  Message-ID `<5BB87CC4-F31B-4213-AAAC-0C0CE738460C@mac.com>`_
+
+* And more discussion starting at message with
+  Message-ID `<00AA7433-C853-4101-9718-060468EBAC54@mac.com>`_
+
 
 
 References
@@ -627,6 +680,12 @@ Copyright
 
 This document has been placed in the public domain.
 
+..  _`<75030FAC-6918-4E94-95DA-67A88D53E6F5@mac.com>`: http://marc.info/?l=python-dev&m=137510220928964&w=2
+
+.. _`<5BB87CC4-F31B-4213-AAAC-0C0CE738460C@mac.com>`: https://mail.python.org/pipermail/python-ideas/2014-July/028420.html
+
+.. _`<00AA7433-C853-4101-9718-060468EBAC54@mac.com>`: https://mail.python.org/pipermail/python-dev/2013-July/127321.html
+
 .. _`Issue 18181`: http://bugs.python.org/issue18181
 
 .. _`super class`: http://docs.python.org/3/library/functions.html#super
@@ -652,3 +711,5 @@ This document has been placed in the public domain.
 .. _`PyObjC`: http://pyobjc.sourceforge.net/
 
 .. _`classmethod`: http://docs.python.org/3/library/functions.html#classmethod
+
+.. _`pep447-micro-bench.py`: http://bugs.python.org/file40013/pep447-micro-bench.py