PEP 575 minor update (#642)

pep-0575.rst

@@ -6,7 +6,7 @@ Type: Standards Track
 Content-Type: text/x-rst
 Created: 27-Mar-2018
 Python-Version: 3.8
-Post-History: 31-Mar-2018, 12-Apr-2018, 27-Apr-2018
+Post-History: 31-Mar-2018, 12-Apr-2018, 27-Apr-2018, 5-May-2018
 
 
 Abstract
@@ -180,7 +180,11 @@ cfunction
 ---------
 
 This is the new version of the old ``builtin_function_or_method`` class.
-It is a copy of ``base_function``, with the following differences:
+The name ``cfunction`` was chosen to avoid confusion with "built-in"
+in the sense of "something in the ``builtins`` module".
+It also fits better with the C API which use the `PyCFunction`` prefix.
+
+The class ``cfunction`` is a copy of ``base_function``, with the following differences:
 
 #. ``m_ml`` points to a ``PyMethodDef`` structure,
    extending ``PyCFunctionDef`` with an additional ``ml_doc``
@@ -277,11 +281,12 @@ with the UTF-8 encoded name.
 
 The ``_ml`` field reserves space to be used by ``base.m_ml``.
 
+A ``base_function`` instance must have the flag ``METH_PYTHON`` set
+if and only if it is an instance of ``function``.
+
 When constructing an instance of ``function`` from ``code`` and ``globals``,
 an instance is created with ``base.m_ml = &_ml``,
 ``base.m_self = NULL``.
-Instances of ``function`` should always have the flag ``METH_PYTHON`` set.
-This is also handled by the constructors.
 
 To make subclassing easier, we also add a copy constructor:
 if ``f`` is an instance of ``function``, then ``types.FunctionType(f)`` copies ``f``.
@@ -296,6 +301,9 @@ This conveniently allows using a custom function type as decorator::
     >>> type(f)
     <class '__main__.CustomFunction'>
 
+This also removes many use cases of ``functools.wraps``:
+wrappers can be replaced by subclasses of ``function``.
+
 bound_method
 ------------
 
@@ -493,9 +501,10 @@ it also indicates a default implementation of ``__call__`` and ``__get__``.
 In particular, such subclasses of ``base_function``
 must follow the implementation from the section `Calling base_function instances`_.
 
-This flag is never inherited.
-Extension types should explicitly specify it if they
-do not override ``__call__`` nor ``__get__`` or if they override them in a compatible way.
+This flag is automatically set for extension types which
+inherit the ``tp_call`` and ``tp_descr_get`` implementation from ``base_function``.
+Extension types can explicitly specify it if they
+override ``__call__`` or ``__get__`` in a compatible way.
 The flag ``Py_TPFLAGS_BASEFUNCTION`` must never be set for a heap type
 because that would not be safe (heap types can be changed dynamically).
 
@@ -533,7 +542,11 @@ Some of these are existing (possibly changed) functions, some are new:
   The old functions are kept as aliases of the new functions.
 
 - ``int PyFunction_Check(PyObject *op)``: return true if ``op``
-  is an instance of ``function``.
+  is an instance of ``base_function`` with the ``METH_PYTHON`` flag set
+  (this is equivalent to checking whether ``op`` is an instance of ``function``).
+
+- ``int PyFunction_CheckFast(PyObject *op)``: equivalent to
+  ``PyFunction_Check(op) && PyBaseFunction_CheckFast(op)``.
 
 - ``int PyFunction_CheckExact(PyObject *op)``: return true
   if the type of ``op`` is ``function``.
@@ -609,14 +622,16 @@ in the ``tp_methods`` array of an extension type).
 Non-CPython implementations
 ===========================
 
-For other implementations of Python apart from CPython,
-only the classes ``base_function``, ``bound_method`` and ``function`` are required.
-The latter two are the only classes which can be instantiated directly
-from the Python interpreter.
+Most of this PEP is only relevant to CPython.
+For other implementations of Python,
+the two changes that are required are the ``base_function`` base class
+and the fact that ``function`` can be subclassed.
+The classes ``cfunction`` and ``defined_function`` are not required.
+
 We require ``base_function`` for consistency but we put no requirements on it:
 it is acceptable if this is just a copy of ``object``.
 Support for the new ``__parent__`` (and ``__objclass__``) attribute is not required.
-If there is no ``defined_function`` type,
+If there is no ``defined_function`` class,
 then ``types.DefinedFunctionType`` should be an alias of ``types.FunctionType``.
 
 
@@ -809,9 +824,14 @@ Backwards compatibility
 
 While designing this PEP, great care was taken to not break
 backwards compatibility too much.
-In particular, Python code not using ``inspect`` or type checks
-should not be affected by this PEP.
-For example, ``staticmethod``, ``functools.partial`` or ``operator.methodcaller``
+Most of the potentially incompatible changes
+are changes to CPython implementation details
+which are different anyway in other Python interpreters.
+In particular, Python code which correctly runs on PyPy
+will very likely continue to work with this PEP.
+
+The standard classes and functions like
+``staticmethod``, ``functools.partial`` or ``operator.methodcaller``
 do not need to change at all.
 
 Changes to types and inspect