Add a mention of IntEnum, summarizing the language summit and later discussions

on the subject.

Note that this is still in draft stage and decisions can change (and have
changed more than once over the past few days).

pep-0435.txt

@@ -34,9 +34,25 @@ The idea of adding an enum type to Python is not new - PEP 354 [2]_ is a
 previous attempt that was rejected in 2005.  Recently a new set of discussions
 was initiated [3]_ on the ``python-ideas`` mailing list.  Many new ideas were
 proposed in several threads; after a lengthy discussion Guido proposed adding
-``flufl.enum`` to the standard library [4]_.  This PEP is an attempt to
+``flufl.enum`` to the standard library [4]_.  During the PyCon 2013 language
-formalize this decision as well as discuss a number of variations that can be
+summit the issue was discussed further.  It became clear that many developers
-considered for inclusion.
+want to see an enum that subclasses ``int``, which can allow us to replace
+many integer constants in the standard library by enums with friendly string
+representations, without ceding backwards compatibility.  An additional
+discussion among several interested core developers led to the proposal of
+having ``IntEnum`` as a special case of ``Enum``.
+
+The key dividing issue between ``Enum`` and ``IntEnum`` is whether comparing
+to integers is semantically meaningful.  For most uses of enumerations, it's
+a **feature** to reject comparison to integers; enums that compare to integers
+lead, through transitivity, to comparisons between enums of unrelated types,
+which isn't desirable in most cases.  For some uses, however, greater
+interoperatiliby with integers is desired. For instance, this is the case for
+replacing existing standard library constants (such as ``socket.AF_INET``)
+with enumerations.
+
+This PEP is an attempt to formalize this decision as well as discuss a number
+of variations that were discussed and can be considered for inclusion.
 
 
 Motivation
@@ -153,9 +169,9 @@ same named values in the derived class::
     >>> Colors.blue is MoreColors.blue
     True
 
-However, these are not doing comparisons against the integer equivalent
+However, these are not doing comparisons against the integer equivalent values,
-values, because if you define an enumeration with similar item names and
+because if you define an enumeration with similar item names and integer values,
-integer values, they will not be identical::
+they will not be identical::
 
     >>> class OtherColors(Enum):
     ...     red = 1
@@ -174,7 +190,7 @@ These enumeration values are not equal, nor do they hash equally::
     2
 
 Ordered comparisons between enumeration values are *not* supported.  Enums are
-not integers::
+not integers (but see ``IntEnum`` below)::
 
     >>> Colors.red < Colors.blue
     Traceback (most recent call last):
@@ -342,6 +358,48 @@ Enumeration values are hashable, so they can be used in dictionaries and sets::
     green -> granny smith
 
 
+IntEnum
+-------
+
+A variation of ``Enum`` is proposed that also subclasses ``int`` - ``IntEnum``.
+Such enumerations behave much more similarly to integers.  In particular, they
+can be compared to integers; by extensions, enumerations of different types can
+also be compared to each other::
+
+    >>> from enum import IntEnum
+    >>> class Shape(IntEnum):
+    ...   circle = 1
+    ...   square = 2
+    ...
+    >>> class Request(IntEnum):
+    ...   post = 1
+    ...   get = 2
+    ...
+    >>> Shape == 1
+    False
+    >>> Shape.circle == 1
+    True
+    >>> Shape.circle == Request.post
+    True
+
+However they still can't be compared to ``Enum``::
+
+    >>> from enum import Enum, IntEnum
+    >>> class Shape(IntEnum):
+    ...   circle = 1
+    ...   square = 2
+    ...
+    >>> class Colors(Enum):
+    ...   red = 1
+    ...   green = 2
+    ...
+    >>> Shape.circle == Colors.red
+    False
+
+For the vast majority of code, ``Enum`` is recommended.  Only if a greater
+degree of interoperatility with integers is required and ``Enum`` does not
+fit the bill, ``IntEnum`` should be used.
+
 Pickling
 --------