Describe the __hash__ changes
diff --git a/Doc/whatsnew/2.6.rst b/Doc/whatsnew/2.6.rst
index 961d637..4340576 100644
--- a/Doc/whatsnew/2.6.rst
+++ b/Doc/whatsnew/2.6.rst
@@ -1595,6 +1595,22 @@
:func:`complex` constructor will now preserve the sign
of the zero. (Fixed by Mark T. Dickinson; :issue:`1507`.)
+* Classes that inherit a :meth:`__hash__` method from a parent class
+ can set ``__hash__ = None`` to indicate that the class isn't
+ hashable. This will make ``hash(obj)`` raise a :exc:`TypeError`
+ and the class will not be indicated as implementing the
+ :class:`Hashable` ABC.
+
+ You should do this when you've defined a :meth:`__cmp__` or
+ :meth:`__eq__` method that compares objects by their value rather
+ than by identity. All objects have a default hash method that uses
+ ``id(obj)`` as the hash value. There's no tidy way to remove the
+ :meth:`__hash__` method inherited from a parent class, so
+ assigning ``None`` was implemented as an override. At the
+ C level, extensions can set ``tp_hash`` to
+ :cfunc:`PyObject_HashNotImplemented`.
+ (Fixed by Nick Coghlan and Amaury Forgeot d'Arc; :issue:`2235`.)
+
* Changes to the :class:`Exception` interface
as dictated by :pep:`352` continue to be made. For 2.6,
the :attr:`message` attribute is being deprecated in favor of the
@@ -3125,6 +3141,10 @@
This section lists previously described changes and other bugfixes
that may require changes to your code:
+* Classes that aren't supposed to be hashable should
+ set ``__hash__ = None`` in their definitions to indicate
+ the fact.
+
* The :meth:`__init__` method of :class:`collections.deque`
now clears any existing contents of the deque
before adding elements from the iterable. This change makes the
@@ -3147,6 +3167,10 @@
functions now default to absolute imports, not relative imports.
This will affect C extensions that import other modules.
+* C API: extension data types that shouldn't be hashable
+ should define their ``tp_hash`` slot to
+ :cfunc:`PyObject_HashNotImplemented`.
+
* The :mod:`socket` module exception :exc:`socket.error` now inherits
from :exc:`IOError`. Previously it wasn't a subclass of
:exc:`StandardError` but now it is, through :exc:`IOError`.
@@ -3182,5 +3206,5 @@
The author would like to thank the following people for offering suggestions,
corrections and assistance with various drafts of this article:
-Georg Brandl, Jim Jewett, Antoine Pitrou.
+Georg Brandl, Nick Coghlan, Jim Jewett, Antoine Pitrou.