[Python-3000-checkins] r58824 - python/branches/py3k/Doc/reference/datamodel.rst
georg.brandl
python-3000-checkins at python.org
Sat Nov 3 09:44:44 CET 2007
Author: georg.brandl
Date: Sat Nov 3 09:44:43 2007
New Revision: 58824
Modified:
python/branches/py3k/Doc/reference/datamodel.rst
Log:
Re-add two paragraphs that seem to have been lost during the merge from trunk.
Modified: python/branches/py3k/Doc/reference/datamodel.rst
==============================================================================
--- python/branches/py3k/Doc/reference/datamodel.rst (original)
+++ python/branches/py3k/Doc/reference/datamodel.rst Sat Nov 3 09:44:43 2007
@@ -1289,8 +1289,7 @@
instances are compared by object identity ("address"). See also the
description of :meth:`__hash__` for some important notes on creating
:term:`hashable` objects which support custom comparison operations and are
- usable as dictionary keys. (Note: the restriction that exceptions are not
- propagated by :meth:`__cmp__` has been removed since Python 1.5.)
+ usable as dictionary keys.
.. method:: object.__hash__(self)
@@ -1307,8 +1306,18 @@
(e.g., using exclusive or) the hash values for the components of the object that
also play a part in comparison of objects.
- :meth:`__hash__` may also return a long integer object; the 32-bit integer is
- then derived from the hash of that object.
+ If a class does not define a :meth:`__cmp__` or :meth:`__eq__` method it
+ should not define a :meth:`__hash__` operation either; if it defines
+ :meth:`__cmp__` or :meth:`__eq__` but not :meth:`__hash__`, its instances
+ will not be usable as dictionary keys. If a class defines mutable objects
+ and implements a :meth:`__cmp__` or :meth:`__eq__` method, it should not
+ implement :meth:`__hash__`, since the dictionary implementation requires that
+ a key's hash value is immutable (if the object's hash value changes, it will
+ be in the wrong hash bucket).
+
+ User-defined classes have :meth:`__cmp__` and :meth:`__hash__` methods
+ by default; with them, all objects compare unequal and ``x.__hash__()``
+ returns ``id(x)``.
.. method:: object.__bool__(self)
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