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Re-add two paragraphs that seem to have been lost during the merge from trunk.

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Georg Brandl 2007-11-03 08:44:43 +00:00
parent a6c04bed1e
commit db62967a46
1 changed files with 13 additions and 4 deletions
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17
Doc/reference/datamodel.rst
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@ -1289,8 +1289,7 @@ Basic customization
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 @@ Basic customization
(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)