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  r58745 | georg.brandl | 2007-11-01 10:19:33 -0700 (Thu, 01 Nov 2007) | 2 lines

  #1364: os.lstat is available on Windows too, as an alias to os.stat.
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  r58750 | christian.heimes | 2007-11-01 12:48:10 -0700 (Thu, 01 Nov 2007) | 1 line

  Backport of import tests for bug http://bugs.python.org/issue1293 and bug http://bugs.python.org/issue1342
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  r58751 | christian.heimes | 2007-11-01 13:11:06 -0700 (Thu, 01 Nov 2007) | 1 line

  Removed non ASCII text from test as requested by Guido. Sorry :/
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  r58753 | georg.brandl | 2007-11-01 13:37:02 -0700 (Thu, 01 Nov 2007) | 2 lines

  Fix markup glitch.
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  r58757 | gregory.p.smith | 2007-11-01 14:08:14 -0700 (Thu, 01 Nov 2007) | 4 lines

  Fix bug introduced in revision 58385.  Database keys could no longer
  have NULL bytes in them.  Replace the errant strdup with a
  malloc+memcpy.  Adds a unit test for the correct behavior.
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  r58758 | gregory.p.smith | 2007-11-01 14:15:36 -0700 (Thu, 01 Nov 2007) | 3 lines

  Undo revision 58533 58534 fixes.  Those were a workaround for
  a problem introduced by 58385.
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  r58759 | gregory.p.smith | 2007-11-01 14:17:47 -0700 (Thu, 01 Nov 2007) | 2 lines

  false "fix" undone as correct problem was found and fixed.
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  r58765 | mark.summerfield | 2007-11-02 01:24:59 -0700 (Fri, 02 Nov 2007) | 3 lines

  Added more file-handling related cross-references.
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  r58766 | nick.coghlan | 2007-11-02 03:09:12 -0700 (Fri, 02 Nov 2007) | 1 line

  Fix for bug 1705170 - contextmanager swallowing StopIteration (2.5 backport candidate)
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  r58784 | thomas.heller | 2007-11-02 12:10:24 -0700 (Fri, 02 Nov 2007) | 4 lines

  Issue #1292: On alpha, arm, ppc, and s390 linux systems the
  --with-system-ffi configure option defaults to "yes" because the
  bundled libffi sources are too old.
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  r58785 | thomas.heller | 2007-11-02 12:11:23 -0700 (Fri, 02 Nov 2007) | 1 line

  Enable the full ctypes c_longdouble tests again.
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  r58796 | georg.brandl | 2007-11-02 13:06:17 -0700 (Fri, 02 Nov 2007) | 4 lines

  Make "hashable" a glossary entry and clarify docs on __cmp__, __eq__ and __hash__.
  I hope the concept of hashability is better understandable now.
  Thanks to Tim Hatch for pointing out the flaws here.
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This commit is contained in:
Guido van Rossum 2007-11-02 23:46:40 +00:00
parent e845c0f922
commit 2cc30daa86
20 changed files with 139 additions and 85 deletions
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56
Doc/reference/datamodel.rst
View file

@ -388,9 +388,10 @@ Set types
Frozen sets
.. index:: object: frozenset
These represent an immutable set. They are created by the built-in
:func:`frozenset` constructor. As a frozenset is immutable and hashable, it can
be used again as an element of another set, or as a dictionary key.
These represent an immutable set. They are created by the built-in
:func:`frozenset` constructor. As a frozenset is immutable and
:term:`hashable`, it can be used again as an element of another set, or as
a dictionary key.
.. % Set types
@ -1242,6 +1243,9 @@ Basic customization
object.__gt__(self, other)
object.__ge__(self, other)
.. index::
single: comparisons
These are the so-called "rich comparison" methods, and are called for comparison
operators in preference to :meth:`__cmp__` below. The correspondence between
operator symbols and method names is as follows: ``x<y`` calls ``x.__lt__(y)``,
@ -1256,14 +1260,16 @@ Basic customization
context (e.g., in the condition of an ``if`` statement), Python will call
:func:`bool` on the value to determine if the result is true or false.
There are no implied relationships among the comparison operators. The truth of
``x==y`` does not imply that ``x!=y`` is false. Accordingly, when defining
:meth:`__eq__`, one should also define :meth:`__ne__` so that the operators will
behave as expected.
There are no implied relationships among the comparison operators. The truth
of ``x==y`` does not imply that ``x!=y`` is false. Accordingly, when
defining :meth:`__eq__`, one should also define :meth:`__ne__` so that the
operators will behave as expected. See the paragraph on :meth:`__hash__` for
some important notes on creating :term:`hashable` objects which support
custom comparison operations and are usable as dictionary keys.
There are no reflected (swapped-argument) versions of these methods (to be used
when the left argument does not support the operation but the right argument
does); rather, :meth:`__lt__` and :meth:`__gt__` are each other's reflection,
There are no swapped-argument versions of these methods (to be used when the
left argument does not support the operation but the right argument does);
rather, :meth:`__lt__` and :meth:`__gt__` are each other's reflection,
:meth:`__le__` and :meth:`__ge__` are each other's reflection, and
:meth:`__eq__` and :meth:`__ne__` are their own reflection.
@ -1276,14 +1282,15 @@ Basic customization
builtin: cmp
single: comparisons
Called by comparison operations if rich comparison (see above) is not defined.
Should return a negative integer if ``self < other``, zero if ``self == other``,
a positive integer if ``self > other``. If no :meth:`__cmp__`, :meth:`__eq__`
or :meth:`__ne__` operation is defined, class instances are compared by object
identity ("address"). See also the description of :meth:`__hash__` for some
important notes on creating 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.)
Called by comparison operations if rich comparison (see above) is not
defined. Should return a negative integer if ``self < other``, zero if
``self == other``, a positive integer if ``self > other``. If no
:meth:`__cmp__`, :meth:`__eq__` or :meth:`__ne__` operation is defined, class
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.)
.. method:: object.__hash__(self)
@ -1293,19 +1300,12 @@ Basic customization
builtin: hash
single: __cmp__() (object method)
Called for the key object for dictionary operations, and by the built-in
function :func:`hash`. Should return a 32-bit integer usable as a hash value
Called for the key object for dictionary operations, and by the built-in
function :func:`hash`. Should return an integer usable as a hash value
for dictionary operations. The only required property is that objects which
compare equal have the same hash value; it is advised to somehow mix together
(e.g., using exclusive or) the hash values for the components of the object that
also play a part in comparison of objects. If a class does not define a
:meth:`__cmp__` 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).
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.

2
Doc/reference/expressions.rst
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@ -267,7 +267,7 @@ in the new dictionary in the order they are produced.
hashable
Restrictions on the types of the key values are listed earlier in section
:ref:`types`. (To summarize, the key type should be hashable, which excludes
:ref:`types`. (To summarize, the key type should be :term:`hashable`, which excludes
all mutable objects.) Clashes between duplicate keys are not detected; the last
datum (textually rightmost in the display) stored for a given key value
prevails.