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Issue 2235: document the ability to block inheritance of __hash__ in the language reference

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Nick Coghlan 2008-08-31 13:10:50 +00:00
parent 4d028570c7
commit 82358691f7
1 changed files with 18 additions and 2 deletions
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Doc/reference/datamodel.rst
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@ -1382,13 +1382,29 @@ Basic customization
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)``.
by default; with them, all objects compare unequal (except with themselves)
and ``x.__hash__()`` returns ``id(x)``.
Classes which inherit a :meth:`__hash__` method from a parent class but
change the meaning of :meth:`__cmp__` or :meth:`__eq__` such that the hash
value returned is no longer appropriate (e.g. by switching to a value-based
concept of equality instead of the default identity based equality) can
explicitly flag themselves as being unhashable by setting
``__hash__ = None`` in the class definition. Doing so means that not only
will instances of the class raise an appropriate :exc:`TypeError` when
a program attempts to retrieve their hash value, but they will also be
correctly identified as unhashable when checking
``isinstance(obj, collections.Hashable)`` (unlike classes which define
their own :meth:`__hash__` to explicitly raise :exc:`TypeError`).
.. versionchanged:: 2.5
:meth:`__hash__` may now also return a long integer object; the 32-bit
integer is then derived from the hash of that object.
.. versionchanged:: 2.6
:attr:`__hash__` may now be set to :const:`None` to explicitly flag
instances of a class as unhashable.
.. method:: object.__nonzero__(self)