Mercurial > cpython-lru_cache-weakref

diff Doc/c-api/typeobj.rst @ 48741:7517bdcc769a

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Merged revisions 65810 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r65810 | nick.coghlan | 2008-08-18 23:14:22 +1000 (Mon, 18 Aug 2008) | 1 line Issue 2235: document PyObject_HashNotImplemented ........
author Nick Coghlan <ncoghlan@gmail.com>
date Mon, 18 Aug 2008 13:18:16 +0000
parents 6a7eb0b70198
children 3302045bae97
line wrap: on
line diff
--- a/Doc/c-api/typeobj.rst	Mon Aug 18 12:31:52 2008 +0000
+++ b/Doc/c-api/typeobj.rst	Mon Aug 18 13:18:16 2008 +0000
@@ -321,6 +321,14 @@
    error occurs during the computation of the hash value, the function should set
    an exception and return ``-1``.
 
+   This field can be set explicitly to :cfunc:`PyObject_HashNotImplemented` to
+   block inheritance of the hash method from a parent type. This is interpreted
+   as the equivalent of ``__hash__ = None`` at the Python level, causing
+   ``isinstance(o, collections.Hashable)`` to correctly return ``False``. Note
+   that the converse is also true - setting ``__hash__ = None`` on a class at
+   the Python level will result in the ``tp_hash`` slot being set to
+   :cfunc:`PyObject_HashNotImplemented`.
+
    When this field is not set, two possibilities exist: if the :attr:`tp_compare`
    and :attr:`tp_richcompare` fields are both *NULL*, a default hash value based on
    the object's address is returned; otherwise, a :exc:`TypeError` is raised.