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- Issue #18440: Clarify that hash() can truncate the value returned from an

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object's custom `__hash__()` method.
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Barry Warsaw 2013-07-15 14:47:29 -04:00
parent 48830035e5
commit 224a599c0c
3 changed files with 27 additions and 8 deletions
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19
Doc/reference/datamodel.rst
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@ -1264,10 +1264,21 @@ Basic customization
Called by built-in function :func:`hash` and for operations on members of
hashed collections including :class:`set`, :class:`frozenset`, and
:class:`dict`. :meth:`__hash__` should return an integer. 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.
:class:`dict`. :meth:`__hash__` should return an integer. 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.
.. note::
:func:`hash` truncates the value returned from an object's custom
:meth:`__hash__` method to the size of a :c:type:`Py_ssize_t`. This is
typically 8 bytes on 64-bit builds and 4 bytes on 32-bit builds. If an
object's :meth:`__hash__` must interoperate on builds of different bit
sizes, be sure to check the width on all supported builds. An easy way
to do this is with
``python -c "import sys; print(sys.hash_info.width)"``
If a class does not define an :meth:`__eq__` method it should not define a
:meth:`__hash__` operation either; if it defines :meth:`__eq__` but not