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Diffstat (limited to 'Doc/reference/datamodel.rst')
| -rw-r--r-- | Doc/reference/datamodel.rst | 19 |
1 files changed, 15 insertions, 4 deletions
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 4f9b8b0..1a48c1f 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -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 |