From f6447e006587fcb632b80ca7e5888cefc1a26d58 Mon Sep 17 00:00:00 2001 From: Victor Stinner Date: Mon, 19 Dec 2016 13:15:35 +0100 Subject: doc: Suggest to hash(tuple of attr) rather than XOR Issue #28383: __hash__ documentation recommends naive XOR to combine but this is suboptimal. Update the doc to suggest to reuse the hash() method using a tuple, with an example. --- Doc/reference/datamodel.rst | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index db2fa17..4adf5fa 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -1409,8 +1409,12 @@ Basic customization 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. + advised to mix together the hash values of the components of the object that + also play a part in comparison of objects by packing them into a tuple and + hashing the tuple. Example:: + + def __hash__(self): + return hash((self.name, self.nick, self.color)) If a class does not define a :meth:`__cmp__` or :meth:`__eq__` method it should not define a :meth:`__hash__` operation either; if it defines -- cgit v0.12