[issue21782] hashable documentation error: shouldn't mention id
New submission from Giacomo Alzetta: The documentation for hashable in the glossary (https://docs.python.org/3.4/reference/datamodel.html#object.__hash__) is incorrect: they all compare unequal (except with themselves), **and their hash value is their id().** It is *not* true that their hash is their id (see relevant SO question: http://stackoverflow.com/questions/24249729/user-defined-class-hash-and-id-a...) Also the documentation for __hash__ (https://docs.python.org/3.4/reference/datamodel.html#object.__hash__) doesn't even mention id(). ---------- assignee: docs@python components: Documentation messages: 220746 nosy: Giacomo.Alzetta, docs@python priority: normal severity: normal status: open title: hashable documentation error: shouldn't mention id type: enhancement versions: Python 3.1, Python 3.2, Python 3.3, Python 3.4, Python 3.5 _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue21782> _______________________________________
R. David Murray added the comment: The statement is poorly worded. The id() is the *input* to the hash function used to compute the default __hash__. This is a CPython implementation detail, though, so perhaps all mention of it should indeed be dropped. ---------- nosy: +r.david.murray _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue21782> _______________________________________
Giacomo Alzetta added the comment: "their hash value is their id()" seems quite clearly stating that:
class A: pass ... a = A() hash(a) == id(a)
should be true, but:
hash(a) == id(a) False
(both in python2 and in python3) The python 2 documentation for the __hash__ special method *does* state that the default implementation returns a value "derived" by id(). I believe there is an inconsistency. In the python2 glossary it should say: "their hash value is derived from their id()" While in python3 it shouldn't mention id() at all, since the documentation for __hash__ doesn't mention it at all. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue21782> _______________________________________
R. David Murray added the comment: Yes, I should have been clearer. By "poorly worded" I meant "id is involved, but the sentence does not correctly describe *how* id is involved". That is, the sentence is wrong as written. The implementation is the same in both Python2 and Python3, though the source code organization and how it becomes the default hash is a bit different. ---------- _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue21782> _______________________________________
Roundup Robot added the comment: New changeset bfaf434a6f10 by Georg Brandl in branch '3.4': Closes #21782: the default hash(x) is not exactly id(x) but derived from it. https://hg.python.org/cpython/rev/bfaf434a6f10 ---------- nosy: +python-dev resolution: -> fixed stage: -> resolved status: open -> closed _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue21782> _______________________________________
participants (3)
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Giacomo Alzetta -
R. David Murray
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Roundup Robot