New submission from Ethan Furman <ethan@stoneleaf.us>:

From http://docs.python.org/py3k/reference/datamodel.html#object.__hash__

Classes which inherit a __hash__() method from a parent class but change the meaning of __eq__() such that the hash value returned is no longer appropriate (e.g. by switching to a value-based concept of equality instead of the default identity based equality) can explicitly flag themselves as being unhashable by setting __hash__ = None in the class definition. Doing so means that not only will instances of the class raise an appropriate TypeError when a program attempts to retrieve their hash value, but they will also be correctly identified as unhashable when checking isinstance(obj, collections.Hashable) (unlike classes which define their own __hash__() to explicitly raise TypeError). If a class that overrides __eq__() needs to retain the implementation of __hash__() from a parent class, the interpreter must be told this explicitly by setting __hash__ = <ParentClass>.__hash__. Otherwise the inheritance of __hash__() will be blocked, just as if __hash__ had been explicitly set to None. ----------------------------------------------------------------------- The first paragraph says the user has to change __hash__ if it's different because of changes to __eq__, the second paragraph says __hash__ is automatically removed if __eq__ is changed; the second paragraph reflects reality. Proposed change: ----------------------------------------------------------------------- Classes which change the meaning of __eq__() (thus losing automatic delegation to the parent class' __hash__) can explicitly flag themselves as being unhashable by setting __hash__ = None in the class definition (which is otherwise done implicity). Having __hash__ set to None, either explicitly or implicitly, means that not only will instances of the class raise an appropriate TypeError when a program attempts to retrieve their hash value, but they will also be correctly identified as unhashable when checking isinstance(obj, collections.Hashable) (unlike classes which define their own __hash__() to explicitly raise TypeError). If a class that overrides __eq__() needs to retain the implementation of __hash__() from a parent class, the interpreter must be told this explicitly by setting __hash__ = <ParentClass>.__hash__. ----------------------------------------------------------------------- Patch attached. ---------- assignee: docs@python components: Documentation, Interpreter Core files: __hash__.diff keywords: patch messages: 158644 nosy: docs@python, stoneleaf priority: normal severity: normal status: open title: confusing docs with regard to __hash__ type: behavior versions: Python 3.1, Python 3.2, Python 3.3 Added file: http://bugs.python.org/file25261/__hash__.diff _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue14617> _______________________________________