[docs] [issue15981] improve documentation of __hash__
report at bugs.python.org
Thu Sep 20 13:00:24 CEST 2012
New submission from Max:
In dev/reference/datamodel#object.__hash__, there are two paragraphs that seem inconsistent. The first paragraph seems to say that a class that overrides __eq__() *should* explicitly flag itself as unhashable. The next paragraph says that a class that overrides __eq__() *will be* flagged unhashable by default. Which one is it?
Here are the two paragraphs:
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.
assignee: docs at python
nosy: docs at python, max
title: improve documentation of __hash__
versions: Python 3.3
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