[docs] [issue14617] confusing docs with regard to __hash__

Ethan Furman report at bugs.python.org
Wed Apr 18 19:55:28 CEST 2012


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

>From http://docs.python.org/py3k/reference/datamodel.html#object.__hash__
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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.
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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:
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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__.
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Patch attached.

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assignee: docs at python
components: Documentation, Interpreter Core
files: __hash__.diff
keywords: patch
messages: 158644
nosy: docs at 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

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue14617>
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