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| -rw-r--r-- | Doc/reference/datamodel.rst | 26 |
1 files changed, 13 insertions, 13 deletions
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst index 7dcd459..2ddb852 100644 --- a/Doc/reference/datamodel.rst +++ b/Doc/reference/datamodel.rst @@ -1255,22 +1255,22 @@ Basic customization by default; with them, all objects compare unequal (except with themselves) and ``x.__hash__()`` returns ``id(x)``. - Classes which inherit a :meth:`__hash__` method from a parent class but - change the meaning of :meth:`__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 :exc:`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 :meth:`__hash__` to explicitly raise :exc:`TypeError`). + A class that overrides :meth:`__eq__` and does not define :meth:`__hash__` + will have its :meth:`__hash__` implicitly set to ``None``. When the + :meth:`__hash__` method of a class is ``None``, instances of the class will + raise an appropriate :exc:`TypeError` when a program attempts to retrieve + their hash value, and will also be correctly identified as unhashable when + checking ``isinstance(obj, collections.Hashable``). If a class that overrides :meth:`__eq__` needs to retain the implementation of :meth:`__hash__` from a parent class, the interpreter must be told this - explicitly by setting ``__hash__ = <ParentClass>.__hash__``. Otherwise the - inheritance of :meth:`__hash__` will be blocked, just as if :attr:`__hash__` - had been explicitly set to :const:`None`. + explicitly by setting ``__hash__ = <ParentClass>.__hash__``. + + If a class that does not override :meth:`__eq__` wishes to suppress hash + support, it should include ``__hash__ = None`` in the class definition. + A class which defines its own :meth:`__hash__` that explicitly raises + a :exc:`TypeError` would be incorrectly identified as hashable by + an ``isinstance(obj, collections.Hashable)`` call. See also the :option:`-R` command-line option. |