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Update docs: mention setting __hash__ = None for custom __eq__ method #2287

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Update docs: mention setting __hash__ = None for custom __eq__ method #2287

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Update classes.rst
ixje Jul 10, 2020
fd12511
Update classes.rst
ixje Jul 10, 2020
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7 changes: 7 additions & 0 deletions docs/advanced/classes.rst
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Expand Up @@ -695,6 +695,13 @@ throwing a type error.

To use the more convenient ``py::self`` notation, the additional
header file :file:`pybind11/operators.h` must be included.

.. note::

A class that overrides ``__eq__()`` and does not define ``__hash__()`` will have its ``__hash__()`` implicitly set.
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Suggestion: When a class overrides __eq__() and does not define __hash__(), pybind11 will automatically add a __hash__(). (I think this rewording makes it more clear that this behavior is something pybind11 does.)

This is a deviation from the standard Python behaviour where ``__hash__()`` is set to ``None`` when overriding ``__eq__()``. To restore the default behaviour
one can use ``.attr("__hash__") = py::none()``. When set ``None`` the class will raise an appropriate TypeError when a program attempts to retrieve their
hash value, and will also be correctly identified as unhashable when checking ``isinstance(obj, collections.abc.Hashable)``.

.. seealso::

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