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gh-144873: Document isinstance Optimization #144997

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bc8102c
Update documentation to describe type(instance) is cls edge case of _…
George-Ogden Feb 19, 2026
038e10c
Update internal documentation to describe type(instance) is cls edge …
George-Ogden Feb 19, 2026
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4 changes: 3 additions & 1 deletion Doc/c-api/object.rst
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Expand Up @@ -433,7 +433,9 @@ Object Protocol

If *cls* has a :meth:`~type.__instancecheck__` method, it will be called to
determine the subclass status as described in :pep:`3119`. Otherwise, *inst*
is an instance of *cls* if its class is a subclass of *cls*.
is an instance of *cls* if its class is a subclass of *cls*. If *inst* is
an instance of *cls*, this will return ``1`` and will not call
:meth:`~type.__instancecheck__`.

An instance *inst* can override what is considered its class by having a
:attr:`~object.__class__` attribute.
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6 changes: 5 additions & 1 deletion Doc/reference/datamodel.rst
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Expand Up @@ -2965,7 +2965,11 @@ ABCs.

Return true if *instance* should be considered a (direct or indirect)
instance of *class*. If defined, called to implement ``isinstance(instance,
class)``.
cls)``.

.. note::
If ``type(instance) is cls`` then ``isinstance(instance, cls)`` is
always ``True`` and ``__instancecheck__`` is not called.


.. method:: type.__subclasscheck__(self, subclass)
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