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gh-159580: Improve performance of isinstance() for misses and improve documentation #159581

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gh-159580: Improve performance of isinstance() for misses and improve documentation #159581

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23f9f6b
Skip the __class__ lookup in isinstance() when it cannot differ from …
eendebakpt Oct 16, 2026
05f4669
Use a stack reference for the __class__ descriptor lookup in isinstan…
eendebakpt Oct 16, 2026
6fad9d7
Document that isinstance() also consults a differing __class__ attribute
eendebakpt Oct 16, 2026
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5 changes: 5 additions & 0 deletions Doc/builtins/functions.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -1174,6 +1174,11 @@ are always available. They are listed here in alphabetical order.
a :exc:`TypeError` exception is raised. :exc:`TypeError` may not be
raised for an invalid type if an earlier check succeeds.

If *object* has a ``__class__`` attribute that differs from
:func:`type(object) <type>` (for example a proxy, or a :mod:`unittest.mock`
object created with a *spec*), ``True`` is also returned when that class
is a subclass of *classinfo*.

.. versionchanged:: 3.10
*classinfo* can be a :ref:`types-union`.

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25 changes: 25 additions & 0 deletions Objects/abstract.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2600,6 +2600,28 @@ check_class(PyObject *cls, const char *error)
return -1;
}

/* Return 1 if inst.__class__ would resolve to object's own getset descriptor,
i.e. evaluate to Py_TYPE(inst). Types with a custom tp_getattro or an
overridden __class__ return 0 and take the generic lookup. */
static inline int
class_attr_is_type(PyObject *inst)
{
PyTypeObject *tp = Py_TYPE(inst);
if (tp->tp_getattro != PyObject_GenericGetAttr) {
return 0;
}
_PyStackRef cref;
_PyType_LookupStackRefAndVersion(tp, &_Py_ID(__class__), &cref);
if (PyStackRef_IsNull(cref)) {
return 0;
}
PyObject *descr = PyStackRef_AsPyObjectBorrow(cref);
int res = (Py_IS_TYPE(descr, &PyGetSetDescr_Type)
&& PyDescr_TYPE(descr) == &PyBaseObject_Type);
PyStackRef_CLOSE(cref);
return res;
}

static int
object_isinstance(PyObject *inst, PyObject *cls)
{
Expand All @@ -2608,6 +2630,9 @@ object_isinstance(PyObject *inst, PyObject *cls)
if (PyType_Check(cls)) {
retval = PyObject_TypeCheck(inst, (PyTypeObject *)cls);
if (retval == 0) {
if (class_attr_is_type(inst)) {
return 0;
}
retval = PyObject_GetOptionalAttr(inst, &_Py_ID(__class__), &icls);
if (icls != NULL) {
if (icls != (PyObject *)(Py_TYPE(inst)) && PyType_Check(icls)) {
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